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International Journal of Intelligent Robotics and Applications (2018) 2:1–28 https://doi.org/10.1007/s41315-017-0042-6

REGULAR PAPER

Hybrid FES–robotic gait rehabilitation technologies: a review on mechanical design, actuation, and control strategies

Francisco Anaya1 · Pavithra Thangavel1 · Haoyong Yu1

Received: 19 September 2017 / Accepted: 27 December 2017 / Published online: 17 January 2018 © Springer Nature Singapore Pte Ltd. 2018

AbstractGait disorders in neurologically disabled people can be treated by various techniques available today which include passive orthoses, functional electrical stimulation (FES) and robot assisted gait training devices (RAGT). However, each system has its own drawback. For example, gait rehabilitation with orthosis is physically taxing for the patient with no significant functional improvement. FES uses muscle powers as physiological actuators to promote balance and improve gait but leads to fatigue, along with poor control of joint trajectories. RAGT devices including powered exoskeletons, gait rehabilitation systems employing programmable footplates and mobile training platforms, have shown significant advantages but the devices are not yet mature due to numerous drawbacks associated with physical and cognitive interaction, energy-management and portability issues. The combination of FES technology and RAGT devices, often named hybrid FES–robot technologies, has arisen as a promising approach to aid in gait restoration. This work reports a comprehensive review on the hybrid FES–robot technologies over the last decades, focusing on different mechanical structures, actuator designs, sensing technologies, and control approaches. The hybrid robotic structures are classified into two categories: (i) orthotic-based hybrid systems, where (a) FES is used to stimulate the muscles and produce joint torque while the robotic system acts as energy dissipating device, and (b) FES and robotic systems are both torque-generating devices; and (ii) non-orthotic based hybrid systems. The review compiles a variety of sources and illustrates the technology’s most important challenges in the fields of hybrid rehabilitation robotics which may contribute towards further development of hybrid robot systems.

Keywords Functional electrical stimulation · FES · Neuroprosthesis · Robotic assisted gait · Robot control · Muscle fatigue · Rehabilitation · Gait · Hybrid control

AbbreviationsFES Functional electrical stimulationRAGT Robot assisted gait trainingCBO Controlled-brake orthosisSBO Spring-brake orthosisGBO Gravity balanced orthosisJCO Joint-coupled orthosisESO Energy storing orthosisVCHM Variable constraint hip mechanismSEAHO Semi-active hybrid orthosisIMU Inertial measurement unitsHyPO Hybrid powered orthosisFES-IM Intramuscular functional electrical stimulation

BWS Body weight supportPID Proportional-integral-derivativeVIKM Variable impedance knee mechanismHNP Hybrid neuroprosthesisGT-FES Gait trainer with functional electrical

stimulationCT Conventional therapyAFO Ankle-foot orthosisLEE Lower-extremity exoskeletonBLERE Bionic lower extremity rehabilitation

exoskeleton

1 Background

According to the World Health Organization, around 15 mil-lion people fall victim to stroke every year around the world, and this number is estimated to grow by 3.4 million people by 2030 (Mackay et al. 2004). The most important body

* Haoyong Yu [email protected]

1 Department of Biomedical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore

http://orcid.org/0000-0002-9876-4863

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2 F. Anaya et al.

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function disability related to stroke is the inability to produce controlled movement. This motor impairment has a major impact on the stroke survivor’s quality of life. Stroke patients undergo rehabilitation therapy in the hope of recovering their lost neuromuscular functions and to regain control over their body movement. However, even after completing conventional therapy only 40–50% of stroke patients benefit from this intensive therapy (Schaechter 2004). In particular, stroke patients who had a unilateral paralysis rarely regain walking function to the point of effective ambulation (Hara 2013).

Many methods of training are available for gait recovery in persons with motor impairment. However, these numer-ous approaches have their own limitation which calls for the need to improve the available rehabilitation interven-tions that aim to recover the walking capability. Functional electrical stimulation (FES) and robotic rehabilitation ther-apy targeting motor function recovery have been gaining attention over the past decade. FES-based approach aims to deliver low intensity electrical stimulation to promote muscle contraction and produce flexion, extension of joints needed for ambulation. In comparison with conventional treatment, FES is reported to produce better results by not only promoting motor improvements but also by inducing changes in motor cortex excitability and functional corti-cal reorganization (Maffiuletti et al. 2011; Popović 2014). However, the high degree of complexity and non-linearity involved in controlling joint movement during the swing phase of FES-induced gait prevents its widespread adop-tion in lower limb rehabilitation. Also, the high metabolic and performance costs of activity caused by the induced recruitment of motor unit results in fast and abrupt incidence of muscle fatigue, which consequently foils proper progres-sion of the treatment (Lynch and Popovic 2008; Zhang et al. 2007; Popovic 2003).

Rehabilitation robots are supposed to be a promising therapy approach that can deliver longer duration of therapy with adjustable training intensity and customized treatment for individual patient’s needs and problems. As the robotic technology can provide accurate kinetic and kinematic measurements, it is easier to quantify a person’s functional improvement with the treatment (Lum et al. 2002; Huang and Krakauer 2009). Nonetheless, robot-assisted training is susceptible to limited effect, where patient’s compliance is restricted, resulting in no functional improvements. Regard-less of the developments in sophisticated systems focused at enhancing rehabilitation outcomes, the use of robotics is still controversial due to the lack of evidence supporting superiority of robotic therapy restoring motor function over standard health care.

To overcome the drawbacks of each individual approach, the combination of FES with robotic devices was proposed; which aims to provide more effective, safe, and robust

rehabilitation therapy. These combined technologies are con-ventionally termed Hybrid Robotic Rehabilitation Systems and are defined as systems that aim to achieve motor recov-ery or compensate motor function by combining electrically stimulated muscle action and torque provision to the joints (del-Ama et al. 2014). A previous review has identified and discussed the key technological aspects concerning hybrid exoskeletons (del-Alma et al. 2012). However, a review not only focusing on the application of this technology to pow-ered exoskeletons but also to robotic footplates systems, mobile platforms and cycling systems is still not recorded in literature. In this paper, we also discuss the limitations of the currently developed systems and future research and development directions of hybrid robotic technologies. We provide detailed comparison for the exoskeleton bio-inspired joint design and highlight the rising hybrid robotic technolo-gies, with focus on their mechanical design, actuator system, and sensing and control strategy.

Our analysis follows two classifications: (i) orthotic-based hybrid systems, where (a) FES is used to stimulate the mus-cles and produce joint torque while the robotic system acts as energy dissipating device, and (b) FES and robotic systems are both torque-generating devices; and (ii) non-orthotic based hybrid systems which include robotic footplates sys-tems, mobile platforms and cycling systems. The analysis is addressed from a technological and a clinical perspective where their main challenges for consolidation is discussed. This may contribute towards further development of hybrid robotic rehabilitation systems.

2 Results

From the search results, 55 papers were selected for the review. This amounted to 28 different hybrid robotic system which were then classified into two groups: orthotic-based hybrid systems (23), and non-orthotic based hybrid systems (5), including robotic footplates systems, mobile platforms and cycling systems.

Relevant and referenced publications were included in the survey if they: (i) Fit into the definition of hybrid rehabilita-tion systems (presented a shared use of robotics with passive or active actuation and functional electrical stimulation), (ii) focused on lower limb rehabilitation and (iii) considered at least one gait outcome measure such as kinematic data, elec-tromyography signal analysis, force measurements, clinical scales, or functional evaluation in either healthy subjects or stroke survivors. Works in which the robotic treatment and FES were prescribed separately were excluded from this review. The sequence of the reviewed technologies follows a time order and the invention year of the robot is added in each summary table.

3Hybrid FES–robotic gait rehabilitation technologies: a review on mechanical design, actuation,…

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2.1 Technical overview of orthosis‑based hybrid systems

In the context of this review, we define FES Hybrid-Robotic gait rehabilitation technologies as those systems that aim to achieve motor recovery or compensate motor function in patients with neurological challenges by combining the effects of functional electrical stimulation and torque pro-vision at joint level. The combination of FES and robotic control is not a new concept. Hybrid robotic systems were introduced for the first time in 1972 by Tomovic R. (1973) after which several hybrid technologies have been intro-duced with varied mechanical designs, actuation, and con-trol approaches. However, a two-fold classification could be identified: (1) Hybrid systems where FES is used to stimulate the muscles and produce joint torque while the robotic system acts as energy dissipating device, and (2) Active hybrid systems where FES and robotic system are both torque generating devices.

2.1.1 FES is used to stimulate the muscles and produce joint torque while the robotic system acts as energy dissipating device

We identified eight main hybrid systems for the recovery of gait related functions in patients with motor impairments where FES is used to stimulate the muscles and produce joint torque while the orthotic robot acts as energy dissi-pating device. Hybrid systems with energy dissipation are built on the bases of systems where FES is used to actively stimulate the muscles of the limb while the robotic systems are mostly passive gait orthosis. Here, the former hybrid system provides joint trajectory control and reduces muscle fatigue with the help of elastic elements and joints brakes. Table 1 shows a summary of the reported orthotic-based hybrid systems where the robotic systems have been mostly used as energy dissipation devices.

2.1.1.1 Hybrid orthosis controlled by joint brakes Durfee and Hausdorff were the first to propose the concept of joint brakes to control joint movements generated by FES in 1990 (Durfee and Hausdorff 1990), by presenting three controllers for position tracking of the free-swinging shank in hybrid FES gait. To build on this concept, the authors introduced the 8 DOFs controlled-brake orthosis (CBO) with four FES channels (Fig. 1) (Durfee et al. 1992; Goldfarb and Durfee 1996; Goldfarb et al. 2003). CBO is a long leg brace with friction controllable brakes at the hip and knee joints. It uses the muscle stimulation as a power source to produce desired joint trajectories while the friction controllable brakes reg-ulate the power generated by the electrical stimulation at each joint. The muscle fatigue was decreased by locking the brakes throughout the entire stance phase and by stopping

stimulation to the quadriceps. The laboratory CBO weighs 6 kg and refers to a two-sided device which interfaces with the hip, knee and ankle joints. The CBO completely relies on the robotic structure to provide support to the axial load-ing of joint movement at the hip. Experiments were per-formed for the evaluation of CBO on SCI patients wearing the device and their ambulation performance was accessed. The results of these experiments showed that the CBO was capable of providing better control over the joint trajecto-ries and significantly reduce muscle fatigue as compared to FES-only gait.

A second solution, named Sensor Walk, was presented by Sharma et al. (2014). This system consists of a novel KAFO with FES programmed to couple hip and knee flexion (Fig. 2). A spring clutch mechanism is built into the system for locking and unlocking of the joints during the stance phase of gait. The KAFO system locks the knee joint just before stance phase and remains locked until early swing phase. Similar to previously reported system (Goldfarb et al. 2003), the KAFO locks the knee joint of both legs during double support phase and unlock the knee joint when swing phase is initiated. The KAFO is completely passive, with the only purpose of providing additional stability and support to the individual’s body weight by unlocking and locking the knee joint while hip flexors/extensors contract to produce hip torque during the swing phase. However, no clinical tests have been done for this technology.

A similar solution to manage muscle fatigue and joint trajectory was introduced by Chang et al. (2016, 2017). The Stimulation-driven exoskeleton system was designed to restore the loss of motor function caused by varying types of paralysis from spinal cord trauma. The hybrid neuro-prosthesis consists of the combination of a passive variable-constraint exoskeleton to provide better support and stability with FES (Fig. 3). The overall control of the sys-tem governed the locking and unlocking of the lower limb exoskeleton’s hip and knee joints as well as the stimulation designs triggering hip and knee flexors and extensors mus-cles across different phases of gait. Attaching controllable brakes to passive gait orthosis during FES-assisted walk-ing aimed to solve the drawbacks of joint trajectory control generated by FES. Unlike other hybrid neuro-prosthesis, the Stimulation-driven exoskeleton, followed the “muscle-first” approach where the stimulated contraction of the muscles produced the joint movements rather than using motor driv-ers to produce these motions.

