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Physiotherapy Theory and Practice, 26(4):240250, 2010

Copyright & Informa Healthcare

ISSN: 0959-3985 print/1532-5040 onlineDOI: 10.3109/09593980903015292

RESEARCH REPORT

The efficacy of two modified proprioceptiveneuromuscular facilitation stretching techniques insubjects with reduced hamstring muscle length

James W Youdas, PT, MS,1 Kristin M Haeflinger, BS,2 Melissa K Kreun, BA,2

Andrew M Holloway, BA,2 Christine M Kramer, BS,2 and John H Hollman, PT, PhD,1

1Associate Professor of Physical Therapy, Mayo Clinic, Rochester, Minnesota, USA2

Doctoral student in the program in physical therapy, Mayo Clinic, Rochester, Minnesota, USA

AB ST RA CT

Difference scores in knee extension angle and electromyographic (EMG) activity were quantified before and after

modified proprioceptive neuromuscular facilitation (PNF) hold-relax (HR) and hold-relax-antagonist contraction

(HR-AC) stretching procedures in 35 healthy individuals with reduced hamstring muscle length bilaterally (knee

extension angle ,1608). Participants were randomly assigned each PNF procedure to opposite lower extre-

mities. Knee extension values were measured by using a goniometer. EMG data were collected for 10 seconds

before and immediately after each PNF stretching technique and normalized to maximum voluntary isometric

contraction (% MVIC). A significant time by stretch-type interaction was detected (F1,34 5 21.1; p , 0.001).

Angles of knee extension for HR and HR-AC were not different prior to stretching (p 5 0.45). Poststretch knee

extension angle was greater in the HR-AC condition than the HR condition (p , 0.007). The proportion of

subjects who exceeded the minimal detectable change (MDC95) with the HR-AC stretch (97%) did not differ

(p 5 0.07) from the proportion who exceeded the MDC95 with the HR stretch (80%). Because EMG activation

increased (p , 0.013) after the HR-AC procedure, it is doubtful a relationship exists between range of motion

improvement after stretching and inhibition of the hamstrings. On average the 10-second modified HR procedure

produced an 118 gain in knee extension angle within a single stretch session.

INTRODUCTION

Muscle flexibility is defined (Zachazewski, 1989) as

the ability of a muscle to lengthen, allowing one joint

(or more than one joint in a series) to move through a

range of motion. Historically, rehabilitation profes-

sionals have advocated that static hamstring muscle

stretching be included in a warm-up to improve

athletic performance (Smith, 1994) and reduce injury

risk during vigorous physical exercise (Safran, Scaber,

and Garrett, 1989). However, recent studies have

questioned the use of static muscle stretching before

exercise or performance events because of a docu-

mented stretching- induced force deficit (Church,

Wiggins, Moode, and Crist, 2001; Cramer et al, 2004;

Fowles, Sale, and MacDougall, 2000; Marek et al,

2005). Furthermore, contrary to traditional beliefs,

scientific evidence fails to support static stretching

before exercise as a way to reduce the risk of sports

related injury (Shrier, 1999; Thacker, Gilchrist,

Stroup, and Kimsey, 2004). Despite this controversystatic hamstring muscle stretching is of keen interest

among physical therapists, athletic trainers, and other

fitness professionals who desire to improve muscle

flexibility in their clients (Decoster, Cleland, Altieri,

and Russell, 2005).

Clinicians use ballistic stretching, static stretching,

and proprioceptive neuromuscular facilitation (PNF)

procedures (Decoster, Cleland, Altieri, and Russell,

2005) to improve hamstring muscle flexibility. PNF

stretching procedures (Kisner and Colby, 2007) include

Address correspondence to James W. Youdas, PT, MS, Mayo Clinic,

200 1st St. SW, Rochester, MN 55905 USA.

E-mail: [email protected]

Accepted for publication 23 April 2009.

240


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four techniques: 1) hold-relax (HR); 2) contract-relax

(CR); 3) antagonist contraction (AC); and 4) hold-relax

with antagonist contraction (HR-AC). The HR and CR

procedures have been considered identical by many

clinicians, whereas the originators of PNF techniques did

not consider them interchangeable (Voss, Ionta, and

Myers, 1985). In classic PNF the procedures are

performed by activating muscle groups in diagonal

patterns. When using CR the rotators of the extremity

are allowed to shorten, whereas all other muscle groups

in the pattern contract under isometric conditions. In

contrast, for HR all muscle groups in the pattern are

activated isometrically. Since 1983 a variety of clinical

investigations that studied the efficacy of PNF stretching

(CR or HR) in subjects with reduced hamstring muscle

length avoided using diagonal movement patterns and

limited thigh and leg motion to the sagittal plane

(Halbertsma and Goeken, 1994; Moller, Ekstrand,

Oberg, and Gillquist, 1985; Osternig, Robertson,Troxel, and Hansen, 1990; Prentice, 1983; Rowlands,

Marginson, and Lee, 2003; Spernoga, Uhl, Arnold, and

Gansneder, 2001; Sullivan, DeJulia, and Worrell, 1992;

Wallin, Ekblom, Grahn, and Nordenborg, 1985;

Wiktorsson-Moller, Oberg, Ekstrand, and Gillquist,

1983; Worrell, Smith, and Winegardner, 1994).

