EMG_X_LENGTH_X_TORQUE_HAMST

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Relationship between wire EMG activity, muscle length,
and torque of the hamstrings
Olfat Mohamed a,*, Jacquelin Perry b, Helen Hislop c
a Department of Physical Therapy, California State University, 1250 Bellflower Boulevard, Long Beach, CA 90840-5603, USA
b Pathokinesiology Laboratory, Rancho Los Amigos National Rehabilitation Center, Downey, CA, USA
c Department of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, CA, USA
Received 8 January 2002; accepted 28 June 2002
Abstract
Objective. To determine the effect of change of muscle length on the torque and wire electromyographic activity of six knee flexor
muscles.
Design. Maximum isometric knee flexion torque and wire EMG data were collected at nine different positions.
Background. In vivo EMG–length–tension relationship is difficult to determine because of the interaction between muscle length
and moment arm. The study of two-joint muscles allows the change of muscle length at one joint while preserving stable mechanical
relationships at the other. This model facilitates understanding of length–tension and EMG–length relationship in vivo.
Methods. Nineteen subjects performed maximum isometric knee flexion contraction at nine positions of varying hip and knee
angles. Wire EMG activity was recorded from semitendionsus, semimembranosus, long and short head of the biceps femoris, gracilis
and sartorious muscles.
Results. As the two-joint hamstrings were lengthened, torque was significantly increased. Maximum isometric torque ranged
from 257 to 716 kg cm. The ratio of the torque values to EMG activity of all muscles was increased at longer muscle lengths. A
change in the muscle length of the two-joint hamstrings did not produce a consistent change of EMG activity. The short head of the
biceps femoris and sartorius muscles increased their activity as the angle of knee flexion increased.
Conclusions. Maximum torque of knee flexion occurs at the most lengthened position of the hamstrings. EMG activity did not
consistently change with the change in muscle length.
Relevance
Understanding in vivo length–tension relationship and associated EMG activity is important for designing rehabilitation pro-
tocols, tendon lengthening and transfer and interpretation of EMG data.
Published by Elsevier Science Ltd.
Keywords: Length–tension relationship; EMG–length relationship; Electromyography; Hamstrings; Knee flexion torque
1. Introduction
Understanding length-tension relationships in human
muscle is important for assisting the design of tendon
transfer, rehabilitation and training procedures. Fur-
thermore understanding the effect of change in mus-
cle length on the electromyographic (EMG) activity
recorded from muscle is essential in the functional in-
terpretation of EMG data during different activities.
Muscles are not attached to the skeleton at their opti-
mum length. The in vivo length–tension relationship is
complicated by several anatomical and mechanical fac-
tors. As a joint rotates, the synchronous change in
muscle length and effective moment arm at the joint re-
sult in torque variations. The interrelationship between
these two variables, namely muscle length and moment
arm appear to be different in different muscles. The in situ
length–tension curves in the cat hindlimb muscles have
all been found to follow the pattern of the ascending limb
of the force–length relationship [1,2]. However, frog
semitendinosus occupy the descending limb of the force–
length curve [3]. In humans, gastrocnemius and wrist
*Corresponding author.
E-mail address: osm@csulb.edu (O. Mohamed).
0268-0033/02/$ - see front matter Published by Elsevier Science Ltd.
PII: S0268-0033 (02 )00070-0
Clinical Biomechanics 17 (2002) 569–579
www.elsevier.com/locate/clinbiomech
flexor torque increased with increased muscle length
[4,5]. Wrist extensors however, followed the classical
length–tension curve with an ascending limb, a plateau
and a descending limb through the physiologic range of
motion [5]. Based on their mathematical model Chang
et al. found that forces generated by the biceps brachii
and brachialis increased in the more flexed joint posi-
tion (descending limb of length–tension relationship)
and the brachioradialis pursued the ascending limb of
length–tension relationship [6].
The fact that joint angles vary during motion, thereby
continually altering the moment arm, is a confounding
problem when examining the effect of changes in muscle
length on EMG and torque. Investigating two-joint
muscles provides an advantage because of the oppor-
tunity to alter the muscle length at one joint while re-
taining stable mechanics at the other joint. With this in
mind, several investigators studied the effect of change
of angle at one joint on torque production at the other
joint. These reports have included the effects of altering
the knee angle on hip extensor torque [7–9], and the hip
angle on knee extensor [10,11] or knee flexor torque
[11,12] and also the knee angle on plantar flexion torque
[13–15]. Elongation of the two-joint muscles was ac-
companied with greater torque production in most
cases.
Measured torque values at any joint, is the sum of
torques of several muscles. The level of participation of
these muscles at different angles is an important factor
that affects the torque values measured at the joint.
Sensing the electrical signal of these muscles indicates
their relative level of participation. An extensive body of
literature is available on the relationship between EMG
activity and graduated muscle force with some authors
reporting a linear relationship [16–22] and others re-
porting more complex curvilinear relationships [22,23].
In these investigations, muscle length generally was kept
constant, a situation which is not analogous to that
present during most functional physical activities.
Studies that have investigated the relationship be-
tween muscle length and myoelectric activity yielded
disparate results. Some researchers reported a decrease
in EMG activity as the muscle length increased [12,16,24]
while others reported no change in EMG activity at
different muscle lengths [22,25]. Only few investigations
have been directed toward a comprehensive analysis of
the interaction of the three factors of muscle length,
muscle torque and concomitant EMG activity [12,15,16].
These studies have used surface, unipolar wire or con-
centric needle EMG methods. Surface and monopolar
electrodes are susceptible to crosstalk [13,26,27], while
concentric needle technique does not allow for sampling
the same motor units in different trials. This investiga-
tion used a fine wire bipolar technique, which has the
advantage of recording the same group of motor units
through the change of joint angle.
This study used a two-joint muscle model in which
the length of the hamstrings was altered at the hip while
keeping a constant lever arm at the knee. The main
hypothesis of this study was that an increase in the
length of the hamstring muscles would result in an in-
creased torque with a decreased EMG output over that
recorded at shorter lengths.