The stimulation-driven exoskeleton developed was tested on an individual with paraplegic gait to (1) evaluate the functional electrical stimulation approach required to induce the desired limb motions and (2) assess the control design by means of gait event detection algorithm for stepping with a finite-state automaton controller. The high-level control of the controller defined the states of the hydraulics and was

4 F. Anaya et al.

1 3

Tabl

e 1

Iden

tified

orth

oses

-bas

ed h

ybrid

syste

ms f

or g

ait f

unct

ion

whe

re F

ES is

use

d to

stim

ulat

e th

e m

uscl

es a

nd p

rodu

ce jo

int t

orqu

e w

hile

the

robo

tic sy

stem

act

s as e

nerg

y di

ssip

atin

g de

vice

Dev

ice

nam

e/gr

oup

Wea

rabl

e ro

bot

Mot

or-n

euro

pros

thes

isC

linic

al e

valu

atio

nD

raw

back

Hyb

rid o

rthos

is c

ontro

lled

by jo

int b

rake

s C

ontro

lled-

brak

e or

thos

is C

BO

(199

2) (D

urfe

e et

al.

1992

; Gol

d-fa

rb a

nd D

urfe

e 19

96; G

oldf

arb

et a

l. 20

03)

Pass

ive

Hip

and

Kne

e: b

rake

. Ank

le:

elas

tic.

Orth

osis

wei

ght:

6 kg

.8

DO

FsRo

M: H

ip -1

0-25

°, K

nee

0–60

°.B

rake

torq

ue: H

ip 3

0 N

m, K

nee

50 N

m

4 FE

S-C

hann

els

Feed

back

con

trol:

mus

cula

r fat

igue

m

onito

ring.

Per

onea

l ner

ve st

imu-

latio

n fo

r hip

and

kne

e fle

xion

. Q

uadr

icep

sG

ait p

hase

con

trol:

pero

neal

ner

ve

stim

ulat

ed d

urin

g sw

ing

phas

e to

fle

x hi

p, k

nee,

and

ank

le; q

uadr

i-ce

ps st

imul

ated

bef

ore

doub

le

supp

ort (

stan

ce) t

o pr

epar

e fo

r leg

ex

tens

ion

1 Su

bjec

t:co

mpl

ete

T6 (G

oldf

arb

and

Dur

fee

1996

)4

subj

ects

: 3 c

ompl

ete

T6-T

7, 1

in

com

plet

e T8

(Gol

dfar

b et

al.

2003

)

Use

of w

ithdr

awal

refle

x

Otto

bock

Sen

sor W

alk

(201

4)

(Sha

rma

et a

l. 20

14)

Pass

ive

Kne

e, A

nkle

and

Foo

t: El

ec-

trom

agne

tic u

nloc

king

mec

hani

sm.

Orth

osis

wei

ght:

3.41

kg

3DO

Fs

4 FE

S-ch

anne

ls.

Hip

flex

ors a

nd e

xten

sors

.G

ait p

hase

con

trol:

stim

ulat

ion

initi

-at

ed d

urin

g sw

ing

phas

e

Not

teste

dEv

alua

tion

of sa

fety

and

usa

bilit

y ha

ve n

ot y

et b

een

repo

rted

Stim

ulat

ion-

driv

en e

xosk

elet

on

(201

6) (C

hang

et a

l.201

6; C

hang

et

al.

2017

)

Pass

ive

Hip

and

Kne

e: c

ontro

llabl

e hy

drau

lic L

ower

ext

rem

ity jo

int

cons

train

ts

12 F

ES-c

hann

els:

hip

flex

ors,

hip

exte

nsor

s, kn

ee fl

exor

s, kn

ee

exte

nsor

s and

ank

le d

orsi

flexo

rsO

pen

loop

/On–

off fe

edfo

rwar

d co

ntro

l.G

ait p

hase

con

trol:

stim

ulat

ion

initi

-at

ed d

urin

g ea

rly sw

ing

1 su

bjec

t:C

ompl

ete

T4 (C

hang

et a

l. 20

16)

Trig

ger b

utto

n.Pr

epro

gram

ed F

ES p

aram

eter

s

Sprin

g-br

ake

orth

osis

Spr

ing-

Bra

ke O

rthos

is S

BO (2

000)

(G

haro

oni e

t al.

2001

; Gha

roon

i et

al.

2000

; Jai

lani

et a

l. 20

11)

Pass

ive

Hip

and

Kne

e: b

rake

and

el

astic

-mec

hani

cal c

oupl

ing

2 FE

S-ch

anne

ls.

Feed

back

con

trol.

Ener

gy st

orag

e fro

m q

uadr

icep

s ele

ctro

stim

ula-

tion.

Gai

t pha

se c

ontro

l: sti

mul

atio

n in

itiat

ed d

urin

g st

ance

pha

se to

aid

kn

ee e

xten

sion

Not

teste

dEv

alua

tion

of sa

fety

and

usa

bilit

y ha

ve n

ot y

et b

een

repo

rted

Gra

vity

Bal

ance

d O

rthos

is G

BO

(200

6) (K

rishn

amoo

rthy

et a

l. 20

08; B

anal

a et

al.

2006

; A

graw

al e

t al.

2007

)

Pass

ive

Hip

, Kne

e &

Ank

le.

Sprin

gs to

bal

ance

gra

vity

effe

ct o

n lim

bs. T

read

mill

-bas

ed.

7 D

OFs

4 FE

S-ch

anne

ls.

Ank

le d

orsi

flexo

r and

pla

ntar

flexo

r m

uscl

es.

Ope

n-lo

op c

ontro

lG

ait p

hase

con

trol:

ankl

e do

rsifl

ex-

ors s

timul

ated

dur

ing

swin

g ph

ase

and

plan

tar fl

exor

s to

aid

knee

fle

xion

1 Su

bjec

t: str

oke

(Kris

hnam

oorth

y et

al.

2008

)Pr

epro

gram

ed F

ES p

aram

eter

s.To

o bu

lky:

onl

y fo

r clin

ic u

se. S

ta-

tiona

ry tr

aini

ng sy

stem


1 3

Tabl

e 1

(con

tinue

d)

Dev

ice

nam

e/gr

oup

Wea

rabl

e ro

bot

Mot

or-n

euro

pros

thes

isC

linic

al e

valu

atio

nD

raw

back

Join

t-cou

pled

orth

osis

Ene

rgy

storin

g or

thos

is E

SO

(200

5) (K

angu

de e

t al.

2009

; D

urfe

e an

d R

ivar

d 20

05; K

an-

gude

et a

l. 20

10)

Pass

ive

Hip

and

Kne

e: b

rake

and

el

astic

and

pne

umat

ic c

oupl

ing.

Orth

osis

wei

ght:

16.8

2 kg

.Ro

M: H

ip -1

0-25

°, K

nee

-5-6

0°.

Bra

ke to

rque

: Hip

17

Nm

, Kne

e 34

Nm

2 FE

S-ch

anne

ls.

Ope

n-lo

op c

ontro

lQ

uadr

icep

s mus

cles

. Use

stim

u-la

ted

mus

cle

ener

gy to

driv

e hi

p ac

tuat

or.

Gai

t pha

se c

ontro

l: sti

mul

atio

n in

itiat

ed d

urin

g st

ance

pha

se to

aid

kn

ee e

xten

sion

1 Su

bjec

t:In

com

plet

e T1

2 (K

angu

de e

t al.

2010

)

Wei

ght a

nd si

ze o

f the

syste

m.

Prep

rogr

amed

FES

par

amet

ers

Hyb

rid jo

int-c

oupl

ed c

ontro

lled

brak

e or

thos

is (2

009)

(Far

ris

et a

l. 20

09; F

arris

et a

l. 20

09)

Pass

ive

Hip

and

Kne

e: b

rake

and

el

astic

-mec

hani

cal u

nidi

rect

iona

l co

uplin

g.O

rthos

is w

eigh

t: 6

kgB

rake

torq

ue: 5

0.7

Nm

max

. 6 D

OFs

2 FE

S-ch

anne

lsO

pen-

loop

con

trol

Ener

gy st

orag

e fro

m q

uadr

icep

s el

ectro

stim

ulat

ion

Gai

t pha

se c

ontro

l: sti

mul

atio

n in

itiat

ed d

urin

g st

ance

pha

se to

aid

kn

ee e

xten

sion

Not

teste

dPr

epro

gram

ed F

ES p

aram

eter

s.Ev

alua

tion

of sa

fety

and

usa

bilit

y ha

ve n

ot y

et b

een

repo

rted

Varia

ble

hip

cons

train

t mec

hani

sms

Var

iabl

e co

nstra

in h

ip m

echa

nism

V

CH

M (2

008)

(Aud

u et

al.

2010

; K

obet

ic e

t al.

2009

; To

et a

l. 20

08)

Pass

ive

Hip

: Con

strai

nt O

rthos

isO

rthos

is w

eigh

t: 22

kg

4 FE

S-ch

anne

ls: h

ip fl

exor

s and

ex

tens

ors

Ope

n-lo

op c

ontro

lG

ait p

hase

con

trol:

stim

ulat

ion

initi

-at

ed d

urin

g sw

ing

phas

e

Not

teste

dW

eigh

t and

size

of t

he sy

stem

.Pr

epro

gram

ed F

ES p

aram

eter

s.Ev

alua

tion

of sa

fety

and

usa

bilit

y ha

ve n

ot y

et b

een

repo

rted

DO

Fs d

egre

es o

f fre

edom

, RoM

rang

e of

mot

ion,

FES

func

tiona

l ele

ctric

al st

imul

atio

n, T

thor

acic

6 F. Anaya et al.

1 3

coded with the sit-to-stand, standing, steeping, and stand to sit stimulation patterns. For the steeping state, the gait event detector determined transitions between early swing, double stance, late swing, and weight acceptance gait phases. At early swing phase, the contralateral knee joint was locked and the ipsilateral knee was unlocked. During double stance, all valves remained in their default state by locking the knees joints. Once the subject pressed the “go” button, the finite automaton shifted to early swing phase where muscle stimu-lation for the ipsilateral leg was initiated. At this point, while the ipsilateral knee mechanics was unlocked to allow the knee to swing freely, the contralateral knee remained locked. During late swing phase, and after an extension threshold was surpassed by the ipsilateral leg, the knee was locked for weight acceptance in stance phase. In this way, the weight acceptance phase ensure that weight was fully accepted prior

to initiating a new step. The individual then initiated the fol-lowing step for the contralateral leg by pressing the “go” but-ton again. Although during testing, the finite automaton con-troller successfully moved through the different states and the subjects showed sufficient joint angles and foot–ground clearance, this hybrid system has been reported to result in high energy cost of gait (del-Alma et al. 2012).

2.1.1.2 Spring‑brake orthosis Unlike those aforementioned technologies, Gharonni et al. (2000, 2001) presented a hybrid orthosis with energy storage (SBO). Here, stimula-tion is provided to the paralyzed muscles while a spring-loaded orthosis store and transfer the energy to produce joint flexion. Therefore, knee flexion during swing phase is achieved by means of the energy stored by stretching a spring which is released from rest whereas hip flexion is achieved by the gravitational force acting upon the system. Consequently, Gharonni et al.’s hybrid orthosis produces maximum possible torque through the stimulated quadri-ceps during knee extension. When excess stimulation is pro-vided to the quadriceps muscles, the spring resists the knee extension and the remaining quadriceps torque is stored in the spring to be discharged later. A break can then contract the spring, discharging its stored energy resulting in knee flexion. A single test on a non-disabled subject was reported using the spring-break orthosis which addresses the knee and hip joint kinematics.

Krishnamoorthy et al. (2008) tested a passive robotic sys-tem and FES on one patient with post stroke hemiparesis.