Although not described by the originators of PNF,

antagonist contraction (AC), the third PNF stretching

maneuver (Cherry, 1980; Condon and Hutton, 1987)

refers to the muscle group that opposes the hamstrings

or the agonists (Voss, Ionta, and Myers, 1985). For

hamstring muscle tightness the subject would con-

centrically activate his/her knee extensors and hold theend-range position for a few seconds. Concentric

activation of the knee extensors is produced without

manual resistance from the therapist or fitness

professional. The fourth procedure, HR-AC, also

termed slow reversal-hold-relax by the originators of

PNF (Voss, Ionta, and Myers, 1985) combines both HR

and AC techniques. The therapist passively moves the

extremity to a point where tightness is perceived in the

hamstrings (range-limiting muscle group). The subject is

instructed to perform a resisted isometric activation of

the range-limiting muscle group-agonist followed by

muscle relaxation and then a concentric activation of the

antagonistthe muscle group opposite to the range-limiting muscle group.

Neurophysiological mechanisms underlying the

effectiveness of PNF stretch procedures have tradi-

tionally included 1) autogenic inhibition via the Golgi

tendon organ (GTO) tension receptor and 2) reciprocal

inhibition through the muscle spindle. GTO tension

receptors within the hamstring muscle-tendon unit (the

target muscle because of its reduced muscle length) are

initially activated during the HR procedure, causing

the hamstring muscle to be inhibited via autogenic

inhibition (Macefield et al, 1991). This phenomenon is

thought to be responsible for the increased joint ROM

immediately following the HR procedure. However,

investigators also contend that GTO influence is limited

to the period of either active or passive tension within

the muscle, because GTO activity following a muscle

contraction occurs at very small levels or is unable to be

recorded (Edin and Vallbo, 1990; Gollhofer, Schopp,

Rapp, and Stroinik, 1998). It has also been hypo-

thesized that active contraction of the quadriceps

femoris (the antagonistic muscle group to the ham-

strings) immediately after the HR period in the HR-AC

technique inhibits the hamstring muscle-tendon unit

via the principle of reciprocal inhibition (Osternig,

Robertson, Troxel, and Hanson, 1990). On the basis of

this information a clinician might expect to observe

a statistically significant reduction in peak hamstring

muscle activation (% maximum voluntary isometric

contraction [MVIC]) after the administration of eitherthe HR or HR-AC techniques. Nevertheless, EMG

activity in the hamstring muscle group following PNF

stretching was reported to be similar to that obtained

after static stretching procedures (Magnusson et al,

1996). This information challenged the belief that

PNF stretching is more effective than static stretching

procedures for increasing joint ROM because of

neurophysiological mechanisms mediated by the GTO

and muscle spindle.

A review of the literature revealed 10 studies

(Decoster, Cleland, Altieri, and Russell, 2005) that

described the effects of PNF stretching procedures on

hamstring muscle length in healthy subjects (Table 1)whose ages ranged between 18 and 32 years. Only three

groups (Spernoga, Uhl, Arnold, and Gansneder, 2001;

Sullivan, DeJulia, and Worrell, 1992; Worrell, Smith,

and Winegardner, 1994) reported their volunteers had

a preexisting limitation in hamstring muscle length. The

most preferred PNF technique was a modified form

of contract-relax (CR). Three investigators (Prentice,

1983; Rowlands, Marginson, Lee, 2003; Sullivan,

DeJulia, and Worrell, 1992) chose techniques similar

to HR-AC whereby subjects initially activated the

hamstrings immediately followed by activation of the

quadriceps femoris. One study (Sullivan, DeJulia, and

Worrell, 1992) used a single 30-second cycle stretch,whereas most required several (three to six) PNF stretch

cycles for total stretch times ranging from about

60 seconds to 10 minutes. The outcome variable was

change in hamstring muscle length measured by either

straight leg raise (SLR) or knee extension angle (KE).

ROM gains ranged from 58 to 348. Three investigators

reported an estimate of measurement error (Spernoga,

Uhl, Arnold, and Gansneder, 2001; Sullivan, DeJulia,

and Worrell, 1992; Worrell, Smith, and Winegardner,

1994) associated with the knee extension outcome

241 Youdas et al.

Physiotherapy Theory and Practice


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variable. Investigators used a variety of stretching

protocols which varied from a single session to

multiple times per week for up to 10 weeks. On the

basis of this information we believe there is a lack of

information about which PNF procedureCR, HR,

and HR-ACis most effective for increasing hamstring

muscle length. Because the HR and CR procedures are

interchangeable when knee ROM is restricted to the

sagittal plane, we believe it appropriate to specifically

compare the efficacy of a single session of modified

PNF stretching consisting of one cycle each of HR and

HR-AC in subjects with reduced hamstring muscle

length as measured by the knee extension angle.

We hypothesized there would be a statistically

significant difference in knee extension ROM for

both modified HR and HR-AC stretch procedures in

subjects with reduced hamstring muscle length

immediately following a single session of stretch

(poststretch) compared to knee extension ROMat baseline (prestretch). In addition, we also

hypothesized a statistically significant stretch by time

interaction whereby the knee extension angle would be

greater in the HR-AC condition than the HR

condition poststretch. Our hypothesis was based on

the fact that HR-AC combines two successive

inhibitory influences upon the hamstrings: the initial

effect of the GTO within the shortened hamstring

muscle-tendon unit (HR technique) immediately

followed by reciprocal inhibition (AC technique) due

to active contraction of the antagonist quadriceps

femoris. We also hypothesized there would be a

statistically significant reduction in peak values ofhamstring EMG activity at rest obtained immediately

after each of the PNF stretch techniques compared to

prestretch measurements.