2. Methods
EMG activity of six knee flexor muscles and isometric
knee flexion torque were recorded from the dominant
lower extremity of a group of normal healthy women.
Data were collected during manual muscle testing and
during maximum isometric knee flexion at varying po-
sitions of the hip and knee.
2.1. Subjects
Nineteen healthy adult women between the ages of 22
and 36 years volunteered for the study. Participants
reported no history or signs of musculoskeletal or neu-
rological dysfunction that would affect motor perfor-
mance involving either lower extremity.
2.2. Instrumentation
Myoelectric activity (EMG) of each muscle, was
sensed by a pair of 50-lm insulated nickel–chromium
alloy wire electrodes with hookedends. The paired wires
were threaded through a single 1.5 in., 25 gauge, dis-
posable needle. The electrode assembly was sterilized
before use and discarded following insertion. The prox-
imal ends of both wires were connected to a 2 cm2 pad
that contained the ground plate taped to the ipsilateral
leg near the insertion site. Shielded cables extending
from the pads (one for each muscle) carried the signals
to a telemetry package worn around the subject’s waist.
Within this package the signals were differentially am-
plified, filtered and transmitted by a six-channel FM–
FM telemetry unit (Model 2600, Biosentry Telemetry
INC, Torrance, CA, USA). The Telemetry system had a
common mode rejection ratio of 58 dB. The modulation
of each channel was adjusted to insure 100–1000 Hz
frequency responses. The signals from these modulated
channels then were telemetered to a multichannel on-
line digital acquisition system using a DEC 11/23 com-
puter (Digital Equipment Co., Maynard MA, USA).
The computer sampled each channel at 2500 Hz. The
filter system band passed the data from 80 to 1000 Hz,
thus eliminating any 60 Hz or low frequency noise.
Calibration was achieved by passing a 1 V, 400 Hz signal
through the ground electrodes; a baseline noise level
above 50 mV was not accepted. The instrumentation
also included seven monitoring oscilloscopes, which
allowed immediate viewing of the data.
570 O. Mohamed et al. / Clinical Biomechanics 17 (2002) 569–579
Knee flexor torque was measured using a Cybex II
dynamometer (Lumex Co., Ronkonkoma, NY, USA).
The dynamometer was attached to a table with an ad-
justable back support which accommodated differing
thigh lengths and could recline backward from 90� to
180�, and forward from 90� to 70�. The resistance arm of
the dynamometer was adjustable to different leg lengths
and was capable of being locked in varying degrees of
knee flexion. The speed adjustment was set at 0 rad/s to
provide an immovable dynamometer arm. Calibration
of the dynamometer was performed using a series of
certified weights placed on the dynamometer’s lever arm
against which the torque output of the device was
compared. A Dual Beam oscilloscope (Model 502A
Tektronix Inc., Oregon, USA) was connected to the
dynamometer and was used to provide an instant visual
feedback of torque values to the subject. The oscillo-
scope had two horizontal fluorescent beams; one was
‘‘static’’ and could be set at any desired level on the
screen while the other beam moved to represent the
amount of torque exerted at the dynamometer arm. A
video camera and cassette recorder were used to docu-
ment and store all steps in the study of each subject.
2.3. Procedure
2.3.1. Subject preparation and electrode insertions
A training session was scheduled about an hour be-
fore testing to familiarize the subject with the dyna-
mometer and the visual feedback system. Height and
weight (without shoes) were measured to the nearest cm,
and to the nearest 0.5 kg, respectively. The leg that the
subject used to kick a ball was determined as the dom-
inant lower extremity. Straight leg raise (SLR) range of
the dominant lower extremity was measured by a hand-
held goniometer with the subject supine. Because of the
study requirement to avoid passively stretching the knee
flexor muscles, the full range of SLR was considered to
be 10� less than the passive full range for each subject.
Thus, for this study SLR equalled passive SLR minus
10�. Subjects with a SLR range equal to, or more than
90� were designated as the ‘‘long’’ muscle group. Those
subjects with a SLR range of <90� were classified as
having ‘‘short’’ hamstrings. All electrode insertions were
in the dominant lower extremity, the wire electrode pairs
were placed using Basmajian’s single needle technique
[28] into the following six muscles; semitendinosus,
semimembranosus, biceps fermoris (long head), biceps
femoris (short head), sartorius and gracilis.
After each insertion, the subject was asked to produce
several contractions of that muscle to secure the wires.
The wires were taped to the skin leaving a loose loop to
assure that under high torque the electrodes would not
be displaced. The proximal ends of the wires were at-
tached to the ground electrode–telemetry system. Elec-
trode placement was confirmed with a mild electrical
current administered through the inserted wires to cause
a visible or palpable contraction of each muscle. After
confirmation of electrode placement the subject was
asked to walk across the walkway at different speeds to
ensure that the electrodes were settled within the muscles
and to observe if discomfort existed that might have
altered the normal pattern of walking. In any such case,
the electrodes causing the discomfort were moved or
replaced.
2.3.2. Manual muscle testing
Manual muscle testing for the medial and lateral
hamstrings and the gracilis muscle was performed with
the subject in the prone lying position [29]. For the
medial hamstring and gracilis muscles, maximum resis-
tance to knee flexion was applied with the subject’s knee
flexed to 75� and the leg medially rotated. For lateral
hamstring muscles, the knee was flexed with the leg
laterally rotated.
The manual muscle test for the sartorius muscle was
conducted with the subject in a sitting position at the
edge of the plinth. Hip flexion was maximally resisted at
the lower one-third of the thigh, while the other hand of
the investigator prevented the pelvis from tilting poste-
riorly. This modified sartorius test was chosen over the
traditional test because a comparison of techniques
showed that the selected procedure generated the highest
EMG.
2.3.3. Dynamometer testing
Torque and EMG data were collected during a series
of nine combinations of hip and knee positions (Fig. 1).