Fig. 1 Controlled-brake orthosis (CBO). Retrieved from (Goldfarb et al. 2003)

Fig. 2 Ottobock Sensor Walk with surface FES electrodes. Retrieved from (Sharma et al. 2014)

Fig. 3 Overview of the Self-contained exoskeleton. Retrieved from (Chang et al. 2017)


1 3

The robotic orthosis employed is known as Gravity Balanced Orthosis (GBO). GBO is a treadmill-based device which allows intension based ambulation of the patient and opti-mizes the effects of gravity on their limbs (Fig. 4). It also assists knee and hip joint movements with specially designed springs that compensate for the limb segment weights. The springs were mounted at a convenient location on the ortho-sis which was found by calculating the center of mass of the user to fully or partially balance gravity effects throughout the joint motion. The level of gravity balance can be adjusted depending on the user’s need. Since it did not have any actu-ators to generate joint motion it worked as a passive device.

This system used FES to stimulate ankle dorsiflexor muscles during swing phase and plantarflexion muscles to assist knee flexion. Foot switches were used to determine gait phases during treadmill walking. Initial test on the subject was performed to determine the stimulation inten-sity required to produce knee flexion\extension and ankle plantar\dorsiflexion. The results of the preliminary study showed an increase in the patient’s gait parameters such as walking speed, weight bearing capacity of the paretic leg, knee and hip joint angles. And the positive changes were still retained during the 1 month follow up evaluation. The researchers also added that the combination GBO and FES can benefit patients with inadequate hip–knee flexion during swing phase from GBO support and electrical stimulation

can compensate for the inadequate muscle strength to gener-ate plantar-dorsiflexion.

2.1.1.3 Joint‑coupled orthosis On top of the spring-break orthosis, the joint coupled orthosis concept was developed based on the elastic-storage principle, but in this case, the elastic element acts across two joints. Durfee et al. (Kangude et al. 2009, 2010; Durfee and Rivard 2005), developed an energy storing orthosis (ESO) that elicits the hip motion by means of a pneumatically driven actuator which stores and transfers energy from knee extension caused by quadriceps stimulation. Thus, the robotic structure not only governs the locking and unlocking of the exoskeletal joints, but also employs the induced quadriceps muscle power to “push on the orthosis” while storing energy in the process. The power transmission systems are added on commercial orthotics, the Newport 4 Hip System belt, and the Donjoy Legend knee brace. The total weight of the system is 16.82 kg. The system description is as follows: The knee and hip joints are held in flexed positions by the elastic energy storage actua-tors. FES generates energy for knee extension by stimulat-ing the quadriceps and the excess energy generated is stored in the spring and the energy transfer element. This stored energy is then transferred and discharged at the hip joint causing hip extension and enabling forward progression. Lastly, release of the hip and knee joints from the straight-leg position to the initial flexed equilibrium position initi-ates a new step. Pre-clinical evaluation on a single 51 year old male with T12 spinal cord injury was performed during fit and standing tasks. Nevertheless, the outcomes of this test were restricted to those of safety and fitting functions of the orthosis, with no major results on functionality.

A similar approach to Durfee’s Energy Storing Ortho-sis was presented by Farris et al. (2009), where a hybrid FES–robot cooperative control system incorporated two FES-channels with a microprocessor-controlled orthosis for gait rehabilitation. In this orthosis, the flexion of the knee leads to hip flexion as both joints were unidirectionally cou-pled. Additionally, the hybrid orthosis included sensors and modulated friction brakes which worked in combination with FES to provide feedback control of joint trajectories. Quadri-ceps muscles were stimulated by two FES channels due to their easy accessibility and strong contraction. The control-lable friction brakes on the orthosis lock the knee joint dur-ing stance and when the knee joint is unlocked during swing, it also releases the stored energy from the biasing spring which causes the knee to flex; and due to the joint coupling effect, the hip joint also flexes. During the late swing phase, the hip brakes lock the hip joint while the quadriceps mus-cles are stimulated causing knee extension. Once the knee is fully extended, it remains locked during the stance phase of the gait. A simulation of the joint-coupled controlled brake orthosis was conducted on ten healthy subjects to determine

Fig. 4 Gravity Balanced Orthosis (GBO) with FES. Retrieved from (Krishnamoorthy et al. 2008)

8 F. Anaya et al.

1 3

the efficiency of spring and joint coupling to generate hip\knee flexion and if quadriceps stimulation generated suf-ficient power without causing muscle fatigue. The results reported that the knee flexion amplitude stabilized at 85% of the mean amplitude which led to the conclusion that this energy storing method might slow down the onset of muscle fatigue. However, experiments were conducted in healthy individuals only and clinical data on neurologically injured patients has not yet been reported.

2.1.1.4 Variable hip constraint mechanism Audu et al. (2010) introduced a variable constraint hip mechanism (VCHM) intended to be worn in combination with FES. The VCHM consists on a hydraulic mechanism in which double acting hydraulic cylinders were connected to the hip joints via custom rack-and-pinion transmissions (Fig. 5). The hip constrain states are achieved by activating specific valve(s) which lock the hips of the VCHM depending on the phase of gait, therefore allowing free movement when the hips are being stimulated by the FES while restricting the joints movement when support is required. In such manner, the hip joints are coupled during double stance phase and locked against flexion during single stance to prevent undesirable anterior trunk tilt. During swing phase, the hip joints are released to move, allowing the leg to swing passively or be

powered by FES, while producing forward progression. The effect of the hip mechanism was studied in five able-bodied subjects with no history of orthopedic illness or any other ailment that would prejudice their walking abilities. Results showed a reduced normalized hip movement suggesting that the VCHM allows for improved unconstrained hip motion. However, future testing with individuals with paraplegia may produce dissimilar results.

2.1.2 FES and robotic system are both torque generating devices

We identified 15 main orthotic-based hybrid technologies for the restoration and support of gait functions where FES and the robotic system are both torque-generating devices (Table 2). With regard to the actuation principle of the wear-able robot joints, hybrid powered wearable robots are clas-sified as either semi-active or fully active (del-Ama et al. 2016). Powered robotic systems for gait rehabilitation often follow the following concept: (1) applying power to the hip, (2) dissipating power at the knee and (3) storing and releas-ing energy at the ankle trough elastic components (Pons et al. 2013). In most existing robotic system designs, electric direct drivers and brushless DC-motor-gearbox combina-tions are mainly employed (Pons et al. 2013).

An electromechanical gait trainer GT II (RehaStim) was developed by Hesse et al. (2000; Hesse 2004). It consists of a BWS system and two-foot plates driven by servo motor controller whose movements simulate symmetric stance and swing (Fig. 6). Ng et al. (2009) tested this device with func-tional electrical stimulation (GT-FES) on 16 stroke patients and compared the results to conventional physiotherapy (CT). The protocol involved 20 min of gait training with GT II and FES was provided simultaneously. Two Chan-nel FES stimulator was linked to the gait trainer control-ler which synchronized the gait phase and the stimulation timings. Subjects received standardized stimulation to the quadriceps during stance phase to facilitate weight accept-ance and the peroneal nerve during swing phase to facili-tate ankle dorsiflexion and knee flexion. The results proved that the GT-FES group showed significant improvement in lower limb strength, mobility, ambulation ability and walk-ing speed as compared to that of CT group.

Vallery et al. (2005) introduced a hybrid motor neuropro-sthesis composed of three parts: an exoskeleton, a stimula-tor, and a data glove for the user interface. The exoskeleton consists of a knee orthosis driven by a DC-motor in combi-nation with a gear box to exert torque on the patient’s knee. The knee angles and angular velocities are fed back to the system for control purposes and the lower thigh is modeled as a pendulum. The control approach proposed is based on a cooperative control, where the low frequency components are given to the motor-neuroprosthesis controller, while the

Fig. 5 Hybrid neuroprosthesis with variable-constraint hip mecha-nism. Retrieved from (Kobetic et al. 2009)


1 3

Tabl

e 2

Iden

tified

orth

oses

-bas

ed h

ybrid

syste

ms f

or g

ait f

unct

ion

whe

re F

ES a

nd ro

botic

syste

ms a

re b

oth

torq

ue-g

ener

atin

g de

vice

s

Dev

ice

nam

e/gr

oup

Wea

rabl

e ro

bot

Mot

or-n

euro

pros

thes

isC

linic

al e

valu

atio

nD

raw

back

Elec

trom

echa

nica

l Gai

t tra

iner

(2

004)

(Hes

se e

t al.

2000

, 200

4;

Ng

et a

l. 20

09; S

chm

idt e

t al.

2007

)

Bod

y w

eigh

t sup

port.

Foot

pla

tes w

ith se

rvo

mot

or c

ontro

l-le

r

2 FE

S-ch

anne

ls: q

uadr

icep

s and

pe

rone

al n

erve

Gai

t pha

se c

ontro

l: qu

adric

eps

stim

ulat

ed d

urin

g st

ance

pha

se to

fa

cilit

ate

wei

ght a

ccep

tanc

e; p

ero-

neal

ner

ve st

imul

ated

dur

ing

swin

g ph

ase

to fa

cilit

ate

ankl

e fle

xion

16 S

troke

subj

ects

(Ng

et a

l. 20

09)

Too

bulk

y: o

nly

for c

linic

use

. Sta

-tio

nary

trai

ning

syste

m.

Una

ble

to p

rodu

ce o

verg

roun

d w

alk-

ing

prac

tice

Hyb

rid m

otor

neu

ropr

osth

esis

(200

5)

(Val

lery

et a

l. 20

05)

Kne

e ac

tive.

Pow

ered

exo

skel

eton

with

a st

imul

a-to

r and

a d

ata

glov

e. D

C m

otor

4 FE

S-ch

anne

ls: q

uadr

icep

s and

ha

mstr

ings

. Fee

dbac

k co

ntro

l.G

ait p

hase

con

trol:

not s

peci

fied

Not

teste

dEv

alua

tion

of sa

fety

and

usa

bilit

y ha

ve n

ot y

et b

een

repo

rted

HyP

O (2

006)

(Obi

nata

et a

l. 20

07;

Fuka

da e

t al.

2006

)H

ip a

ctiv

e, K

nee

activ

e.Fo

ur D

C m

otor

s. Pa

ralle

l lin

kage

s co

nnec

ted

to th

e ac

tuat

ors f

or

trans

ferr

ing

the

gene

rate

d to

rque

.O

rthos

is w

eigh

t: 9.

2 kg

.Ro

M: H

ip -1

0-12

0°,

Kne

e 0–

120°

.Pe

ak T

orqu

e: 1

40 m

Nm

2 FE

S-ch

anne

ls:

Qua

dric

eps

Ope

n-lo

op c

ontro

l.G

ait p

hase

con

trol:

stim

ulat

ion

initi

ated

dur

ing

swin

g ph

ase

with

sti

mul

atio

n pa

ttern

tune

d ac

cord

-in

g to

subj

ect r

espo

nse

Not

teste

dPr

epro

gram

ed F

ES p

aram

eter

s.Ev

alua

tion

of sa

fety

and

usa

bilit

y ha

ve n

ot y

et b

een

repo

rted

Mot

ionM

aker

TM st

atio

nary

robo

tic

syste

m (2

006)

(Met

raill

er e

t al.

2006

)

Hip

, Kne

e an

d A

nkle

act

ive.

3-D

OFs

4 FE

S-ch

anne

ls:

Glu

teus

max

imus

and

qua

dric

eps.

Feed

back

con

trol.

Gai

t pha

se c

ontro

l: fle

xors

and

ex

tens

ors m

uscl

es st

imul

ated

to

aid

leg

pres

s

5 SC

I sub

ject

s: 4

inco

mpl

ete,

1

com

plet

e (M

etra

iller

et a

l. 20

06)

Doe

s not

pro

vide

upr

ight

bip

edal

w

alki

ng. S

tatio

nary

trai

ning

syste

m.

Eval

uatio

n of

safe

ty a

nd u

sabi

lity

have

not

yet

bee

n re

porte

d

Loko

mat

(200

8) (M

cCab

e et

al.

2008

; Doh

ring

and

Dal

y 20

08)

Hip

act

ive;

Kne

e ac

tive.

DC

Mot

ors a

nd b

all s

crew

.Tr

eadm

ill-b

ased

with

exo

skel

eton

s an

d vi

rtual

real

ity

16 F

ES-I

M-c

hann

els:

Tibi

alis

ant

erio

r, Pe

rone

us lo

ngus

, G

astro

cnem

ius,

Bic

eps f

emor

is,

Sem

itend

inos

us, S

emim

embr

ano-

sus,

Vastu

s lat

eral

is, a

nd G

lute

us

med

ius.