METHODS

Subjects

We recruited 35 healthy subjects (23 women and

12 men) with a mean (6SD) age, body mass, height,

and body mass index (BMI) of: 32.3 years610.5,

82.3 kg612, 17966.4, 25.4 kg/m263.1, respectively,for men and 26 years67.8, 68.9kg69.3, 16969.4,

24.1 kg/m263.8, respectively, for women. Subjects

comprised a sample of convenience and were recruited

primarily from our institutions School of Health

Sciences. Measurements were obtained between

12 noon and 1 PM while subjects were not attending

class. All subjects were engaged in personal fitness

programs consisting of aerobic exercise and strength

training. For the 2 hours immediately prior to the

testing session each subject had been sitting in aTABLE1Criticalfeaturesofstud

iesthatexaminedtheeffectsofproprioce

ptiveneuromuscularfacilitation(PNF)s

tretchingonhamstringmusclelengthinhealthysubjects

Author

HM

Lflexibility

p

restretch

Technique

Duration

Outcome

ROM

gain

(8)

Estimateof

measurementerror

Protocol

Prentice

N

otstated

Slow-reversal-ho

ld(SRH)

3320-scycles

S

LR

12

Notstated

3/wkfor10wks

Wiktorsson-Molleretal.

N

otstated

Contract-relax(CR)

6316-scycles

S

LR

9

Notstated

Singlesession

Molleretal.

N

otstated

Contract-relax(CR)

5316-scycles

S

LR

6

Notstated

Singlesession

Wallinetal.

N

otstated

Contract-relax(CR)

5319-scycles

S

LR

9

Notstated

3/wkfor4wks

Osternigetal.

N

otstated

Contract-relax(CR)

5320-scycles

K

E

5

Notstated

Singlesession

Sullivanetal.

,708S

LRbilaterally

Contract-relax-con

tract(CRC)

1330-scycles

K

E

11

ICC5

.99

SEM

5

1.8

8

4/wkfor2wks

Worrelletal.

,

208KE

Contract-relax-con

tract(CRC)

4320-scycles

K

E

10

ICC5

.93

SEM

5

2.918

5/wkfor3wks

Halbertsmaetal.

N

otstated

Contract-relax(CR)

1310min

S

LR

5

Notstated

2/dfor4wks

Spernogaetal.

,

208KE

Hold-relax(HR)

5326-scycles

K

E

8

ICC5

.96

SEM

5

2.3

8

Singlesession

Rowlandsetal.

N

otstated

Contract-relax-agonist-contract(CRAC)

3320-scycles

S

LR

34

Notstated

2/wkfor6wks

Abbreviations:HML,hamstringm

usclelength;ICC,intraclasscorrelation

coefficient;KE,kneeextension;SEM,st

andarderrorofmeasurement;SLR,straightlegraise.

Physiotherapy Theory and Practice 242

Copyright & Informa Healthcare USA, Inc.


4/12

classroom, studying, or using a personal computer.

Inclusion criteria were bilaterally reduced hamstring

muscle length as measured by the knee extension angle

and the absence of any self-reported trunk or lower

extremity pathology in the past 6 months. We defined

reduced hamstring muscle length as a knee extension

angle less than 1608 to use a muscle-tendon unit that

could be lengthened and inclusion criteria that could be

met by the general population (DePino, Webright, and

Arnold, 2000). Investigators have questioned the

validity of using the more familiar straight leg test to

assess hamstring muscle length due to problems

associated with controlling pelvic motion and because

the initial intent of a straight leg raise was to assess the

length of the sciatic nerve (Fredrikson, Dagfinrud,

Jacobsen, and Maehlum, 1997). The technique for

measurement of knee extension angle has been

described by previous investigators (Gajdosik and

Lusin, 1983). This study was approved by the authorsinstitutional review board, and each subject signed an

approved consent form.

Procedure

Pilot study

A pilot study consisting of 40 subjects (8 men, 32 women;

age 2130 years) was performed to estimate the intra-

tester reliability and minimal detectable change (MDC95)

of an examiner when obtaining measurements of the

knee extension angle with a goniometer. To help with

alignment of the goniometer the fixed arm was extendedfrom 31.3 (12.3 in) to 47 (18.5 in), whereas the move-

able arm was extended from 31.3 (12.3 in) to 61.6

(24.4 in). The scale of the protractor was marked in

18 increments. The examiners were year two doctoral

of physical therapy (DPT) students experienced with

applying the PNF stretch procedures of HR and HR-AC.

Main study: HR technique

Each subject was informed of the testing procedure

through verbal instruction and serial pictures of the

process. Subjects selected a card that determined the

lower extremity to be tested first (right or left). Next,

subjects selected a card that designated the PNFtechnique (HR or HR-AC) to be used on the previously

chosen extremity. The opposite procedure was per-

formed on the opposite lower extremity. Next, the

subject was instructed to lie supine on the treatment

table while the examiner positioned the bracing device

made from 3.8-cm (1.5-in) polyvinylchloride (PVC)

pipe so the trunk-thigh angle was 908 (Figure 1).