Maximum isometric knee flexion was performed with
the knee at 0�, 45� and 90� while the hip was at 0� of
flexion. The same knee angles were tested with the hip
flexed to 90� and to the angle of SLR range minus 10�.
Fig. 1. The nine test positions.
O. Mohamed et al. / Clinical Biomechanics 17 (2002) 569–579 571
For ease of recording and comprehension the nine
positions were coded in a four-digit sequence. The first
two digits indicate hip angle and the last two digits
represent knee angle (Fig. 1). For example: hip 0�, knee
0� ¼ 00–00, hip SLR, knee 45� ¼ SLR–45 and hip 90�,
knee 90� ¼ 90–90.
2.3.3.1. Test sequence. A preset randomization order for
maximum testing was prepared for each subject, which
ensured: (1) that the same number of subjects performed
each sequence and (2) avoidance of frequent changes of
subject position between sitting and lying. Following
recording knee flexor torque and EMG activity at all
three knee angles in the first randomly selected hip po-
sition, the same test procedure was then repeated at the
second and then the third hip position with the knee
angle again randomly ordered.
For every position, the axis of the dynamometer arm
was aligned with the axis of the knee joint (at the level of
the femoral epicondyle). A baseline measurement was
recorded with the leg resting on the cuff so that the
torque produced by the weight of the leg would not be
added to that produced by the contraction of the knee
flexor muscles. After securing the test position, the
monitoring oscilloscope used by the subject was set so
that its two horizontal fluorescent beams were super-
imposed at the baseline. The subject then was instructed
to try as hard as she could to move the ‘‘dynamic torque
beam’’ as high on the screen as possible. Verbal en-
couragement was given during all tests. In each position,
two maximum isometric contractions, of 5 s each, were
performed. The highest torque value of the two testing
trials was used for analysis.
Stabilization of subjects was provided in a variety of
ways, a firm wedge shaped pad was placed under the
kneesurface, a pelvic strap was used to counteract dis-
placement of the pelvis and another strap was placed
firmly over the midthigh. The back support and seat
depth were adjusted individually for each subject adding
to her stability and comfort. The subject was encour-
aged to grip the table for further support.
2.3.3.2. Target torque. Target torque testing was per-
formed after the maximum knee flexion test in the po-
sition of hip at 0�, knee at 90� (the shortest position for
the hamstrings). The torque value achieved in that po-
sition (00–90) was recorded and then the subject was
asked to match it in the two following positions: hip at
90�, knee at 90�, and hip at SLR, knee at 0�, using the
same visual feedback from the oscilloscope.
2.4. Data analysis
2.4.1. EMG data processing and quantification
Digitization of the data was accomplished with the
aid of a DEC 11/23 minicomputer with an AD converter
to transfer the EMG signal into a discrete numerical
output. All EMG data were digitized at 2500 samples
per second. The digitized data were integrated using a
VAX computer 750. The integration program first cal-
culated the mean signal from the resting EMG trial to
establish a noise threshold for each channel of EMG.
The noise threshold value which then was subtracted for
the remaining trials to correct for any DC interference in
the signal form. The corrected values were rectified and
summed to produce the integrated value of the EMG for
each 1/50th second. Data from the manual muscle test
were processed to yield a normalization factor against
which subsequent EMG data were compared. EMG
values from the isometric testing were expressed as a
percentage of the maximum EMG obtained during the
manual muscle test and abbreviated as %MMT. Torque
data for each trial were printed out at 0.02 s intervals for
the 5-s duration of the test. The mean value of the
highest torque in any 1 s, and the EMG data associated
with that interval, were used for analysis.
2.4.2. Statistical analysis
A detailed screening analysis was performed on tor-
que data and on the EMG data for all muscles. A two-
way repeated measures analysis of variance (ANOVA)
was employed to determine if there was a significant
difference in torque or EMG data from all tested mus-
cles between the group with long muscles (>90�) and the
group with short muscles (<90�). The other factor was
the testing angle.
A two-way repeated measures ANOVA was used to
determine the effect of hip and knee position on the
torque and EMG data. Preliminary analysis revealed a
significant interaction between the positions of the two
joints. Consequently the effect of each joint was studied
separately using one-way repeated measures ANOVA.
Because separate analysis was done for each joint which
required examining the same data twice, Bonferroni
adjustment was made to ensure an overall alpha level of
0.05. A repeated measures ANOVA was also conducted
to compare the EMG at target torque. All statistical
procedures were conducted with the BMDP software
program (Statistical Software Inc, Los Angeles, USA).
3. Results
3.1. Subject description
The mean age for the group of 19 subjects was 27
years (range: 22–36 years), the mean height was 162 cm
(range: 141–173 cm) and the mean weight was 61 kg.
The maximum SLR measurement ranged from 70 to
106� with mean of 90�. There were two subjects with a
left dominant lower extremity whereas in 17 subjects the
right lower extremity was dominant.
572 O. Mohamed et al. / Clinical Biomechanics 17 (2002) 569–579
3.2. Repeated trials
The EMG and torque values from the first trial of
maximum isometric knee flexion in all nine tested posi-
tions were significantly correlated with those obtained
during the second trial (P < 0:0001). The correlation
coefficients ranged from 0.91 to 0.97 for the torque
values and from 0.77 to 0.99 for the EMG of all tested
muscles. The majority (80%) of the correlations coeffi-
cients were equal to or greater than 0.90 for EMG of all
muscles in all positions.
3.3. Effect of inherent hamstring length on EMG and
torque
SLR range was significantly different between the
long and short group (P < 0:00001). Age, weight and
height data, however, did not contribute to significant
differences between the two groups. The mean SLR
range for the short group was 82.6 (SD, 6.3�) and for the
long group was 98.8 (SD, 4.1�). According to responses
on the screening questionnaire there was no discernible
difference between the two groups in their customary
lifestyle or recreational activity patterns.