Ope

n-lo

op c

ontro

l.G

ait p

hase

con

trol:

knee

flex

or m

us-

cles

stim

ulat

ed d

urin

g sw

ing

phas

e fo

r act

ive

flexi

on

1 A

ble-

bodi

edsu

bjec

t (D

ohrin

g an

d D

aly

2008

)Re

stric

tion

of p

elvi

c m

ovem

ent.

Prep

rogr

amed

FES

par

amet

ers

Wal

kTra

iner

(200

9) (S

tauff

er e

t al.

2009

)Pe

lvis

, Hip

, Kne

e &

Ank

le: D

C

mot

or.

6 D

OFs

20 F

ES-c

hann

els

Feed

back

con

trol:

min

imiz

atio

n of

in

tera

ctio

n fo

rces

Gai

t pha

se c

ontro

l: sti

mul

atio

n in

itiat

ed a

t toe

-off

for a

gre

ater

foot

cl

eara

nce

durin

g sw

ing

phas

e

6 Su

bjec

ts: 2

com

plet

e, 4

inco

m-

plet

e pa

rapl

egia

(Sta

uffer

et a

l. 20

09)

Too

bulk

y: o

nly

for c

linic

use

. No

mus

cula

r fat

igue

mon

itorin

g.Li

mite

d im

prov

emen

ts in

forc

e or

co

ordi

natio

n

10 F. Anaya et al.

1 3

Tabl

e 2

(con

tinue

d)

Dev

ice

nam

e/gr

oup

Wea

rabl

e ro

bot

Mot

or-n

euro

pros

thes

isC

linic

al e

valu

atio

nD

raw

back

Mec

hatro

nic

low

er li

mb

(200

9)

(Pob

oron

iuc

et a

l. 20

09)

Bru

shle

ss D

C se

rvom

otor

.R

ated

torq

ue: 0

.637

Nm

4 D

OFs

6 FE

S-ch

anne

ls:

Qua

dric

eps,

ham

strin

gs a

nd g

lute

al

mus

cles

Ope

n-lo

op c

ontro

l: m

axim

um p

ulse

w

idth

.G

ait p

hase

con

trol:

not s

peci

fied

Not

teste

dFo

r sta

ndin

g-up

, sta

ndin

g an

d si

tting

-do

wn

exer

cise

s onl

y.Pr

epro

gram

ed F

ES p

aram

eter

s.Ev

alua

tion

of sa

fety

and

usa

bilit

y ha

ve n

ot y

et b

een

repo

rted

Vand

erbi

lt In

dego

(201

1) (Q

uint

ero

et a

l. 20

11; H

a et

al.

2016

; Far

ris

et a

l. 20

11; H

a et

al.

2012

)

Hip

& K

nee

activ

e; A

nkle

pas

sive

.Po

wer

ed e

xosk

elet

on w

ith b

rush

less

D

C m

otor

s at b

oth

hip

and

knee

jo

ints

, and

nor

mal

ly-lo

cked

bra

kes

Orth

osis

wei

ght:

12 k

g.Ro

M: H

ip -3

0-10

5°,

Kne

e -1

0-10

5°.

Peak

Tor

que:

40

Nm

4 FE

S-ch

anne

ls:

Qua

dric

eps a

nd h

amstr

ings

Feed

back

con

trol.

Gai

t pha

se c

ontro

l: ha

mstr

ings

sti

mul

ated

to a

id h

ip e

xten

sion

, qu

adric

eps s

timul

ated

to a

id k

nee

exte

nsio

n (s

tanc

e)

3 Su

bjec

tsM

otor

com

plet

e T6

-T10

(Ha

et a

l. 20

16)

Prep

rogr

amed

FES

par

amet

ers

Varia

ble

impe

danc

e kn

ee m

echa

nism

V

IKM

-HN

P (2

011)

(Bul

ea e

t al.

2011

, 201

3a, b

, 201

4)

Kne

e ac

tive.

Line

ar M

R fl

uid

dam

per w

ith fo

ur-

bar l

inka

ge tr

ansm

issi

onO

rthos

is w

eigh

t: 11

.1 k

g.Ro

M: K

nee

-10-

90°,

Peak

Tor

que:

65

Nm

4 FE

S-ch

anne

ls:

Hip

& K

nee

exte

nsor

s.O

pen-

loop

con

trol.

Gai

t pha

se c

ontro

l: no

t spe

cifie

d

1 Su

bjec

t: M

otor

com

plet

e T7

(B

ulea

et a

l. 20

13a,

b)

Prep

rogr

amed

FES

par

amet

ers.

Trig

ger b

utto

n

Hyb

rid-m

ultic

hann

el F

ES W

alki

ng

Ass

istiv

e D

evic

e (2

012)

(Kur

ok-

awa

et a

l. 20

12)

Hip

act

ive,

act

uate

d w

ith c

om-

pres

sed

air a

ctua

tor

8 FE

S-ch

anne

ls:

Gas

trocn

emiu

s, so

leus

, tib

ialis

ant

e-rio

r and

qua

dric

eps.

Ope

n-lo

op c

ontro

l.G

ait p

hase

con

trol:

gastr

ocne

-m

ius,

sole

us, a

nd ti

bial

is a

nter

ior

stim

ulat

ed d

urin

g in

itial

swin

g to

aid

kne

e fle

xion

; qua

dric

eps

stim

ulat

ed d

urin

g st

ance

pha

se to

st

abili

ze k

nee

join

t

7 H

ealth

y su

bjec

ts2

SCI p

atie

nts

Inco

mpl

ete

C5

(Kur

okaw

a et

al.

2012

)

Prep

rogr

amed

FES

par

amet

ers

Lim

ited

impr

ovem

ents

in h

ip a

nd

ankl

e jo

int k

inem

atic

s.

iLeg

low

er li

mb

reha

bilit

atio

n ro

bot

(201

3) (C

hen

et a

l. 20

13)

Hip

act

ive;

Kne

e ac

tive

Leg

robo

t sys

tem

2-D

OFs

4 FE

S-ch

anne

ls:

Glu

teus

max

imus

& q

uadr

icep

s fe

mor

is.

Feed

back

con

trol.

Gai

t pha

se c

ontro

l: no

t spe

cifie

d

Not

teste

dD

oes n

ot p

rovi

de u

prig

ht b

iped

al

wal

king

. Sta

tiona

ry tr

aini

ng sy

stem

.Ev

alua

tion

of sa

fety

and

usa

bilit

y ha

ve n

ot y

et b

een

repo

rted


1 3

Tabl

e 2

(con

tinue

d)

Dev

ice

nam

e/gr

oup

Wea

rabl

e ro

bot

Mot

or-n

euro

pros

thes

isC

linic

al e

valu

atio

nD

raw

back

Sem

i-act

ive

hybr

id o

rthos

isSE

AH

O (2

014)

(Kirs

ch, e

t al.

2014

)H

ip a

ctiv

e; K

nee

sem

i-act

ive;

Ank

le

pass

ive

Elec

tric

mot

ors l

ocat

ed

at th

e hi

p jo

ints

, and

wra

p sp

ring

clut

ches

atta

ched

at t

he k

nee

join

ts.

Peak

Tor

que:

40

Nm

.2D

OFs

6 FE

S-ch

anne

ls:

Gas

trocn

emiu

s, qu

adric

eps,

and

ham

strin

gs.

Ope

n lo

op –

Ban

g-ba

ng c

ontro

l (o

nly

on–o

ff st

ates

).G

ait p

hase

con

trol:

gastr

ocne

miu

s sti

mul

ated

dur

ing

stan

ce p

hase

to

aid

kne

e ex

tens

ion;

qua

dric

eps

and

ham

strin

gs st

imul

ated

dur

ing

swin

g ph

ase

to a

id k

nee

flexi

on

Not

teste

dPr

epro

gram

ed F

ES p

aram

eter

s. St

imul

atio

n am

plitu

de e

qual

s to

max

imum

con

tract

ion

stim

ulat

ion.

Eval

uatio

n of

safe

ty a

nd u

sabi

lity

have

not

yet

bee

n re

porte

d

Kin

esis

(201

4) (d

el-A

ma

et a

l. 20

14,

2015

)K

nee

activ

e; A

nkle

pas

sive

Con

trolle

r- 4

mai

n co

mpo

nent

s: (1

) a

robo

tic o

r joi

nt ro

botic

, (2)

a F

ES

cont

rolle

r, (3

) a m

uscl

e fa

tigue

es

timat

or (M

FE) a

nd (4

) a fi

nite

-st

ate

mac

hine

(FSM

).D

C fl

at m

otor

s.Ro

M: K

nee

5-12

0°.

1 D

OFs

4 FE

S-ch

anne

ls:

Qua

dric

eps a

nd h

amstr

ings

Feed

back

con

trol.

PID

con

trol o

f kn

ee e

xten

sors

mus

cles

. ILC

for

the

knee

flex

or m

uscl

es.

Gai

t pha

se c

ontro

l: no

t spe

cifie

d

4 H

ealth

y su

bjec

ts (d

el-A

ma

et a

l. 20

14).

3 M

otor

inco

mpl

ete

SCI (

del-A

ma

et a

l. 20

14, 2

015)

PID

con

trolle

r dec

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12 F. Anaya et al.

1 3

high frequencies are directed to the motor controller (Fig. 7). In this way, each actuator is exploited individually regarding its own requirements, resulting in improvements in perfor-mance, energy consumption and muscle fatigue. However, validation results have not yet been published.

Obinata et al. (2007) proposed HyPO, a new hybrid pow-ered orthosis which comprises a functional neuromuscular stimulator and a control system with four actuators. The controller of the actuators achieved preprogrammed joint

trajectories while the functional neuromuscular stimulator system generates muscular forces through stimulation. In the prosthesis, two active joints are placed at the knee and hip joints in each leg. As reported by the authors, the HyPO has two novel mechanical features: (1) the use of parallel linkages instead of gearing system or timing belt systems to transfer generated torque from the exoskeleton’s joint actua-tors and (2) the HyPO system can be worn from the front of the body while staying seated in a wheelchair. The link-age mechanism (Fig. 8b), refers to simple parallel four links which are linked to the actuator for transferring the gener-ated torque. This allows to place the actuator at the front side of the lower limb while reducing the sidewise ledge. Furthermore, the linkage mechanism restricts the range of motions from 0 to 120 degrees for the knee, and from − 10 to 120 degrees for the hip, preventing excessive extension and flexion at the joints.

A preliminary experiment of unaffected walking has been reported with the developed HyPO system. FNS stimula-tion patterns were based on case by case according to the subject’s maximum tolerated amplitudes of the quadriceps muscles of both lower limbs. Results showed a robust and precise tracking of the reference joint trajectories through the torques generated by both the FNS and the actuators. Moreover, the generated torques by FNS were reported to be useful to reduce energy consumption of the actuators with-out deteriorating the tracking performance. However, the experimental results are not enough to confirm its effective-ness in SCI patients.

A hybrid system comprising of MotionMaker™ and FES was proposed by Patrick et al. (Metrailler et al. 2006). The MotionMaker™ includes two robotic orthoses com-bined to achieve 3 DOFs (knee, hip and ankle, Fig. 9). Each joint electrical actuator is mounted with force and position

Fig. 6 Electromechanical Gait Trainer with movable footplates. Retrieved from (Schmidt et al. 2007)

Fig. 7 Based on the intention prediction reference angle, an optimal torque trajectory is predicted by the model-based torque controller, which subsequently is split up to the two actuators. Retrieved from (Vallery et al. 2005)

Fig. 8 a Hybrid Powered Orthosis (HyPO). b Side view of links of HyPO. Retrieved from (Obinata et al. 2007)


1 3

sensor for the control of FES. 20-channel self-designed FES stimulator called “StimWave2” with closed loop control is employed in this system. The hybrid system was evaluated on five SCI patients who underwent 2 months training with the system. The training program included 1 h of leg press exercise once in 2 days. The FES stimulated the extensor and flexor muscles in the leg to aid leg press. The targeted muscles were gluteus maximus, quadriceps and gastrocne-mius, hamstrings and tibialis anterior. The results showed to increase the muscle strength of the patients. After the train-ing, three of the participants were able to voluntarily exert 150 N of force during leg press which they were unable to do earlier. Though the FES displayed positive results, there were no clear benefits of the robotic system mentioned in the paper.