A separate investigator used a goniometer to ensure

both hip and knee joint of the tested extremity were

flexed to 908 (90:90 position). It was imperative that the

subjects anterior thigh stay in contact with the

58.4-cm-wide crossbar of the PVC frame throughout

the procedure to maintain 908 of hip flexion. The

subjects nontested hip and knee joint were positioned in

neutral. If necessary, an examiner would manually

extend the subjects nontested thigh if the hip joint

began to flex during the PNF procedure. With one

hand supporting the subjects distal thigh and the other

hand cupping the subjects heel, examiner 1 passively

extended the subjects knee joint to the end point, where

firm resistance was detected in the hamstring muscles

(Figure 1), but the subject did not verbally acknowledge

discomfort. Examiner 2 measured the baseline knee

extension angle with a masked 3608 goniometer where-

upon the value was recorded by examiner 3. In addition

to recording the knee extension angle before and after

the PNF stretch, examiner 3 also used a stopwatch to

verbally signal the start and stop points of the PNF

stretch. The HR procedure required subjects to producea 10-second resisted isometric activation (Figure 2) of

the hamstrings against manual resistance provided by

examiner 1 (Kisner and Colby, 2007). Upon completing

the HR contraction, the examiner instructed the subject

to relax while he passively extended his/her knee joint

until the examiner consistently felt a firm end point with

each subject. The knee extension angle was remeasured

with the masked goniometer by examiner 2 and the value

recorded by examiner 3.

Main study: HR-AC technique

For the HR-AC procedure the opposite lower extre-mity was used. Because of randomization, either

technique may have been performed first. Again, knee

FIGURE1 After the hip and knee were placed in a 908:908positions using the polyvinylchloride (PVC) framework, exami-

ner 1 passively extended the subjects knee until firm resistance

was felt at the end point. Examiner 2 measured the knee

extension angle by using a masked goniometer.

243 Youdas et al.



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extension angle was measured passively, followed by

the same procedure of isometric activation of the

hamstrings. This time (Figure 3), following the iso-

metric hamstring contraction, the subject was instructed

to immediately begin a 10-second concentric activation

(Kisner and Colby, 2007; Magnusson et al, 1996) of the

ipsilateral quadriceps femoris known as the antagonistic

contraction (AC). The AC of the quadriceps femoris

distinguished this procedure from the previous HR.

While actively extending the leg, the subject was asked

to maintain contact of the distal anterior thigh of the test

extremity with the crossbar of the PVC frame. Upon

completion of the AC portion of the PNF procedure,

examiner 1 grasped the subjects thigh and leg being

careful to maintain the new knee extension angle. With

examiner 1 holding the subjects thigh and heel, the

subject was instructed to relax the quadriceps and

examiner 2 measured the knee extension angle with the

masked goniometer.

Main study: EMG measurements

Parameters of EMG signal detection. Raw EMG data

were collected by using D-100 bipolar surface elec-

trodes (Therapeutics Unlimited, Inc., Iowa City, IA).

The active Ag-AgCl electrodes had an interelectrode

distance of 22mm and were encased within a pre-

amplifier assembly measuring 35 3 17 3 10 mm. The

preamplifiers had a gain of 35. The combined pre-

amplifier and main amplifier permitted a gain of 100to 10,000 with a bandwidth of 40 Hz to 6 KHz.

Conductive gel (Signa Creams Electrode Cream;

Parker Laboratories, Inc., Fairfield, NJ) was applied to

the electrodes to conduct the electrical signal from the

skin. The common mode rejection ratio was 87 dB at

60 Hz, and input impedance was greater than 15 MOat

100Hz (Maarsingh et al, 2000). Signals were filtered

with a 20 HZ high-pass filter. Data were collected at

a sampling frequency of 1,000 Hz. Raw EMG signals

were processed with WinDaq data acquisition software

(DATAQ Instruments, Inc., Akron, OH) using the

root mean square algorithm at a time constant of

55 milliseconds.

Normalization of EMG signals. Prior to the HR and

HR-AC procedures, subjects were positioned prone

on a standard treatment table. The skin of each

posterior thigh was prepared by shaving (if necessary),

abrading, and cleaning with isopropyl alcohol wipes to

reduce skin impedance in preparation for placement of

the EMG electrodes. Surface electrodes were placed

over the center of the posterior thigh in a vertical

plane one-half the distance between the gluteal fold

and popliteal fossa of both lower extremities and

attached to the skin with double-sided adhesive tape(Cram and Kasman, 1998). The ground electrode was

positioned over the skin of the right acromion process.

Subjects were then instructed in the procedure for

obtaining a maximum voluntary isometric contraction

(MVIC) of the hamstring muscles (Figure 4). Each

subject was positioned prone with the hip in neutral

and the knee joint of the side to be tested flexed to

458. The examiner instructed the subject to maintain

the 458 of knee joint flexion while he applied a knee

extension external moment of force to the posterior

FIGURE3 For the hold-relax with antagonist contraction (HR-

AC) procedure, the subject maintained the anterior thigh against

the polyvinylchloride (PVC) framework and concentrically acti-

vated the quadriceps for 10 seconds immediately following the

hold-relax (HR) procedure. The arrow represents the direction

of leg extension.