The two groups did not vary significantly with respect
to EMG activity in any of the investigated muscles
(P > 0:05). Neither were there significant differences in
torque values between the two groups (P > 0:05). The
lack of any significant differences related to inherent
muscular length led to the dissolution of the grouping
and all 19 subjects were treated thereafter as a single
group.
3.4. Effect of change of hip positions on maximum
isometric knee flexion torque
Regardless of knee position, the extended hip posi-
tion (0�) was always associated with significantly less
torque than that recorded in the two flexed hip positions
(SLR and 90) (Table 1, Fig. 2). For example, with the
knee at 0�, peak torque with the hip at 0� was 581 kg cm,
which was significantly lower than the 680 and 637
kg cm obtained in the SLR and 90� hip angles respec-
tively. Similarly with the knee at 45� and hip at 0�, the
value for maximum torque was 467 kg cm, which was
significantly lower than the 719 and 716 recorded at the
SLR and 90�. There was no significant difference be-
tween the two sitting positions (SLR and 90�) with the
knee at 45� (Table 1).
3.5. Effect of change of knee positions on maximum
isometric knee flexion torque
Flexion at 90� was the only knee position that influ-
enced flexor muscle torque independent of hip angle, as
the maximum isometric flexor torque was the least at all
three hip positions. At the extended hip position (0�), all
subjects generated their highest torque when the knee
was fully extended (0�). The group mean was 580 kg cm.
The magnitude of torque values declined significantly as
knee flexion increased to 45� (467 kg cm) and fell to its
lowest value at 90� (257 kg cm). Differences between the
three knee angles were statistically significant (P < 0:05)
(Table 1, Fig. 2).
In the two sitting positions (hip SLR and 90�) the 90�
knee angle, was associated with significantly lower mean
torque values than the other two knee positions (0� and
45�). The highest mean knee flexion torque values were
generated when the knee was at 45� with the hip at SLR
and 90�. For example, with the hip at 90�, the three knee
angles of 0�, 45� and 90� were associated with torque
values of 637, 716 and 462 respectively; the differences
Table 1
Maximum isometric knee flexion torque (kg cm), comparison of three hip and three knee positions
Hip position Mean (SEM) Significant multiple comparison
Knee 0� Knee 45� Knee 90�
0� 580 (33.4) 467.6 (30.5) 256.5 (24.1) Knee 0� versus 45�, 90�
Knee 45� versus 90�
SLR 680 (38.5) 719.2 (41.6) 437.1 (31.6) Knee 90� versus 0�, 45�
90� 637 (32.6) 716.1 (47.1) 461.5 (32.5) Knee 0� versus 45�, 90�
Knee 45� versus 90�
Fig. 2. Maximum isometric knee flexion torque at three positions of
the knee and two hip angles (mean, SEM).
O. Mohamed et al. / Clinical Biomechanics 17 (2002) 569–579 573
were significant between the three knee angles (Table 1,
Fig. 2).
3.6. Effect of the most lengthened position on knee flexion
torque
When the hamstring muscles were tested in their most
elongated positions (knee fully extended), the slight
differences in length incurred when the hip varied be-
tween SLR and 90� did not significantly alter knee
flexion torque (P > 0:05). Fourteen of the 19 subjects
regardless of their range of SLR, produced equal or
slightlygreater torque at the SLR–00 versus the 90–00
position.
3.7. Effect of change of hip and knee position on EMG
activity of the two-joint hamstrings
The EMG activity of the two-joint hamstrings at
maximum effort was not altered significantly with the
change in hip angle at the three knee positions
(P > 0:05). The only exception was the decreased ac-
tivity of the semimembranosus where the mean value
was 75% MMT with the hip at 0� as opposed to 90%
MMT at the other two hip angles (Table 2). Similarly,
the change in knee position did not significantly affect
the EMG activity of the two-joint hamstrings, except
for the semitendinosus where the mean EMG values
were 68% MMT in the most lengthened position (90–00)
compared to 86% in the most shortened position
(00–90).
3.8. Effect of change of hip and knee positions on EMG
activity of the other three knee flexors
The change in hip position did not affect the EMG
activity of the short head of biceps femoris. Knee angle
change, however, resulted in significantly less EMG at
the extended knee position (0�) compared to the other
two knee-flexed positions (45� and 90�). This pattern
was consistent in all three hip positions (Table 3).
Altering the hip position affected the sartorius muscle
only when the knee was at 0�.
Significantly less activity was recorded when both the
hip and knee were at the extended position 00–00,
compared to the other hip-flexed positions (90� and
SLR) (Table 3).
Regardless of hip position, the extended knee angle
(0�) was always associated with significantly less EMG
activity in the sartorious muscle compared to the other
flexed knee positions (Table 3). Gracilis activity did not
significantly change with the change in hip position at the
three tested knee angles. The change in knee position
significantly affected its activity only with the hip at 0�, as
the knee 0� position was associated with significantly less
EMG than the other two positions (45, 90) (Table 3).
3.9. EMG Activity at target torque
Reproduction of the maximum knee flexion torque
recorded at the most shortened hamstring position (00–
90) in an intermediate (90–90) and then at the length-
Table 2
EMG of the hamstring muscles Comparison of three knee and three
hip positions
EMG %MMT Mean (SEM)
Knee 0� Knee 45� Knee 90�
Hip at 0�
STEN 77.7 (6.5) 89.4 (6.6) 87.3 (7.6)
SMEM 86.2 (5.9) 94.3 (6.3) 75.3 (7.5)a
BFLH 87.0 (6.8) 91.7 (7.9) 75.7 (8.1)
Hip at SLR
STEN 77.3 (7.0) 84.0 (8.4) 88.8 (7.6)
SMEM 82.8 (6.3) 79.1 (5.8) 90.2 (5.9)
BFLH 76.5 (4.8) 79.2 (6.0) 66.8 (7.4)
Hip at 90�
STEN 67.9 (6.3)b 80.2 (7.8) 86.2 (6.4)
SMEM 82.2 (5.1) 81.6 (5.1) 90.0 (5.5)
BFLH 74.6 (5.9) 75.9 (5.6) 70.8 (6.2)
Abbreviations: BFLH¼ long head of biceps femoris, BFSH¼biceps
femoris short head, GRAC¼ gracilis, MMT¼manual muscle test,
SART¼ sartorius, SEM¼ standard error of the mean, SMEM¼
semimembranosus, STEN¼ semitendinosus, TQ¼ torque.
a Significant multiple comparison, with Bonferoni adjustment with
knee 90�; hip 0� versus hip SLR, 90�.
b Significant multiple comparison, with Bonferoni adjustment with
hip 90�; knee 0� versus 90�.