Two attempts to synchronize the Lokomat gait robot system with a multichannel functional electrical stimulator were also found. Firstly, McCabe et al. (2008) combined the Lokomat gait robot system (Hocoma Inc.) with a multichan-nel functional electrical simulator using intramuscular elec-trodes (FES-IM) to provide a more coordinated gait training (Fig. 10). Lokomat system comprised of a robotic orthosis, body weight support (BWS) system and a treadmill which provides the subjects with knee joint, hip joint and sagittal plane motion. The FES-IM system comprised of eight intra-muscular electrodes placed on tibialis anterior, semimem-branosus, peroneus longus, vastus lateralis, gastrocnemius, biceps femoris, gluteus medius, and semitendinosus, in both legs. Custom FES-IM patterns were generated for each sub-ject to eliminate deficient components of stance and swing phases of gait and keep the gait pattern as close to normal as possible. Six Ischemic stroke subjects participated in the evaluation of the system. During the initial session, each subject was trained with the Lokomat alone to record the swing and stance durations for the customization of FES-IM stimulation patterns. During the next session, the Lokomat

and FES-IM were tested together. The results proved the fea-sibility of delivering FES-IM patterns simultaneously with Lokomat assistance. The Lokomat provided close to normal movements during the swing phase knee flexion while FES-IM stimulated the knee flexor muscles for active flexion of the knee joint. The same combined effect was also noted during the stance phase.

Secondly, Dohring and Daly (2008) reported the auto-matic synchronization of the Lokomat robotic gait orthosis and functional electrical stimulation using intramuscular electrodes (FES-IM). In this study, the Lokomat was syn-chronized with the stimulation of eight targeted muscles during the gait pattern by making use of the pulse produced by the Lokomat at each right heel strike. Results from two 30-min test trails in one able-bodied subject revealed a more feasible, accurate and repeatable synchronization of the FES gait patterns than manually delivered FES during Lokomat training.

Despite the recent achievements in integrating the Lokomat system with FES control, earlier studies (Hidler et al. 2009; Regnaux et al. 2008) have shown that Lokomat restricts the pelvic rotation and weight shifting between legs, which results in abnormal acceleration and deacceleration during the swing phase. Furthermore, due to the linkages in the Lokomat, the upper extremity motion is also restricted, which makes it difficult to swing the arms while walking.

Stauffer et al. (2009) developed the WalkTrainer as a hybrid orthosis with feedback FES control based on the Cyberthosis concept, which relies on the active participation of the subject’s muscles while the motions applied by the orthoses must closely mimic natural movements (Fig. 11). With this purpose, the Walk trainer consists of leg and pelvic

Fig. 9 MotionMakerTM prototype with an able-bodied subject. Retrieved from (Metrailler et al. 2006)

Fig. 10 Combination of Lokomat and multichannel FES with intra-muscular electrodes. Retrieved from (Schuck et al. 2012) and (Querry et al. 2008)

14 F. Anaya et al.

1 3

orthosis, an active BWS system and motorized wheels for over ground ambulation. The leg orthosis is designed with the idea of having a parallel mechanical leg that is placed just behind the human leg. Linkages connect the powered leg to the device and modified shoes interface the feet with the motor driver. The linear axis unit control the flexion and extension of the hip joint while the knee joint is actuated by a two-stage crank and connecting rod systems. Utilizing the same actuation strategy as the knee, the movement of ankle joint is powered by parallelogram.

The principal benefit of this leg orthosis consists of hav-ing the mechanical parts, cables and motors at the back of the machine, allowing the arms to swing freely. Six subjects with paraplegia participated in the preliminary evaluation of the WalkTrainer. The experiments consisted of short term preliminary clinical trial of 60 min a week training over 3 months which resulted in a drop in the Ashworth Spasticity Scale but with no significant increases in force or coordination.

Poboroniuc et al. (2009) designed a mechatronic lower limb to replicate the lower limbs of the human body, espe-cially the sitting and standing motions (Fig. 12). It consists of five lower limb joints which are actuated by a brushless servomotor driven by servo drivers providing position, velocity and torque control modes. The robotic structure also includes a functional electrical stimulator module to provide active joint movements. The electrical stimulations to the muscles are given in increasing and decreasing pulse widths to gluteal, quadriceps and hamstrings depending on the sitting or standing task. However, this system has not been tested on human subjects.

Vanderbilt exoskeleton, developed by Quintero et al. in 2011, is a power exoskeleton for gait rehabilitation in per-sons with paraplegia (Quintero et al. 2011). It consists of hip and knee joint actuators and weighs around 12kgs. The prototype of the powered orthosis is shown in Fig. 13, which

intends to deliver power-driven support in the sagittal plane at the hip and knee joints. Individual joints are driven by a brushless DC motor which provides a maximum continu-ous torque around 12 Nm. The knee actuators additionally

Fig. 11 The CAD model of the active bodyweight support system and the prototype of the WalkTrainer. Retrieved from (Stauffer et al. 2009)

Fig. 12 The schematic structure of a mechatronic lower limb. Retrieved from (Poboroniuc et al. 2009)

Fig. 13 Powered orthosis prototype. Retrieved from (Quintero et al. 2011)


1 3

employ electrically driven locked brakes, such that the joints continue to be locked in case of power failure and during the stance phase of gait, and are freed during the swing phase of gait. The hip joint presents a range of motion of 105° in flexion and 30° in extension, while the knee joint presents a range of motion of 105° in flexion and 10° in extension. The proposed orthosis is aimed to be worn in combination with a passive ankle foot orthosis, to provide stability to the ankle and to correct foot drop during the swing phase of gait.

The orthosis controller consists of state-flow system with the following four states: (1) right step forward, (2) double-support with right foot forward, (3) left step forward and 4) double-support with left foot forward. The cooperative controller combining FES with the exoskeleton is based on the idea that the hamstring should assist the exoskeleton to extend the hip when extension torques are required while not disturbing the system when flexion torques are desired. Additionally, quadriceps generate extension torques about the knee. To achieve this, the controller employs a constant activation level and varies the timing based on the finite-state machine and the torques on the hip and knee joints from previous steps. The hamstrings are stimulated in their corresponding stance phases and during double support due to the significant amount of hip extension required by the exoskeleton to maintain torso stability while the quadriceps are stimulated during the swing phase. Experiments were performed on 3 SCI patients to evaluate the performance of the controller. Results showed good trajectory tracking and considerable reduction of 20% on the motor power require-ments in both the knee and hip joints (Ha et al. 2016).

The variable impedance knee mechanism (VIKM) was presented by Bulea et al. (Bulea et al. 2013a, b, 2014). The VIKM concept was proposed as a part of a hybrid neuropro-sthesis (HNP) for restoring locomotor functions of individu-als with paralysis in the lower limbs from SCI (Fig. 14). The primary design goal of the VIKM is to replace the controlled eccentric contractions of the knee extensor muscles during the stance phase throughout a step-by-step descent approach. To meet these specifications and to control knee trajectories, the VIKM is based on a linear MR fluid damper with a four-bar linkage transmission.

During the stance phase, the VIKM provides torque of 51 N-m to lock the knee joint and allows knee motions under high loads with minimal resistance of knee motion during swing phases of the gait. The torque generated by the VIKM during the active mode is a function of the cur-rent supplied to the damper. When no current is given to the damper during the inactive mode, VIKM provides the required torque to the knee joint for its motion. The func-tional neuromuscular stimulation controller consists of a two-button finger trigger worn on the subject’s thumb. The VIKM aims for restoration of standing, stair climbing and walking functions. By pressing the “GO” button during

the seated position, the pre-programmed FES pattern for standing is activated to help the user stand. Once in a standing position, by pressing the “GO” button again the user can activate the walking stimulation pattern to walk. By pressing the “STOP” button the current step is com-pleted, and FES stimulation is initiated by providing cur-rents to the knee and hip extensor muscles to recommence standing. To evaluate the VIKM system linkage model, experiment on one paraplegic SCI patient with complete loss of motor and sensory function below the thoracic (T7) region was performed. The results of the experiment were compared with the results of walking with neuromuscular stimulation alone. The VIKM-HNP produced an almost normal knee flexion during stance phase and reduced the stimulation required for knee extension by 40%.

Kurokawa et al. (2012) described a feasibility study of a hybrid FES walking assistive system based on the funda-mental principle of passive walking (Fig. 15). The joints of the hybrid walking system were actuated with compressed air actuator and were synchronized with foot switches. Preliminary test results were reported from 7 normal con-trol subjects and 3 SCI patients with incomplete paralysis

Fig. 14 VIKM prototype: Computer-aided design representation of stance control knee mechanism. Retrieved from (To et al. 2011)

16 F. Anaya et al.

1 3

at C5 level. Electrical stimulations were delivered to the gastrocnemius, soleus, and tibialis anterior for stiffness control of the ankle joint and for assisting the knee flexion in the initial swing. Quadriceps were stimulated for sta-bilizing the knee joint during stance phase. M-wave was monitored for control and fatigue monitoring purposes. However, results of this approach were controversial, as only certain improvements were reported in ankle and hip kinematics.

Chen et al. (2013) proposed a 2 DOF (hip, knee) self-developed exoskeleton for lower limb rehabilitation. The joints of the exoskeleton were equipped with DC motors, encoders, and torque sensors to drive the joints. The leg-robot system is simplified as shown in Fig. 16. The EMG measurements detect the voluntary movement intensions of the patients, depending on which the FES module generates the stimulation pattern for contracting the gluteous maxi-mums and quadriceps femoris to produce torque at the hip and knee joints, respectively. The control approach of this hybrid robot was split into two components, the impedance control, and the FES control. The mass-damper-spring rela-tionship between force and position, determined during leg

press exercises, helps the impedance controller to produce the desired active compliance on the robot. The FES control-ler generates the desired torque at the joints and controls the intensity of stimulation depending on the need. Although the authors make a promising research on FES-assisted strategy combined with impedance control for conducting leg press exercise, validation results of this interesting approach have not yet been presented.

A semi-active hybrid orthosis (SEAHO) was presented by Kirsch et al. (2014). SEAHO, shown in Fig. 17, consists of three main components: electric motors, FES and wrap spring clutches. The DC-motors, which are placed at the hip joints, can discharge up to 40Nm to produce hip flex-ion while plantar flexion of the foot, knee extension and knee flexion which is produced by electrically stimulating the gastrocnemius, quadriceps muscle and the hamstrings, respectively. The stimulation parameters used for each muscle are preprogrammed as a function of the maximum tolerated intensity, which may lead to fatigue. The relative triggering of the stimulation for the three muscle groups is strictly defined by the hip angles. The final components of the SEAHO refers to the wrap spring clutches attached at the knee joints. With the goal to prevent knee flexion when locked, while still allowing a certain amount of extension, the wrap spring clutch is only unlocked when knee flexors are stimulated. This feature reduces the energy demand of the user to maintain standing position between steps or to maintain the stance leg extension during the step. For the initial device testing, only three states were explored in an able-bodied subject. Since the subject started the experiment in standing position with both feet together, the first state was a half-step. The other two states were defined for both

Fig. 15 Hybrid multichannel electrical stimulator. Retrieved from (Kurokawa et al. 2012)

Fig. 16 Photograph of the prototype of the rehabilitation robot and the simplified model, where qii s the angle for joint i; mr

i , mh

i and li

represent the mass of the robot, the mass of the leg and the length for link i, respectively. Fr2h and Fh2r represent the interaction forces between the robot and the human leg. Retrieved from (Chen et al. 2013)

Fig. 17 A semi-active hybrid orthosis (SEAHO). Retrieved from (Kirsch et al. 2014)


1 3

left and right step. The results of the initial testing suggested that the device is ready for testing in paraplegic subjects with the addition of inertial measurements units (IMU) for the implementation of feedback control of the electrical stimu-lation based on the knee angles. However, to the best of our knowledge, such a protocol has not been developed yet.