FIGURE2 For hold-relax (HR) and hold-relax with antagonist

contraction (HR-AC) procedures subjects isometrically activated

their hamstrings against the examiners resistance for 10 seconds,

while maintaining the anterior thigh against the polyvinylchloride

(PVC) framework. The arrow represents the line-of-force pro-

duced by the subjects heel against the examiners hand.




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surface of the calcaneus. Hamstring muscle EMG

activity was recorded for 10 seconds during an iso-

metric manual muscle test of the hamstrings (Hislop

and Montgomery, 2007). The peak amplitude of the

hamstring muscle activity for the 10-second MVIC

was determined post hoc. During PNF stretch pro-

cedures EMG signals from the hamstring muscles

were specifically recorded during a 10-second duration

rest/baseline period with no intended muscle activation

immediately before and after the hamstring stretch.Examiner 3 used a stopwatch to signal the start and

end of the EMG recording period. Normalized peak

EMG activity during rest was determined post hoc.

Statistical Analyses

Pilot study

An intraclass correlation coefficient (ICC2,1) was

performed on the pilot study data to estimate intratester

reliability for measurement of knee extension angle

(Shrout and Fleiss, 1979). The precision of knee

extension was estimated by the standard error ofmeasurement (SEM). We used the following formula

to calculate the SEM: SEM 5 SD 3 [O (1-ICC)](Nunnally, 1978). The ability of a goniometer to detect

change in knee joint extension ROM resulting from

lengthening or shortening of hamstring muscle length is

known as the devices responsiveness and one popular

measure of responsiveness is minimal detectable change

(MDC). The MDC is defined as the magnitude of

change over and above measurement error of two

repeated measurements at a designated confidence level

(Beaton, 2000; Schmitt and DiFabio, 2004; Stratford,

Binkley, Riddle, and Guyatt, 1998). Therefore, changes

in passive knee joint extension ROM caused by ham-

string muscle length gain or loss may be attributed to

such things as measurement error, unexplained varia-

tion due to random changes in joint ROM, or real

change not accounted for by measurement error or

random variability. A rehabilitation professional can be

confident a patients change in passive knee joint

extension ROM over time represents real change in

hamstring muscle length if the measurement value

increases or decreases by a value greater or equal to the

MDC. Minimal detectable change (MDC95) of knee

extension was also calculated from this pilot data using

the standard formula: MDC95 z-scorelevel of confidence

SDbaseline ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi21 rtestretest

p (Schmitt and DiFabio,

2004; Stratford, Binkley, Riddle, and Guyatt, 1998).

Main study: knee extension angleThe dependent variable for assessment of hamstring

muscle flexibility was knee extension angle obtained

before and immediately after the PNF stretch procedure.

A two-way repeated measures analysis of variance

(ANOVA) was conducted to assess the efficacy of the

stretch procedures on hamstring muscle flexibility. The

two independent variables included stretch-type (HR and

HR-AC) and time (pre- and poststretch measures). A

stretch type by time interaction was analyzed with the post

hoc Bonferroni-corrected paired t-test. The McNemar x2

test for correlated samples also was used to test if the

proportion of subjects who exceeded the MDC95with the

HR-AC stretch was different from the proportion ofsubjects who exceeded the MDC95with the HR stretch.

Statistical significance was established with p # 0.05.

Main study: EMG considerations

The dependent variable for assessment of the activation

level of the hamstring muscles before and after the

PNF stretch procedures was peak normalized EMG

(%MVIC) obtained during a period of muscle inactivity.

A paired t-test was used to determine if a significant

difference existed between activation levels of the

hamstring muscles before and after each PNF stretch

procedure. Statistical significance was established with

p # 0.05. SPSS 15.0 statistical software was used for

all data analysis (SPSS, Inc., Chicago, IL).

RESULTS

Pilot Study

The intratester reliability (ICC2,1) for obtaining measure-

ments of popliteal angle was 0.92. The SEM and

MDC95were 38 and 78, respectively.

FIGURE 4 Prior to the stretching procedures, a maximum

voluntary isometric contraction (MVIC) value of hamstring

electromyographic (EMG) activity was recorded according to

standard muscle testing postioning procedures. The examiner

is attempting to extend the subjects knee while she maintains

the knee joint in 458 of knee flexion.

245 Youdas et al.



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Main Study: Knee Extension Angle Changes

Mean pretest knee extension angles for HR and

HR-AC PNF stretch procedures were 1498678 and

1488678, respectively, whereas mean posttest knee

extension angles for HR and HR-AC were 1608688

and 1638678, respectively. Mean gain in knee exten-

sion for HR was 118648 and 158658 for HR-AC.

A significant time effect occurred between pretest

and posttest knee extension measurements (F1, 34 5

350.2; p , 0.001). In addition, a significant time by

stretch-type interaction was detected (F1,34 5 21.1;

p , 0.001). Paired t-tests revealed angles of knee

extension for HR and HR-AC were not different prior

to stretching (p 5 0.45); nevertheless, poststretch

(Figure 5) knee extension angle was greater in the

HR-AC condition than the HR condition (mean

difference 5 3.58, 95% CI 5 18 to 68, t34 5 2.858;

p , 0.007). Furthermore, for the HR procedure 80%(28 of 35) of subjects demonstrated a knee extension

angle difference (poststretch prestretch) of at least 78

(MDC95), whereas for the HR-AC procedure 97%

(34 of 35) of subjects demonstrated a knee extension

angle difference of at least 78(MDC95). The McNemar

x2 test (x2 5 0.257,p 5 0.07) revealed the proportion

of subjects who exceeded the MDC95with the HR-AC

stretch (97%) did not differ from the proportion of

subjects who exceeded the MDC95 with the HR

stretch (80%).