Table 3
EMG of the other knee flexor muscles Comparison of three hip and
three knee positions
EMG %MMT Mean (SEM)
Knee 0� Knee 45� Knee 90�
Hip at 0�
BFSH 66.8 (6.8)a 104.1 (7.7) 97.8 (8.4)
SART 26.1 (4.9)a ;b 50.3 (7.8) 61.6 (11.4)
GRAC 84.1 (7.6)a 102.8 (8.1) 104.4 (7.6)
Hip at SLR
BFSH 76.9 (6.0)c 98.0 (6.3) 104.7 (9.5)
SART 43.5 (6.7)c 61.2 (8.4) 63.6 (7.3)
GRAC 84.7 (8.4) 90.4 (9.7) 97.8 (8.3)
Hip at 90�
BFSH 67.4 (5.7)d 90.4 (7.8) 107.0 (10.8)
SART 38.7 (6.6)d 61.6 (8.4) 69.8 (8.4)
GRAC 81.4 (8.1) 89.3 (9.7) 102.0 (10.8)
Abbreviations: see Table 2.
a Significant multiple comparison with Bonferroni adjustment with
hip at 0�; knee 0� versus 45�, 90�.
b Significant multiple comparison with Bonferroni adjustment with
knee at 0�; hip 0� versus SLR, 90�.
c Significant multiple comparison with Bonferroni adjustment with
hip at SLR; knee 0� versus 45�, 90�.
d Significant multiple comparison with Bonferroni adjustment hip
90�; knee 0� versus 45�, 90�.
574 O. Mohamed et al. / Clinical Biomechanics 17 (2002) 569–579
ened position (SLR–00), showed significantly decreased
EMG activity with lengthening of all investigated mus-
cles. The difference was statistically significant between
the three positions (P < 0:0001) with only one exception
for the sartorius muscle (Fig. 3). There was no significant
difference in the EMG activity of the sartorius between
the most shortened and the intermediate positions,
however, there was a statistically significant drop of ac-
tivity at the SLR–00 position. The mean activity of the
semitendinosus muscle decreased from 86% MMT at
the most shortened position (00–90) to 60% MMT when
the muscle was at an intermediate length position (90–
90). At the lengthened position (SLR–00) the semiten-
dinosus exhibited an activity level of only 22% MMT (a
76% drop from the value recorded at the shortened po-
sition; 00–90) (Fig. 3). The mean EMG activity of the
semimembranosus muscle at the shortened position (00–
90), the intermediate position (90–90) and the lengthened
position (SLR–00) were 78%, 53% and 28% of MMT,
respectively. The long head of the biceps femoris muscle
exhibited mean EMG values of 76% at SLR–00; 38% at
90–90; and 23% at 00–90. The short head of the biceps
femoris and gracilis muscles followed the same pattern.
4. Discussion
4.1. Effect of inherent muscle length on isometric knee
flexor torque
Comparison of maximum isometric torque values
between the long hamstring group and those with short
hamstrings revealed no differences in any test position.
This finding is in disagreement with data reported by
Gossman et al., who found that subjects with short
hamstrings produced greater peak torque than did those
with long hamstrings [30]. In a later study however, they
collected similar data on men and women and found no
difference due to hamstring length in both groups [31].
Gossman et al. alluded to a gender bias in the first study
as the reason for the disparate results.
Hornsby et al. reported that women with ‘‘tight’’
triceps surae muscles produced significantly greater
maximal plantar flexion torque than comparable women
with loose calf muscles [32]. These differences held true
both with the knee flexed and extended. Hornsby con-
cluded that the triceps surae muscles of the tight group
exerted more force as a result of the combined action of
active and passive elements of muscle; the contribution
of the passive elements (connective tissue, etc.) was es-
pecially significant since two of the test positions (0� and
7� dorsiflexion) required greater than the available
dorsiflexion range of the tight group ()15� to )6� of
neutral) [32]. Thus, they had to force the foot of the tight
group into the test position, which caused force re-
cording before the development of any voluntary effort.
In our study, a baseline measurement was taken with
the subject in the test position. Recording of data started
during voluntary contraction. We found no significant
reduction in the EMG activity of the two-joint hamstring
muscles between the short and long groups. Neither was
there evidence to support an increased contribution of
the one-joint muscle (biceps femoris short head), which
suggests that both groups had a similar strategy to reach
maximum knee flexion torque.
4.2. Effect of change of hip angle on isometric torque
At all tested knee angles there was a significant drop in
isometric knee flexion torque when the hip was at 0� as
compared to the two sitting positions. The significant
decline in knee flexor torque values in supine (hip at 0�)
was consistent with previous reports for isometric knee
flexion of normal subjects [9,11,12,33,34] and with re-
ports on hemiparetic patients [35]. With the knee at a
constant momentarm length, two interrelated factors
explain the increased torque values found in lengthened
muscle. One factor is the non-contractile component of
the muscle, which has two elements; one is parallel with
the muscle fibers (the endomysium, perimysium and
epimysium), while the other is in series with the muscle
(tendons and tendon bundles within the muscle, etc.).