Another hybrid wearable robot reported in 2014 is Kinesis, a knee–ankle–foot exoskeleton consisting of an active actuator at the knee and a passive elastic actuator at the ankle, which prevents drop-foot during swing phase (Fig. 18). The system also interfaces with a PC-controlled stimulator (Rehastim, Hasomed GmbH) for biphasic cur-rent controlled stimulation to knee extensors (rectus femoris and vastus lateralis) and flexors (semitendinosus and biceps femoris) muscles. The kinesis controller consists of electri-cal muscle stimulator and the robotic controller. The FES control is achieved by means of an iterative learning con-trol algorithm for knee flexor muscles and a PID controller for knee extensor muscles, which allows customizing FES parameters through a closed-loop control of stimulation. The robotic joints controller corresponds to an admittance con-troller, which permits the regulation of robotic assistance to the joints during gait. The Kinesis was evaluated on three SCI patients, in which improvements in gait profiles were reported during treatment and follow-up (del-Ama et al. 2014). Despite evaluation of the Kinesis showing good patients acceptance with a good perception over the device, it was reported that the PID controller decreased the stimula-tion performance by interfering with the iterative learning control of the FES parameters.

FEXO Knee proposed by Ren et al. is a powered hybrid exoskeleton which targets single joint rehabilitation for the right knee (Ren et al. 2014). It has a self-developed knee exoskeleton and a commercial functional electrical stimu-lator- RehaStim 2 (Hasomed, Germany). The system also consists of a central pattern generator which predicts the phases by eliminating phase conflicts between the human

shank and exoskeleton to generate the reference trajectories and provide FES at the right phases (Fig. 19).

To provide active torque for the knee joint, two muscle groups- Vasti and Hamstring, are stimulated by FES. The exoskeleton uses DC servo motor to generate torque for knee flexion and extension. A model-based feedforward controller was designed to control the FES parameters (pulse width), whereas the control of the knee joint trajectory of the exo-skeleton was achieved by a proportional-integral-derivative (PID) controller. Preliminary tests were performed on four healthy subjects where the results showed that the phase regulation based on a biologically inspired control strat-egy, maintains the synchronization between the FES refer-ence trajectory and the actual knee angle trajectory. There were unexpected torque conflictions when the FEXO knee exceeded the desired maximum trajectory. The mutual

Fig. 18 Kinesis hybrid exoskel-eton and cooperative control approach. Retrieved from (del-Ama et al. 2014)

Fig. 19 Schematic of the exoskeleton in FEXO Knee. Eight main components are displayed: (1) DC servo motor, (2) driven pulleys, (3) Bowden cables, (4) encoder, (5) shank wraps, (6) interactive force sensors, (7) linear springs, and (8) tension sensors. Retrieved from (Ren et al. 2014)

18 F. Anaya et al.

1 3

torque in flexion and extension of the knee varied in each trial due to different stimulation response of the two muscle groups. As the experiments were performed in a sitting posi-tion the results and parameters may vary drastically with over ground walking experiments. Further trials with para-plegic subjects need to be performed for proper evaluation of the system.

Lastly, Tu et al. (1759; 2016) proposed a lower limb exo-skeleton system with a treadmill and BWS system (Fig. 20). Each pneumatic limb has two degrees of freedom allow-ing flexion and extension of knee and hip joints. A pair of pneumatic muscle (Festo, MAS-40) actuate the joints where inflation/deflation is controlled by a proportional valve with pressure sensor. The CVRC algorithm was employed to con-trol the hip and knee trajectories for treadmill-based gait training, whereas FES stimulation signals control ankle movement. Electrical stimulation was delivered to the tibi-alis anterior, soleus, and gastrocnemius under closed-loop control. One single healthy subject participated in the test-ing of the model-based hybrid cooperative control. The trajectory tracking errors did not show obvious differences between 2 phases of training of 0–5 s and 50–55 s, respec-tively at the hip, knee and ankle joints. The results of this study suggest that pneumatic muscle is a feasible approach in developing lower limb rehabilitation robots as it is light-weight and a low-cost actuator. However, future research in this approach is sorely needed.

2.2 Technical overview of non‑orthosis based hybrid systems, including robotic footplates systems, mobile platforms, cycling wheelchairs and related technologies

Apart from those orthotic-based technologies, technologies such as robotic footplates systems, pelvic-supporting robots,

treadmill based rehabilitation technologies and cycling wheelchairs have also been reported. We identified 5 main robots which incorporate FES for lower limb rehabilitation (Table 3).

A passive therapeutic arm to support the pelvis dur-ing treadmill training of gait with FES was introduced by Cikajlo et al. (2007) in 2007. The passive therapeutic arm was intended to regulate and stabilize the pelvis move-ments during walking in a treadmill. It consists of a steel cylinder, a vertical rod, two hinge joints and a pelvic belt fixed to a horizontal rod (Fig. 21). The pelvic belt was designed to provide balancing force when subjects deviate vertically in sagittal and frontal planes, depending on the adjusted stiffness, allowing physiological rotation of the pelvis. Both hinge joints were passive and permit vertical support.

The FES gait pattern was determined via a shank accel-eration sensor. At the end of terminal swing, the stored accelerations were recorded and compared with a reference physiological acceleration. Based on the comparison output, FES intensity was decreased with a minimal FES intensity of 50%, or increased up to 10%. The FES stimulation was then applied to the peroneal nerve and stopped at the end of the swing phase. An initial clinical assessment of the passive therapeutic arm was completed on a single subject with chronic incomplete SCI (T10). The experimental trial resulted in moderate improvements of gait performance in the studied subject. However, the training period was short-ened because of the appearance of muscle fatigue induced by FES stimulation. The subject was unable to complete the expected gait exercises during walking on the treadmill and the experiment was interrupted.

Kuznetsov et al. (2013) tested a Robotic Tilt Table Sys-tem (Erigo, Hocoma AG, Switzerland) with FES, naming it ROBO-FES. The system consists of a tilting stretcher which can be tiltled between 0° and 80° and foot plates with springs to perform stepping motions. This device is used for mobilization out of bed, body verticalization, and rhythmic motions with cyclic loading of the leg.

A six-channel FES stimulator (Motionstim 8, Medel GmbH, Germany) was incorporated with the robotic system. Biceps femoris and quadriceps femoris of both legs were stimulated during leg flexion and gastrocnemius muscles of both legs were stimulated during leg extension. Intensity of the stimulation was maintained in a range between 5 and 100 mA. This system was tested on 73 hemiparetic ischemic stroke survivors where 38 were treated with ROBO-FES (Fig. 22) and 35 with ROBO and no FES. The study showed training with ROBO-FES may increase leg strength, cerebral blood flow and blood pressures as compared to the training without FES.

Ye et al. (2014) introduced a rehabilitation system for hemiplegic patients based on a pelvis-supporting robotic

Fig. 20 Hip-Knee exoskeleton system concept diagram. Retrieved from (Tu et al. 2016)


1 3

Tabl

e 3

Iden

tified

robo

tic fo

otpl

ates

syste

ms,

mob

ile p

latfo

rms,

and

cycl

ing

syste

ms f

or g

ait f

unct

ion

with

FES

FES

func

tiona

l ele

ctric

al st

imul

atio

n, T

thor

acic

, VD

C v

olts

of d

irect

cur

rent

Dev

ice

nam

e/gr

oup

Wea

rabl

e ro

bot

Mot

or-n

euro

pros

thes

isC

linic

al e

valu

atio

nD

raw

back

Pass

ive

ther

apeu

tic a

rm fo

r pel

vis

supp

ort (

2007

) (C

ikaj

lo e

t al.

2007

)St

eel c

ylin

der w

ith a

djus

tabl

e sti

ff-ne

ss h

elic

alsp

ring,

ver

tical

rod,

two

hing

e jo

ints

an

d pe

lvic

bel

t

2 FE

S-ch

anne

ls: P

eron

eal n

erve

Feed

back

con

trol.

Gai

t pha

se c

ontro

l: pe

rone

al n

erve

sti

mul

ated

dur

ing

the

swin

g ph

ase

1 Su

bjec

t: T1

0 (C

ikaj

lo e

t al.

2007

)D

oes n

ot p

rovi

de p

ower

to jo

ints

.Li

mite

d to

the

pelv

is su

ppor

t

ROBO

-FES

(201

3) (K

uzne

tsov

et a

l. 20

13)

Foot

plat

es w

ith sp

rings

.M

obili

zatio

n, b

ody

verti

caliz

atio

n,

cycl

ic lo

adin

g of

leg

6 FE

S-ch

anne

ls: B

icep

s fem

o-ris

, gas

trocn

emiu

s. St

imul

atio

n be

twee

n 5

and

100

mA

.O

pen-

loop

con

trol.

Gai

t pha

se c

ontro

l: bi

ceps

fem

oris

an

d qu

adric

eps f

emor

is st

imul

ated

du

ring

leg

flexi

on; g

astro

cnem

ius

stim

ulat

ed d

urin

g le

g ex

tens

ion

38 h

emip

aret

ic is

chem

ic st

roke

su

bjec

ts (K

uzne

tsov

et a

l. 20

13)

Prep

rogr

amed

FES

par

amet

ers.

Doe

s not

pro

vide

upr

ight

bip

edal

w

alki

ng

Pelv

is su

ppor

t rob

ot (2

013)

(Ye

et a

l. 20

13, 2

014;

)Ro

bot b

ase

and

a pe

lvic

supp

ort

mec

hani

sm4

FES-

chan

nels

:Q

uadr

icep

s and

tibi

alis

ant

erio

r.O

pen-

loop

con

trol.

Gai

t pha

se c

ontro

l: qu

adric

eps a

nd

tibia

lis a

nter

ior s

timul

ated

dur

ing

swin

g ph

ase

Not

teste

dD

oes n

ot p

rovi

de p

ower

to k

nee

and

ankl

e jo

ints

.Pr

epro

gram

ed F

ES p

aram

eter

s.Ev

alua

tion

of sa

fety

and

usa

bilit

y ha

ve

not y

et b

een

repo

rted

Prof

hand

(201

3) (W

atan

abe

et a

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system with functional electrical stimulation (Fig. 23). The pelvis support robot combines a pelvic support mechanism and a robot base integrated with a treadmill. The vertical and lateral motions of the pelvis are actively supported by the pelvic support brace which is attached to a motor actu-ated device. The vertical unloading and lateral motion forces exerted by the user during walking are detected by two load cells employed by the system. For natural walking, the pelvic support brace provides three passive rotational degrees of freedom at the pelvis.

Electrical stimulations were delivered to the quadriceps and tibialis anterior muscles, to assist and stimulate the leg swing. A preliminary evaluation was reported for two healthy subjects wearing an ankle–foot orthosis (AFO) to simulate hemiplegic gait. However, further evaluation with patient data is required.

Targeting the hemiplegic patients, a lower limb reha-bilitation cycling wheelchair “Profhand” for rehabilitation and locomotion was developed in Japan (Watanabe et al. 2013). The Profhand is a pedaled wheelchair propelled by the lower limbs. The stimulation timings are estimated by crank angle recordings obtained from a wireless iner-tial measurement unit (WAA-010) attached centrally in the crankshaft. Electrical stimulations are provided to the quadriceps (for knee extension) and medial hamstrings or the gluteus maximus with two different patterns. For the first pattern, the quadriceps femoris muscle is stimulated together with hamstrings of the contralateral side caus-ing knee extension and contralateral knee flexion. For the second pattern, the quadriceps femoris is stimulated in conjunction with gluteus maximum of the ipsilateral side causing knee and hip extension of the lower limb. By recording the crank angle θ, the configuration of stimu-lated muscles is adjusted. The electrical stimulation to the right and left quadriceps femoris are switched at the crank angle θR and θL, respectively, where θL was set as θL = θR + 180°. Experimental tests were performed on 3 healthy subjects which proved that the Profhand provided effective propulsion by stimulating the quadriceps femoris and the gluteus maximus of the same limb. However, the cycling speeds were reported to be smaller than that of voluntary cycling.