Main study: EMG changes

A statistically significant increase (t33 5 2.6; p , 0.013)

in peak hamstring EMG activation values during a rest

period (0.6% MVIC; 2.461.5%1.861.0%) was found

post-HR-AC (Figure 6).

DISCUSSION

Knee Extension Angle and Hamstring

Muscle Length

We supported our research hypothesis and detected a

significant difference in knee extension angle for both

HR-AC and HR conditions immediately following

a single session of PNF stretch compared to their

respective baseline knee extension angles. Moreover,

we also detected a statistically significant time by

stretch interaction whereby the mean poststretch value

of knee extension for HR-AC was larger than the

poststretch HR value. However, despite a statistically

significant difference we do not believe there is a

meaningful clinical difference between the modified

PNF techniques of HR-AC and HR because the

proportion of subjects who exceeded the MDC95 (78)

with the HR-AC stretch did not differ from the

proportion of subjects who exceeded the MDC95 (78)

with the HR procedure.

On the basis of a common outcome variable-knee

extension we compared and contrasted five featuresfrom the present study with those from four previously

reported studies (Osternig, Robertson, Troxel, and

Hansen, 1990; Spernoga, Uhl, Arnold, and Gansneder,

2001; Sullivan, DeJulia, and Worrell, 1992; Worrell,

Smith, and Winegardner, 1994) that investigated the

efficacy of modified PNF stretching procedures on

140

145

150

155

160

165

AfterBefore

Stretch Time

KneeExtensionAngleinDegrees(Mean)

HR

HR-AC

*

FIGURE 5 Mean difference values and standard error bars

(SE) of knee extension are displayed for each proprioceptive

neuromuscular facilitation (PNF) stretching technique. Data

indicate a statistically significant time by stretch-type interac-

tion (p, 0.001). Angles of knee extension for HR and HR-ACwere not different prior to stretching (p 5 0.45). Poststretchknee extension angle was greater in the HR-AC condition than

the HR condition (p , 0.007). The asterisk (*) represents astatistically significant difference between the poststretch values

of knee extension for HR-AC and HR PNF procedures.

0

0.5

1

1.5

2

2.5

3

HR-ACHR

PeakHamstringEMG(%M

VIC)

Pre-Test

Post-Test

*

FIGURE6 There was no significant difference pretest to

posttest in peak hamstring electromyographic (EMG) activity

for hold-relax (HR) but hold-relax with antagonist contraction

(HR-AC) produced a 0.6% increase in hamstring activation

(1.82.4%), which was statistically significant (p , 0.05).




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hamstring muscle length in healthy subjects. The five

features included the following: 1) PNF technique;

2) duration of PNF stretch procedure; 3) gain in knee

extension ROM; 4) responsiveness of outcome measure-

ment; and 5) duration of PNF protocol.

PNF Technique

The present study compared both HR-AC and HR

conditions, whereas previous reports examined either

CR (Osternig, Robertson, Troxel, and Hansen, 1990),

HR (Spernoga, Uhl, Arnold, and Gansneder, 2001), or

contract-relax-contract (CRC) (Sullivan, DeJulia, and

Worrell, 1992; Worrell, Smith, and Winegardner, 1994)

PNF stretching procedures. The CR and HR proce-

dures were identical. After a prestretch measurement of

knee extension ROM, both CR and HR techniques

used an isometric contraction of the hamstrings in the

absence of hip or knee rotation. Next, the subject

experienced a short relaxation period followed by

the poststretch measurement of knee extension. The

sequence of CRC alternated between an isometric

contraction of the hamstrings followed by the quad-

riceps femoris. However, the activation sequence varied

between the two studies (Sullivan, DeJulia, and Worrell,

1992; Worrell, Smith, and Winegardner, 1994).

Duration of PNF Stretch Procedure

The present study used a 10-second cycle for the HR

technique and a 20-second cycle for HR-AC. The

next shortest duration was a single 30-second cycle

(Sullivan, DeJulia, and Worrell, 1992) followed by

4 3 20-second cycles (Worrell, Smith, and Winegardner,

1994); 5 3 20-second cycles (Osternig, Robertson,

Troxel, and Hansen, 1990); and 5 3 26-second cycles

(Spernoga, Uhl, Arnold, and Gansneder, 2001).

Gain in Knee Extension ROM.

In the present study mean gain in knee extension

ROM ranged from 118648 (HR) to 158658 (HR-AC).

For CR Osternig, Robertson, Troxel, and Hansen(1990) found a gain in knee extension of 58; for HR

Spernoga, Uhl, Arnold, and Gansneder (2001)

demonstrated an 88 gain; and CRC demonstrated

a knee extension gain of 108 (Worrell, Smith, and

Winegardner, 1994) and 118 (Sullivan, DeJulia, and

Worrell, 1992). The present study and the other four

studies used a crossbar (Spernoga, Uhl, Arnold, and

Gansneder, 2001; Sullivan, DeJulia, and Worrell,

1992; Worrell, Smith, and Winegardner, 1994) or

straps (Osternig, Robertson, Troxel, and Hansen,

1990) to maintain the hip in a fixed position during

the measurement of the knee extension angle.

