Eficacia Fnp en Estiramiento de Biceps Femoral
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8/12/2019 Eficacia Fnp en Estiramiento de Biceps Femoral
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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: youdas.james@mayo.edu
Accepted for publication 23 April 2009.
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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
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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
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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.
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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.
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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,
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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
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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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