These connective tissue sleeves, when stretched, produce
passive tension that is added to the active tension gen-
erated by muscle contraction. At shorter lengths these
passive elements are slack and their contribution to
tension gradually decreases as the amount of slack in-
creases. The second factor is the effectiveness of the
contractile element of the muscle; i.e. a length–tension
relationship as explained by Huxley’s sliding filament
model [36,37]. In this model there is an optimal position
at which maximum overlap between the thick and thin
filaments occurs which results in production of maximum
tension. Lengths shorter or longer than this optimal
length accommodate fewer actin–myosin bonds and,
therefore, there is a decrease in the produced tension. The
hip 0� position lengthens the sartorious muscle. Its small
Fig. 3. EMG of all tested muscles at target torque (mean, SEM).
O. Mohamed et al. / Clinical Biomechanics 17 (2002) 569–579 575
cross sectional area however, limits its contribution to
the knee flexor torque when compared to the hamstrings.
4.3. Effect of change of knee angle on isometric torque
The maximum knee flexor torque recorded at 90� of
knee flexion was consistently lower than that recorded at
the other two knee angles. With the hip extended and
the knee at 90� (the most shortened position) the mean
torque was only 44% of that recorded at the same hip
position with the knee at 0�.
In some subjects, the decrement in torque in the most
shortened position (00–90�) was more dramatic than
others as evidenced by the large standard deviation of
torque values at this position. The decreased ability of
some subjects to produce torque at the most shortened
hamstring length could not be related to discomfort as a
result of a tight rectus femoris. Subjects for this study
were evaluated for a tight rectus femoris muscle; with
the hip at 0� all of the subjects were able to actively flex
the knee at least 15� past the 90� required for the
shortest test position. Active insufficiency is a possible
explanation for decreased flexion torque in the most
shortened position. The term active insufficiency refers
to a decrease in production of muscle tension when the
muscle is contracting at a very short length due to a
disadvantageous positioning of the actin and myosin
filaments [36,38]. Gordon et al. proposed that at ex-
tremely short muscular lengths, the filaments from the
opposite Z lines overlap resulting in a marked decrease
of tension [38].
The effect of shorter muscle length was more pro-
nounced when the hamstring muscles were shortened at
both joints than at one joint. In supine with the hip joint
at 0�, a decrease in knee flexion angle from 90� to 45� to
full extension, resulted in a gradual increase in maxi-
mum isometric torque values which reached maximum
at full extension. These results are in agreement with
previous reports on the knee flexors [9,33,34,39]. With
the hip at 0�, the two-joint hamstrings are shortened at
the proximal end and additional shortening is caused by
increasing the knee flexion angle which greatly reduced
the efficiency of muscle contraction. This insufficiency
was shown by the 56% reduction in torque values at the
90� flexion angle when compared to full extension.
These results are similar to those of Smidt et al. who
reported a 43% decrease in peak torque at the 90� angle
of knee flexion as compared to the peak torque recorded
at 5� of knee flexion [39].
At different angles of knee flexion, Smidt measured
lever arm lengths of the knee flexors from serial X-rays.
He found that the maximum moment arm of 4.08 cm
occurred at 45� of knee flexion. Moment arm lengths
decreased with both increase and decrease of knee flexion
angle from the 45� position. Although the 45� of knee
flexion provided the best mechanical advantage for the
hamstrings [39], the maximum isometric torque occurred
at full knee extension when the hip was held in extension.
It appears that with the hip extended (0�), the hamstrings
were more affected by the length change than by the
mechanical change of the moment arm at the knee joint.
During sitting, however, the knee angle at which the
highest mean flexor torque occurred was 45�, and this
held true at hip angles of both 90� and SLR. That angle
of maximum torque varied, however, between subjects;
12 of 19 subjects produced their maximum torque at the
45� knee angle in the sitting position with the hip at 90�,
while the remaining seven subjects generated more tor-
que at the 0� knee angle. The variation of the angle at
which peak torque occurred during knee flexion between
subjects has been pointed out by a number of investi-
gators [33,34,40,41]. Walsmsley and Yang commented
that nine of their 10 subjects generated their maximum
torque at a knee angle of 30�, while one registered the
greatest torque at the 0� position. In another study,
Bender and Kaplan noted that 13 of 20 subjects created
greater torque at 0� but the remaining seven subjects
were able to generate higher torques at the 45� angle [40].
Inconsistency of the angle of peak torque also has
been noted in patients with knee injuries. Mendler et al.
wrote that in the sitting position the 45� knee angle was
associated with maximum isometric force in half of her
subjects [42]. The other half reached their peak torque at
10� of knee flexion (the 0� angle was not tested). These
findings suggest an interaction between optimum muscle
length and moment arm. The torque values obtained
here at the maximum muscle length (90–00) were not
different from those obtained at an intermediate length
where the muscles had their maximum moment arm at
the knee (90–45�). This means that the increased ham-
string lever arm compensated for the decrease in muscle
length that occurred as a result of flexing the knee from
0� to 45�. That compensatory mechanism was greatly
impaired when shortening the hamstrings by extending
the hip with the knee flexed, as evidenced by torque
decline, which compromised both factors.
4.4. Effect of hip and knee angle on EMG activity of the
knee flexors
In general, the two-joint hamstring muscles as a
group did not show any significant or consistent change
in EMG activity with alteration in the length of the
muscles by hip or knee joint position changes. One
exception was the 16% decline of EMG of the semi-
membranosus at the most shortened position (00–90)
compared to the two sitting positions (90, SLR). The
second exception is the decreased activity of the semi-
tendinosus in sitting (hip at 90) with the knee at 0
compared to the other two flexed knee positions.