Responsiveness of Knee Extension

Measurement

To our knowledge, the current study is the first

investigation to report a MDC95 for hamstring muscle

length (78) after a brief, modified PNF stretch in

healthy subjects with reduced hamstring muscle length.

Using reported estimates of intratester reliability and

measurements of precision (SEM), we calculated the

MDC95 for three previous studies (Spernoga, Uhl,

Arnold, and Gansneder, 2001; Sullivan, DeJulia, and

Worrell, 1992; Worrell, Smith, and Winegardner,

1994). MDC95 values ranged from 68 (Spernoga, Uhl,

Arnold, and Gansneder, 2001; Sullivan, DeJulia, and

Worrell, 1992) to 88 (Worrell, Smith, and Winegardner,1994). The present study used a standard universal

goniometer to record measurements of knee extension,

whereas the three previous reports used a gravity

inclinometer (Spernoga, Uhl, Arnold, and Gansneder,

2001; Sullivan, DeJulia, and Worrell, 1992; Worrell,

Smith, and Winegardner, 1994). According to respon-

siveness data from this study and the three previous

reports, a rehabilitation professional can expect real

improvement in hamstring muscle length after HR or

HR-AC PNF stretching procedures when knee exten-

sion angles exceed baseline measurements by 6888.

Duration of PNF Protocol

The current study administered the modified HR and

HR-AC PNF stretches within a single session. Similarly,

the CR (Osternig, Robertson, Troxel, and Hansen,

1990) and HR (Spernoga, Uhl, Arnold, and Gansneder,

2001) procedures also used a single session, whereas

the CRC PNF procedure was performed between 8

(Sullivan, DeJulia, and Worrell, 1992) and 15 sessions

(Spernoga, Uhl, Arnold, and Gansneder, 2001).

In summary, we believe the present study shows the

efficacy of modified HR and HR-AC PNF stretching

procedures when used to increase knee extension ROMin healthy subjects with reduced hamstring muscle

length. A mean gain in knee extension ROM ranging

from 118 (HR) to 158 (HR-AC) was accomplished

within a single session using a stretch cycle no longer

than 20 seconds. The other two reports (Osternig,

Robertson, Troxel, and Hansen, 1990; Spernoga, Uhl,

Arnold, and Gansneder, 2001) that used single session

stretching protocols incorporated five cycles each lasting

20 seconds (Worrell, Smith, and Winegardner, 1994) or

26 seconds (Spernoga, Uhl, Arnold, and Gansneder,

247 Youdas et al.



9/12

2001). We believe our modified technique of PNF

stretching was successful in terms of gain in knee

extension ROM within one session using a single

stretch cycle of less than 30 seconds because of the

position in which the stretch was performed. Modified

PNF stretches were performed in supine with the hip

stabilized at 908 of hip flexion using the crossbar

apparatus with hamstring resistance and subsequent

stretch supplied by the examiners manual contacts.

Knee extension ROM was measured immediately upon

conclusion of the stretch. In contrast, the CRC stretch

procedure was performed in standing (Sullivan, DeJulia,

and Worrell, 1992; Worrell, Smith, and Winegardner,

1994), whereas the HR procedure was performed by

using a straight leg raise test with the knee straight

(Spernoga, Uhl, Arnold, and Gansneder, 2001). In both

conditions the outcome measurement of knee extension

(poststretch) necessitated that a subject alter the position

of the hip and knee joint, thus changing the newlyobtained hamstring muscle length while transitioning to

the 908:908 start position for the knee extension test.

EMG Activity

For both HR and HR-AC PNF procedures we

hypothesized to observe a statistically significant reduc-

tion in peak hamstring activation level (%MVIC)

between the 10-second poststretch and prestretch

intervals with no intended muscle activation. We based

this expectation upon neurophysiological rationale

whereby the HR procedure would reflexively inhibitthe hamstrings through autogenic inhibition (Edin and

Vallbo, 1990; Gollhofer, Schopp, Rapp, and Stroinik,

1998; Osternig, Robertson, Troxel, and Hanson, 1990),

whereas the AC procedure would reflexively inhibit the

hamstrings via reciprocal inhibition (Magnusson et al,

1996). We reasoned that by reducing contractile activity

in the hamstrings at rest as measured by their normal-

ized EMG activation levels then the hamstring muscles

resistance to passive elongation would result in an

improved knee extension angle. Paradoxically, HR-AC

produced a statistically significant (p , 0.05) change in

peak hamstring EMG activity of 0.6% MVIC (2.4%

MVIC [poststretch]1.8% MVIC [prestretch]) betweenthe rest periods. Although this change was statistically

significant, we would question its clinical relevance

because of the meager change in the level of normalized

hamstring EMG activation. Data from this present

study cast doubt on the role of autogenic and reciprocal

inhibition as putative mechanisms accounting for knee

extension angle improvement (hamstring muscle flexi-

bility) following brief PNF stretch maneuvers.

Besides examining the effect of static and PNF

(contract-relax [CR] and contract-relax with agonist

contraction [CRAC]) stretching on hip flexion ROM,

investigators (Moore and Hutton, 1980) also recorded

EMG activity of the hamstrings during three stretch

procedures. Hamstring EMG activity was expressed as

a percentage of the static stretch activity. Paradoxically,

despite producing the greatest improvement in supine

straight leg raise (hip flexion ROM) the CRAC

procedure demonstrated median values of 710%

more EMG activity than the static stretch maneuver.