Lastly, in (Bellman et al. 2017), Bellman et al. introduced a motorized FES-cycling test bed which incorporates func-tional electrical stimulation with switched control input. In the switching strategy applied here, the electric motors pro-vide assistance during crank cycle only when the effective-ness of the kinematics due to the rider’s muscle activity is low. A modified recumbent tricycle (TerraTrike Rover) with 24 VDC electric motor (Unite Motor Co. Ltd. MY1016Z2) coupled to the drive chain mounted at the mechanical frame

Fig. 21 Passive therapeutic arm based on a belt for supporting the subject’s pelvis to increase stability during treadmill walking. Retrieved from (Cikajlo et al. 2007)

Fig. 22 The ROBO-FES System. Retrieved from (Hacoma 2017)

Fig. 23 Motions of pelvis support robot. Retrieved from (Ye et al. 2014)


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(Fig. 24) was used for the FES trials. The orthotics pedals were affixed to the custom pedals to fix the riders pedals by preventing dorsi/plantar flexion of the ankles and to ensure proper sagittal alignment of the lower limbs. A current-controlled stimulator delivered biphasic, rectangular, sym-metric pulses to the subject’s muscle groups. The stimula-tion frequency was stablished at 60 Hz with amplitude fixed at 90 mA for the quadriceps and 80 mA for the hamstring muscles. Experimental results from five able-bodied subjects successfully validated the ability of the controller with an average cadence tracking error of 0.00 ± 5.82%.

3 Discussion

To the best of our knowledge, the twenty-eight hybrid robotic systems reviewed in this paper describe the state of the art systems that combines rehabilitation robotics with FES for lower limb rehabilitation. The hybrid robotic sys-tems reviewed here demonstrated the viability of combining FES with a wearable robotic device.

Hybrid robotic systems integrate two technologies, that complement each other, onto one platform so they can over-come the drawbacks of individual approaches. In such sce-nario, FES and gait rehabilitation robots promises to provide more safe, robust, and efficient neurological rehabilitation. To achieve this, it is important to determine the contribu-tion of each individual system and establish the impact it will have on the results of rehabilitation. While the wear-able robotic devices are designed to assist movement during the swing phase of the gait and to prevent joint movement during the stance phase, the FES takes benefit of the mus-cle power produced to reduce energy demand of the robotic device. This reduces the need for more powerful and bulky joints actuators, resulting in lighter systems. Moreover, such hybrid systems claim to promote more efficient motor learning as compared to other conventional practices such as

treadmill walking, due to the high demanding, community-based locomotion practice involved.

Over the years, hybrid wearable robots have proven themselves as a promising rehabilitation method by aiding affected individuals to regain their lost functional mobility and improving their quality of life. Regarding the imple-mentation of the motor neuroprosthesis, hybrid wearable robots were classified into either open or closed loop and regarding their actuation principle, they were classified into purely dissipating, semi-active or fully active, depending on the actuator’s ability to add or dissipate energy to the joint.

3.1 Orthotic robot‑based hybrid systems

A number of approaches for hybrid robotic gait restoration are currently available; however, the most widely developed hybrid robotic systems combined an exoskeletal robot and an FES device. Exoskeletal robots are worn by the patients to enhance the motor function of a limb or to fully substitute it while the FES targets the person’s paralyzed muscles to evoke actions that would not be possible otherwise. Differ-ent approaches were proposed to combine FES and lower limb exoskeletons, where the FES system reviewed were as complex as intramuscular systems with up to 8 stimulated muscles (McCabe et al. 2008), and as simple as systems incorporating surface stimulation of a single muscle (Kan-gude et al. 2009).

3.1.1 Hybrid exoskeletons where FES is used to stimulate the muscles and produce joint torque while the robotic system acts as energy dissipating device

Hybrid exoskeletons under control of joint brakes (Goldfarb and Durfee 1996; Chang et al. 2016) employ FES for deriv-ing the power to generate locomotion, by attaching brakes to the passive orthosis joints. The control strategy is based on employing joint brakes to delimit position and movement speed of joints produced by FES. Furthermore, the addi-tion of controllable brakes prevents the need to stimulate the muscle groups during the stance phases of gait, which results in a delayed musculature fatigue onset. As conse-quence, this combined approach acts as an FES regulator for the following step.

Efforts towards a hybrid orthosis with energy storage has also been proposed in (Gharooni et al. 2000, 2001; Sharma et al. 2014), where a spring-loaded mechanism stores and transfers energy from the stimulated paralyzed muscles to the corresponding joint, resulting in joint flexion. On top of the spring-break energy storing orthosis, joint-coupled orthosis has also been reported. These systems are similar to the spring-loaded mechanism orthoses, which are based on what is called the elastic-storage principle, but here the

Fig. 24 Model of the motorized FES cycle-rider system. a Electric motor. b Stimulator. c Orthotic pedals. Retrieved from (Bellman et al. 2017)

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elastic element was employed across two joints (hip and knee joints). Examples of the joint-coupled orthosis were presented by Farris et al. in (2009) and by Durfee WK in (Kangude et al. 2009), where hip motion was driven by the energy generated at the knee joint. Even though this may seem to be an energy efficient method, it may not be an effective rehabilitation method as it considers only an individual muscle in the leg and the stimulation pattern is nowhere close to the normal activation pattern.

Despite the decent evidence reported in literature on hybrid exoskeletons controlled by joint brakes, there are some drawbacks that prevents the widespread adoption of such technologies: hybrid exoskeletons controlled by brakes are unable to fully control the joint trajectories due to the inability of joint brakes to deliver torque. Therefore, limb movement is achieved entirely by the contraction of the stimulated muscles. Here, FES systems are considered to be robust and reliable to generate lower limb movements effi-ciently. However, as stimulation approaches corresponds to a majority of systems performing open-loop FES control, the quality of the motion generated was deemed poor in terms of joint trajectories and velocities. Also, the control system results in inadequacy due to the insufficient torque power produced. This control strategy reports no information about joint movements exerted by FES being fed back into the FES controller showing lack of direct control over muscle stimulation. Furthermore, due to the low joint power exerted and the poor control of joint trajectory, it is difficult to man-age the early appearance of muscle fatigue caused by FES. Consequently, a robust FES controller would play an impor-tant role in successfully deploying a hybrid exoskeleton con-trolled by joint brakes. Though the adequate administration of FES relies on the control strategy (preferably closed-loop control), the number of stimulating channels and the pre-cise electrode placement; the controller approach should also be able to compensate the null joint power exerted by the exoskeleton. FES-control strategies such as open-loop and linear feedback controllers, are mostly insufficient for controlling the limb excursion, mainly because of the high muscle response variability (Lynch and Popovic 2008).

Unlike hybrid exoskeletons employing joint brakes, active hybrid exoskeletons permit better control over the amount of power being added to the joint, which establishes an effec-tive feedback control of limb motions between the combined end-effector and the FES device.

3.1.2 Hybrid exoskeletons where FES and robotic systems are both torque‑generating devices

Two conditions essential for gait restoration are: joint stabil-ity and their control. Here, fully powered exoskeletons that combines the control of an end-effector and a FES device seems to give an alternative to the previously reviewed

hybrid systems with energy dissipating orthoses. The imple-mentation of active actuators on the exoskeleton joints does not only have the advantage of increasing joint power but also to assist the joint trajectory when the muscle is not capable of generating enough torque for the joint move-ment. Consequently, active actuator implementation on the exoskeleton joints has been reported as an effective strategy to reduce muscle fatigue caused by continuous exaggerated muscle stimulation.

Considering the fully active hybrid exoskeletons, mechanical design of the exoskeleton includes treadmill based exoskeletons and over-ground exoskeletons with DC motors as it produces controllable output with quick response time. The treadmill based exoskeletons widely employ the BWS system that provides uplifting force to maintain balance and suspension to the subject walking on the treadmill. Lokomat (McCabe et al. 2008) is a nota-ble system reviewed in this paper that uses the BWS sys-tem with the treadmill. Moreover, WalkTrainer (Stauffer et al. 2009) also employs BWS with motorized wheels for over ground movement and the Electromechanical Gait Trainer (Hesse 2004) employs BWS with movable foot-plates. Other exoskeletons systems include the portable over ground exoskeletons that provide more freedom of motion as compared to the treadmill based systems.

In addition to the aforementioned capability of fully powered exoskeletons to compensate joint trajectory, handling early muscle fatigue is also an aspect of impor-tance to design a fully powered hybrid exoskeleton due to the alteration in muscular activation with current FES systems. Following the principle in which the active exoskeleton can provides torque to the joints and where the FES intensity and duration can be adjusted with the proper control to allow time for muscle relaxation, vari-ous approaches have been proposed. Comparable to the joint brakes based exoskeletons, fully powered exoskeleton have been developed with open-loop control (Hesse 2004; Obinata et al. 2007; McCabe et al. 2008; Poboroniuc et al. 2009; Kurokawa et al. 2012; Kirsch et al. 2014; Bulea et al. 2011) and closed-loop control (Vallery et al. 2005; Stauffer et al. 2009; Chen et al. 2013; Tu et al. 2016; del-Ama et al. 2015; Quintero et al. 2012) of the FES param-eters. However, pre-programmed FES systems result in delivering of excessive electrical stimulation to the mus-cles causing either early fatigue or exaggerated gait pat-tern. Whereas in closed-loop control systems the residual physical abilities of the patients are yet to be implemented in the feed control of the hybrid exoskeletons.

Alternatively, learning feedforward loop controllers considered in (Ren et al. 2014) characterizes an interesting method that feats the cyclical ability of robotic rehabilitation to learn from deviations of preceding trials. This learning competence offers an alternative to assist and adjust to the


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physiological changes of the patients (i.e. Muscle activation variance due to muscle spasticity or fatigue) by integrating the residual physical abilities of the patients to the controller. Therefore, the main challenge for the full spread of hybrid fully powered exoskeletons is developing an optimally shared control by integrating the FES system, the exoskel-eton, and the patient’s residual movement capacity, in order to produce limb movement of enough range and adequate joint trajectory.

3.2 Non‑orthotic robot‑based hybrid systems

Although FES-based technologies have been mostly devel-oped in the field of hybrid exoskeletons, technologies includ-ing robotic footplate systems, mobile platforms and cycling systems have also been proposed. However, a number of drawbacks arise from such systems, such as the inability to add power at the joint level (Cikajlo et al. 2007; Ye et al. 2013, 2014) or to provide upright bipedal walking (Kuznet-sov et al. 2013; Watanabe et al. 2013; Bellman et al. 2017).

Wheelchairs continue to be employed as a mobility option for people with high levels of motor disability. However, they remain as the less desirable options due to the lack of upright bipedal walking. It has been suggested that standing itself has physical and psychological benefits that are not replicated by wheelchairs (Esquenazi et al. 2017).

3.3 Future research and development directions of hybrid robot assistive devices

The following subsections briefly discuss some non-exhaus-tive potential future research directions and challenges in hybrid robot assistive devices.

3.3.1 Understanding biological joint kinematics: compliant joint mechanisms for lower limb exoskeletons

The progressive developments in robotic exoskeletons have resulted from advances in sensors, actuators, and processors. However, there still exists a substantial gap in our understanding of the physiology and biomechanics of human movement (Ferris et al. 2007). This lack of knowl-edge impediments our ability to estimate how exoskeletons will impact the locomotion of cognitively impaired patients (Cenciarini et al. 2011).

Regardless of different design approaches, orthotic robot-based technologies share a main objective, that is, to transfer power to the user’s limb. Moreover, the robots must guar-antee its wearer’s safety, while maintaining or improving its own efficiency. In such a context, the robot must provide mechanical support so that the added loads do not affect the user’s condition. It also must adjust to the wearer’s anatomy. All of this while maintaining a relatively diminutive size.