Regarding semimembranosus, similar results have
been reported for the gastrocnemius muscle at the knee-
576 O. Mohamed et al. / Clinical Biomechanics 17 (2002) 569–579
flexed position [13,15]. Using single motor unit record-
ings Kennedy et al. reported decreased recruitment of
the medial gastrocnemius muscle during plantar flexion
when the knee was flexed to 90� [15]. This was attributed
to the diminished force-producing capabilities of the
muscle as a result of its reduced length. When compar-
ing the architectural characteristics of the human semi-
membranosus to the other two hamstrings, Wickiewicz
et al. reported that it had the largest angle of pennation
and the shortest fiber length [43]. It is possible that in the
most shortened position, some motor units were not
active because they are in a position where they can no
longer produce active force. An important factor to
consider is the change of angle of pennationunder
tension; Fukunaga et al. found that the angle of pen-
nation increased during isometric contraction in the
human vastus lateralis muscle [44]. In other words the
capability of motor unit to produce force is affected not
only by their inherent architectural features but also by
the change in these features during contraction.
The decreased activity of the semitendinosus muscle
in the most lengthened position (90–00) as compared to
the other two knee-flexed positions might be explained
by the interaction of the muslce’s length and the ana-
tomical location of its tendon. When the knee is fully
extended, the semitendinosus tendon lays very close to
the axis of the knee joint, providing a poor lever arm for
knee flexion. This suggests its insufficiency as a knee
flexor; in addition, the muscle is elongated at the both
joints. The interaction of these two factors might have
caused the observed decline of EMG activity.
The general consistency of the EMG activity of
the two-joint hamstrings at different muscle lengths
at maximum effort, reported in this paper, agree with
other reports of normal subjects for the biceps brachii
[22,45,46] and the quadriceps muscles [47–50]. Data
from a substantial number of investigations are in dis-
agreement in that they cite increases of EMG activity at
shorter muscle lengths [12,16,20,24,51–53]. The level of
force at which EMG was recorded in the various studies
is one source of conflict. Andriacchi et al. recorded
EMG from the knee flexors and extensors as they re-
sponded to constant unidirectional loads that tended to
flex or extend the knee [16], more EMG activity was
recorded in the hamstring muscles as the knee angle
increased. The fact that Andriacchi used constant loads
did not mean that the muscle exerted constant effort. A
change of joint angles alters both the muscle’s lever arm
and length. Resisting a constant load might demand a
maximum effort from the muscle at one position, where
at other positions it requires only a submaximal effort,
which could explain the decrease in EMG activity. The
studies that investigated maximum isometric effort at
various angles reported no difference between EMG
activity as the muscle length changed [22,46–48,50]
which is in agreement with the results of this study.
Lunnen’s data were in conflict with those of the
present study. They found a consistent trend of de-
creased activity in the hamstring muscles as the hip
angle was increased, but which was only significant
when the hip was flexed to 135� (subject sitting and
leaning forward) [12]. The decreased EMG activity in
Lunnen’s study might have occurred because of the use
of surface electrodes. As the hip angle was increased
from supine (0�) to sitting (135�), the tendon of the
muscle might have been pulled closer to the recording
electrodes which may have caused a decrease in the
amount of EMG that could be sensed. Significant re-
duction of EMG activity has been demonstrated when
the recording electrodes were moved away from the
center of the muscle and closer to its tendon [27]. In-
tramuscular electrodes have been shown to move with
the muscle during contraction and length change [45].
Studies on cineplastic muscles also have reported
decreased EMG activity with increased muscle length
[20,24]. The difficulty of extrapolating results from such
a model to normal muscle is that a cineplastic muscle
has been detached from its original insertion site and its
new attachment confers major shortening on the mus-
cle. It has been well documented that induced muscle
shortening not only decreases maximum muscle force
but also changes both active and passive length–tension
relationships as the sarcomeres adapt to the new length
[54–56].
The literature does not shed any light on the function
of the short head of the biceps femoris with which the
results of this study could be compared. Such reports as
do exist on that one-joint knee flexor muscle, concern its
function during normal walking and similar submaximal
functional activities. The same comment can be made
regarding the sartorius and gracilis muscles.
All non-hamstring knee flexors (short head of biceps
femoris, sartorius and gracilis) decreased their myo-
electric activity at full knee extension. Since the short
head of the biceps femoris does not cross the hip, it
follows that hip position should have no effect on its
function. This was indeed the case, as a change of hip
angle at the three tested knee positions did not alter its
level of EMG activity. The extended knee angle, how-
ever, caused a 34% decrease in activity of the short head
compared to the knee-flexed positions.
The sartorius on the other hand, should have been
influenced by alterations in both hip and knee angles. The
maximum activity of the sartorius muscle occurred dur-
ing sitting with the knee at 90�, the other more extended
hip positions did slightly decrease its level of activity. The
difference only was statistically significant between the
fully extended and fully flexed hip position with the knee
at 0�. This pattern of activity suggested that the flexed hip
position favored the sartorius, which is, after all, a flexor
muscle at both joints. The gracilis muscle was continu-
ously active during maximal efforts and the activity
O. Mohamed et al. / Clinical Biomechanics 17 (2002) 569–579 577
seemed independent of hip angle and only weakly de-
pendent on knee angle. The gracilis is primarily a hip
adductor, and according to some sources [29,57] it con-
tributes to knee and hip flexion. The data in this study
support its contribution to knee flexion. Change in hip
angle however, did not result in any major changes in
gracilis EMG activity. One possible flaw in this discus-
sion may arise from the fact that these two strap-like
muscles (sartorius and gracilis) have such a small cross-
sectional area that a small EMG difference and a small
torque differences might be magnified in importance.
The semitendinosus, sartorius and gracilis muscles
share a common insertion site at the medial side of
upper part of the tibia ‘‘pes anserinus’’. These three
muscles had a common response of decreased EMG
activity when the knee was fully extended. It is easy to
demonstrate on cadaver specimens that during full knee
extension the tendons of these three muscles are placed
very close to the knee joint axis. The lever arm of these
muscles decreases which place them in a less efficient
position to produce knee flexion, which may explain
their decreased activity at the extended knee angle.