Moore and Hutton (1980) concluded that full

hamstring muscle relaxation was not required for

effective stretching of the hamstring muscles. Lastly,

in addition to examining passive torque and stretch

perception during a 10-second bout of either static

stretch or 6-second PNF HR stretch, Magnusson et al

(1996) also recorded EMG activity of the hamstrings

during the stretch procedures. The authors noted

when the knee joint of the test side was passively

extended to the onset of a subjects perceived pain by adynamometer, the EMG responses were similar for

both static and HR PNF stretch procedures despite

the fact that the HR procedure produced an increase in

knee joint extension ROM. The authors (Magnusson

et al, 1996) concluded that EMG responses were not

altered by the type of stretch but PNF stretching

produced an increase in the subjects stretch tolerance

or perception.

Limitations

We acknowledge five limitations within this study.1) We did not quantify the lasting changes in

hamstring muscle length produced by one cycle of

modified HR or HR-AC PNF stretch. Physical-fitness

or rehabilitation personnel would be able to make a

more informed clinical decision about the usefulness of

HR or HR-AC stretching procedures if additional

studies were conducted that described long-term

ROM changes. 2) We did not exclude subjects with

lower extremity injury 7 months or more prior to the

study. Potentially, subjects may have presented with

reduced hamstring muscle length or knee joint range of

motion due to chronic lumbar spine or lower extremity

injury. 3) We used a subjective end point for thepassive knee extension test procedure. The examiner

passively extended the subjects knee joint until firm

resistance was detected without a painful stretch

response from the posterior thigh. Perhaps a dynam-

ometer could have been used to ensure a consistently

applied external knee extension force from the

examiner during the prestretch and poststretch condi-

tions. However, this is not a common clinical practice.

We did document the measurement error and respon-

siveness associated with performance of the knee




10/12

extension test (ICC2,1 5 0.93; MDC95 5 78) and both

values appeared clinically acceptable. 4) Hip positional

changes between prone and supine may have influ-

enced hamstring EMG activation values. For example,

MVIC of the hamstrings was obtained in prone (hip

extension), whereas hamstring stretch interventions

were performed supine in 908 of hip flexion. Such

positional changes could potentially alter neuro-

physiological activity of the hamstring muscles and

hence surface EMG activation patterns. 5) Lastly, we

acknowledge the following methodological design flaw.

Prior to the HR-AC stretch (pretest) we used the

passive knee extension test to assess hamstring muscle

length. Upon completing the antagonist contraction

phase (active contraction of the quadriceps femoris)

we immediately measured the knee extension angle.

We did not use the passive knee extension test. We

reasoned that by having the subject relax after the

completion of HR-AC we may contaminate the ham-string muscle length just achieved by the HR-AC.

Despite this flaw, our reported mean gain in hamstring

muscle length after the HR-AC procedure was 158658.

This value was comparable to the 138 gain in knee

extension reported by Sullivan, DeJulia, and Worrell

(1992) and the 108 gain in knee extension reported by

Worrell, Smith, and Winegardner (1994). Both studies

used active knee extension to measure hamstring

muscle length after a contract-relax-contract PNF

stretch procedure that is comparable to the modified

PNF HR-AC stretch. Finally, we did not quantify the

lasting changes in hamstring muscle length produced

by one cycle of modified HR or HR-AC PNF stretch.This would be an important question for further

research on the efficacy of PNF when applied to

subjects with reduced hamstring muscle length.

CONCLUSION

We observed statistically significant improvements in

hamstring muscle length using a single, 10-second

(HR) or 20-second (HR-AC) cycle of modified PNF

stretching in healthy subjects with reduced hamstring

muscle length. Mean difference values in hamstring

muscle length as measured by the knee extension anglewere 118648 for HR and 158658 for HR-AC. For both

PNF stretch procedures, the mean difference in knee

extension angle exceeded the MDC95 value of 78.

Nevertheless, despite a statistically significant differ-

ence, we do not believe there is a meaningful clinical

difference between the modified PNF techniques of

HR-AC and HR because the proportion of subjects

who exceeded the MDC95 (78) with the HR-AC

stretch did not significantly differ from the proportion

of subjects who exceeded the MDC95 (78) using the

HR procedure. We were unable to attribute improve-

ments in knee extension angle to neurophysiological

factors such as autogenic and reciprocal inhibition,

because activation levels of hamstring EMG activity

(%MVIV) during a 10-second rest period immediately

before and after the PNF stretch procedures were

clinically unchanged. Each modified PNF procedure

examined in the present study produced a meaningful

gain in knee extension angle after a single stretch

session lasting no more than 20 seconds. In contrast,

other investigators reported using multiple sessions

and several stretch cycles per session to accomplish

comparable gains in hamstring muscle length. Future

research is necessary to quantify the lasting changes

in hamstring muscle length produced by one cycle of

modified HR or HR-AC PNF stretch.

ACKNOWLEDGMENT

Funding was provided by the authors Program of

Physical Therapy.

Declaration of Interest: The authors report no

conflicts of interest. The authors alone are responsible

for the content and writing of the paper.

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