Thus, there is one main concern that rises when designing the physical part of an exoskeleton: there is a kinematic incompatibility caused by the mismatch between the human and the robotic joints.

Taking into consideration the diversity of morphology among individuals and the inherent complexity of the human joints, it is unreasonable to try to determine a general design that maximizes performance while reducing the mechanical interference for each user (Cenciarini et al. 2011). There-fore, it is commonly accepted to simplify the wearer’s joint kinematics by employing simple robotic elements such as revolute or spherical joints to model the human anatomy, which translates not only into obtaining the mechanical sim-plicity, but also into achieving simpler actuation, controllers, and subsequently cheaper prototypes. However, revolute or spherical joints only reflects the dominant rotations and neglects the small dose of concealed translations (Vukobra-tovic 2007), making it impossible to properly comply with the user (Scott and Winter 1993).

While walking, the whole body participates, including toes and arms. Nevertheless, the hip, knee and ankle joints have been recognized to be more relevant than any other joint, as they produce the greater motions and torques. As an example, the knee joint, is defined as a condyloid joint (Hamill and Knutzen 2009) formed by the proximal portion of the tibia and the distal end of the femur. It performs flex-ion and extension, and internal or external rotations; none-theless, when loaded under body weight or during full exten-sion, internal and external rotations are greatly constrained (Celebi et al. 2013). When knee flexes or extends, tibia rolls and slides on femur causing the instant center of rotation to be displaced up to 3 cm (Lee and Guo 2010). Convention-ally, the knee joint is merely modeled as a revolute joint, as reported in (Sharma et al. 2014; Farris et al. 2009; Pons et al. 2013; Schuck et al. 2012; Querry et al. 2008; Quintero et al. 2011). This estimation significantly simplifies the mechani-cal and kinematic considerations during exoskeletal design but sacrifices the ergonomics and mechanical compliance with the wearer’s anatomy. However, according to (Wang et al. 2014), this oversimplification results in “parasitic” forces and torques which may lead to prolonged discomfort or risk of injury over time.

To avoid the above-mentioned problems and to give bet-ter user experience, numerous scientists are proposing new mechanisms that comply with the users’ anatomy. One of the pioneer designs to do so are those presented by Wang et al. (2014). They proposed five exoskeleton designs to investigate the internal forces and torque generated in the knee with combinations of adaptive kinematic components: pin and fixed end, pin and slider, cam and slider, pin and pinned slider, and cam and pinned slider. They used an arti-ficial model of the knee (A82 Functional Knee Joint Model, 3B Scientific GmbH, Germany) to emulate the knee joint

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mechanics in experiments. Experimental results suggest that the implementation of a pin slider/cam can effectively reduce internal joint forces and torque during the human–robot interaction.

To build on this concept and trying to reduce pain for people with knee osteoarthritis, Lee and Wang (Lee et al. 2015) (Wang et al. 2016) expanded their work by building a weight-support lower-extremity-exoskeleton (LEE) with a compliant joint to release the high-load during gait to the knee while supporting human bodyweight during walking. Unlike prior designs in which it is almost impossible to pro-vide body weight support due to the use of passive DOFs, the design of this compliant joint uses deformable rings to perform compliant coupling (θ > 20°) for adjusting mis-alignments of the rotating centers of the human–machine knee joint. Testing results showed a reduced plantar force by 27 and 21% at the 1st and 2nd peak forces, respectively; thus, showing its effectiveness in supporting the human weight by 20% during level walking.

Parallel to Lee and Wang et al.’ work, different compli-ance solutions to the ankle and hip joints have also been proposed. However, according to Gálvez-Zúñiga and Ace-ves-López in their review on compliant joint mechanisms for lower limb exoskeletons (Gálvez-Zúñiga and Aceves-López 2016), only one lower limb exoskeleton with more than one active complaint joint has been reported: the prototype of Yang et al. (2014) known as Bionic Lower Extremity Reha-bilitation Exoskeleton (BLERE). Their design consists of a novel exoskeleton with compatible hip and knee joints that aid patients to perform level ambulation. They used a pinned slider alongside cam and a curved slider, and two revolute joints for the knee and hip joint, respectively; allowing free movement in all the 3 DOFs mechanisms. The analytical results showed good compliance of the design.

To the best of our knowledge, despite the good perfor-mance reported on compliant joint mechanisms for lower limb exoskeletons, no system has integrated compliant joints with FES for restoration and rehabilitation of motor func-tion. Thus, we believe further efforts should be made to incorporate compliant joint mechanisms into the mechanical design of hybrid robotic systems. In the foreseeable future, it is expected to advance towards the development of tech-nologies in this area as a way to reduce the misalignment between the human and robot joint axes which can cause undesired interaction forces.

3.3.2 Improving functional outcomes in physical rehabilitation

It is common knowledge that countless patients with neu-rological disorders, including stroke and SCI, experience significant continuous improvement of motor, language, and cognitive function. The residual physical abilities of

the patient (i.e. voluntary muscle contraction, range of joint motion, bioelectrical residual activity) and adapta-tion, urge us to vouch for long-term rehabilitation in place of, or at least in conjunction with, motor substitution. This suggests the inclusion of principles in motor reha-bilitation, including “assist-as-needed” (Cai et al. 2006) and “challenged-based” in the development of hybrid robotic systems.

To enhance the welfares of FES-aided gait in functional motor recovery, the stimulation approach must modulate stimulation considering user intent, active participation (e.g., to start the gait) and adaptation, including changes in spasticity and muscle fatigue, while preserving a significant challenge. If insight into the functional movement disorders is fully understood, the design of the robot controller can effectively imitate the actions of a particular patient, thus the robotic controllers (FES and robot) can be personalized to the user impairments. We therefore advocate the inclusion of the principles of “assist-as-needed” and “challenge-based” to the customization of the motor control scheme from the very beginning, in the field of hybrid robotics for lower limb rehabilitation.

3.3.3 Clinical challenges and the relevance of feedback

Research on hybrid robot assistive devices still has to prove its effectiveness when they are used in the rehabilitation of motor function. The upcoming research requires enhanced clinical evidence to support not only effectiveness but to address issues like safety, acceptability, interaction between the wearer and the device and its social constraints. Most of the hybrid robotic devices studied in this article have pre-sented initial evaluation based on safety and energy perfor-mance. However, although these systems have proved to be functional, no evidence supporting major influence of the hybrid systems on the rehabilitation of lower limb impaired individuals has been reported.

Furthermore, having in mind the idea of considering functional active participation of the patient as a com-ponent to motor recovery, feedback provision results in essential component of motor rehabilitation. Today, there is vivid discussion on future research on the way to most effectively provide augmented feedback to better improve motor skills in patients with neurological disorders. The pace of technological development in augmented feedback seems to accelerate more every year, and the last dec-ade saw its share of major studies focused to investigate not only visual, but also auditory, haptic or multimodal augmented feedback. It may be interesting to investigate whether hybrid systems with the more complex, realistic motor tasks of augmented feedback enhances relearning of motor skills in recovery motor function after neurological injury such as stroke.


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3.3.4 Major technical challenges

Reduced metabolic cost along with reduced muscular fatigue while using hybrid robots is one of the key areas to improve. For this to become reality, closed-loop based FES controllers must be exploited. The investigation of new user-based con-trol strategies may give solutions to some of the challenges associated with early fatigue or exaggerated gait pattern.

Likewise, the investigation of modular architectures of mechanisms for rehabilitation robots may address some of the issues associated with increased metabolic costs. Modular robots aim to actuate singular joints like the hip, knee, or ankle; thus, resulting in light weight robots which allows the users to direct physical efforts towards the motor mechanisms that are impaired. Also “soft robotics” that are fabricated with polymeric elastomers and claims to be portable, self-directed, lightweight and with sufficient torque-generating capability to apply biologically relevant forces to the lower limb joints of the wearer (Asbeck et al. 2015), may give a solution. In this review paper, only two modular robotic systems (for the knee joint) were reported in (Bulea et al. 2013a, b; Ren et al. 2014), and none of the cited articles reported the use of any sort of soft robot. In the near future, lightweight rehabilitation robots in which the natural passive dynamics of human gait is preserved need to be developed. Given the current drawbacks in actuator technologies and control approaches, the concept of func-tional ambulation seems fundamental for reduction of the high metabolic energy expenditure required of users. It is projected future research in this area as a way of improving both mechanically and neuronal human–robot interaction.

4 Conclusion

This paper presents the overview of Hybrid Robotic systems which combines FES with lower limb rehabilitation devices for motor recovery. It is a unique approach which aims to enhance the effectiveness of the currents capabilities of these individual techniques.

The systems reviewed in this paper show the feasibility of this technology to provide good control over joint move-ment by minimizing the system’s energy demands. The inclusion of active actuators on the exoskeletons manages the muscle fatigue by optimizing the electrical stimulation needed to produce a joint motion and enables a more natural movement. The three major concepts implemented by these hybrid technologies to produce the desired joint motion are stimulating the gastrocnemius muscles to assist in push-off, locking of the knee joint during stance phase and by provid-ing external energy to the hip joints. However, only few of the systems reported in this review were evaluated with non- disables subjects. The potential of novel hybrid approaches

to rehabilitate motor functions with repeatability and flexi-bility (adaptation), makes them an attractive area of research and development; however, based on an insufficient amount of evidence supporting function recovery after brain injury, the hybrid robotic systems are not yet at a level to be listed as a solution for functional rehabilitation in motor disorders.

Lastly, this paper identifies not only the future research directions but also the scientific, technological, and clinical challenges involved with these hybrid technologies. Also, a new concept including hybrid bio-inspired joint exoskel-etons and FES for motor rehabilitation has been proposed. A better understanding of how humans performs motion would be able to better support this concept. Then technologies can be analogously designed and concepts such as augmented feedback and “assist-as-needed” revisited.

Funding The work was funded in part by the FRC Tier 1 Grant R-397-000-218-112, Faculty of Engineering, National University of Singapore, and in part by the NMRC B&B Grant No. NMRC/BnB/0019b/2015, Ministry of Health, Singapore.

Compliance with ethical standards

Conflict of interest The authors declare that they have no conflict of interest.

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Francisco Anaya received the B.S. degree in Biomedical Engineer-ing from Monterrey Institute of Technology and Higher Educa-tion, Mexico, in 2014. He is cur-rently working towards his Ph.D. at National University of Singa-pore, with research interests relating to biomechanics and muscle coordination of human walking, functional electrical stimulation (FES) and lower-limb robotic rehabilitation. He completed internships with the Center for Technology and

Research in Biomedicine, in Mexico and Holland Bloorview Kids Rehabilitation Hospital, in Toronto ON, Canada, prior to joining the Biorobotics lab at National University of Singapore. He is currently engaged in the development of systems to improve walking and mobil-ity for people with musculoskeletal and neuromuscular conditions by integrating FES with a novel robotic platform that enables patients post stroke to perform over-ground gait training at home or community rehab centers.

Pavithra Thangavel received the B.S. degree in Biomedical Engi-neering in 2013 from SRM Uni-versity, India, and the Master’s degree in Biomedical Engineer-ing in 2016 from VIT University, India. She is a Research Engi-neer in the Biorobotics Lab at National University of Singa-pore. Her current research is focused on the biomechanical study and evaluation of the lower limb powered exoskeleton.

Haoyong Yu received the B.S. and M.S. degrees in Mechanical Engineering from Shanghai Jiao Tong University, Shanghai, China, in 1988 and 1991, respec-tively. He received the Ph.D. degree in Mechanical Engineer-ing from Massachusetts Institute of Technology, Cambridge, Mas-sachusetts, USA, in 2002. He was a Principal Member of Tech-nical Staff at DSO National Lab-oratories, Singapore, until 2010. His research areas in DSO included exoskeleton and humanoid robots, intelligent

ground and aerial robots, and bio-inspired robots. Dr. Yu joined the Department of Biomedical Engineering in 2010. He is also a Principal Investigator of the Singapore Institute of Neurotechnology (SINAPSE), and the Advanced Robotic Centre at the National University of Singapore.

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