4.5. EMG activity at target torque
When the subjects were asked to reproduce, at
lengthened positions, the same amount of torque gen-
erated in the most shortened position (00–90) the EMG
activity significantly decreased. This implied that pro-
duction of a given torque in a lengthened position re-
quires fewer motor units than the same torque in the
shortened position. These results are in agreement with
other investigations [12,16]. Using surface electrodes,
Lunnen et al. found that the EMG activity of the biceps
femoris muscle declined when his subjects reproduced a
submaximal torque at a lengthened position [12].
In this study when the two-joint hamstring muscles
were elongated, not only were their activity reduced but
the short head of the biceps femoris, the sartorius and
the gracilis also had a significant drop-off in their level of
participation. The reduction of activity was dispersed
among the active muscles and no one muscle was re-
sponsible alone to meet the torque demand at any po-
sition, even when the demand was well below maximum.
Major knee flexors contributed more to torque by virtue
of their greater cross-sectional area. The minor flexors
contributed less by virtue of their smaller cross-sectional
area. Contribution of individual muscles to a given
effort cannot be measured where multiple muscles act
synergistically.
4.6. Generalrelationship of maximum flexion torque to
EMG
The ratio between torque output and EMG activity
may be considered as indication of ‘‘efficiency’’. EMG
indicates the number of active muscle fibers and torque
is a measure of muscle output. Thus muscles that pro-
duce high torque with low EMG are more energy effi-
cient than the opposite case. The two-joint hamstrings
contributed the most to knee flexion torque by virtue of
their large cross-sectional area. When all test positions
were ranked by muscle length from short to long and the
EMG values of the two-joint hamstrings were plotted
against the knee flexion torque values, it became ap-
parent that the main contributing factor to the change in
torque was the increase in muscle length as the EMG
values did not change as drastically (Fig. 4).
References
[1] Herzog W, Leonard TR, Renaud JM, Wallace J, Chaki G,
Bornemisza S. Force–length properties and functional demands
of cat gastrocnemius, soleus and plantaris muscles. J Biomech
1992;25:1329–35.
[2] Rack PM, Westbury DR. The effects of length and stimulus rate
on tension in the isometric cat soleus muscle. J Physiol 1969;
204:443–60.
[3] Mai MT, Lieber RL. A model of semitendinosus muscle sarco-
mere length, knee and hip interaction in the frog hindlimb.
J Biomech 1990;23:271–9.
[4] Herzog W, Read LJ. Lines of action and moment arms of the
major force-carrying structures crossing the human knee joint.
J Anat 1993;182:213–30.
[5] Guillard DM, Yakovenko S, Cameron T, Prochazka A. Isometric
muscle length–tension curves do not predict angle–torque curves
of human wrist in continuous active movements. J Biomech
2000;33:1341–8.
[6] Chang YW, Su FC, Wu HW, An KN. Optimum length of muscle
contraction. Clin Biomech 1999;14:537–42.
[7] Nemeth G, Ekholm J, Arbrrelius U. Influence of knee flexion on
isometric hip extensor strength. Scand J Rehab Med 1983;15:97–
101.
[8] Waters R, Perry J, McDaniel J. The relative strength of the
hamstrings during hip extension. J Bone Joint Surg 1974;56-A:
1592–7.
[9] Williams M, Stutzman L. Strength variation through the range
of joint motion. Phys Ther Rev 1959;39:145–52.
Fig. 4. Relationship of the maximum isometric knee flexion torque and
EMG of the two-joint hamstrings. Positions are ranked by from short
to long muscle length (hip position–knee position).
578 O. Mohamed et al. / Clinical Biomechanics 17 (2002) 569–579
[10] Currier DP. Positioning for knee strengthening exercises. Phys
Ther 1977;57:148–52.
[11] Fedler C. Effect of hip position on quadriceps and hamstring
force. Med Sci Sports 1978;10:64 (Abstract).
[12] Lunnen J, Yack J, LeVeau B. Relationship between muscle length,
muscle activity and torque of hamstring muscles. Phys Ther
1981;63:1597–605.
[13] Cresswell AG, Loscher WN, Thorstensson A. Influence of
gastrocnemius muscle length on triceps surae torque development
and electromyographic activity in man. Exp Brain Res 1995;
105:283–90.
[14] Kawakami Y, Ichinose U, Fukunaga T. Architectural and
functional features of juman triceps surae muscles during contrac-
tion. J Appl Physiol 1998;85:398–404.
[15] Kennedy PM, Cresswell AG. The effect of muscle length on
motor-unit recruitment during isometric plantar flexion in hu-
mans. Exp Brain Res 2001;137:58–64.
[16] Andriacchi TP, Andersson GBI, Ortengern R. A study of factors
influencing muscle activity about the knee joint. J Orthop Res
1984;1:266–75.
[17] Bigland B, Lippold OCJ. The relation between force, velocity and
integrated electrical activity in human muscles. J Physiol
1954:214–24.
[18] Bouisset. Quantitative relationship between surface EMG and
intramuscular electromyographic activity in voluntary movement.
Am J Phys Med 1972;51:285–95.
[19] De Vries HA. Method for evaluation of muscle fatigue and
endurance from electromyographic fatigue curves. Am J Phys
Med 1968;17:125–35.
[20] Inman VT, Ralston HJ, Saunders JB, Feinstein B, Wright EW.
Relation of human electromyogram to muscular tension. Electro-
encephalogr Clin Neurophysiol 1952;4:187–94.
[21] Knowlton GC, Hines TF, Keever KW, Bennett RL. Relation
between electromyographic voltage and load. J Appl Physiol
1956;9:476–9.
[22] Vredenbregt J, Rau G. Surface electromyography in relation to
force, muscle length and endurance. New Develop Electromyogr
Clin Neurophysiol 1973;1:607–22.
[23] Zuniga EN, Simons DG. Nonlinear relationship between aver-
aged electromyogram potential and muscle tension in normal
subjects. Arch Phys Med Rehab 1969:613–20.
[24] Heckathorne CW, Childress DS. Relationships of the surface
electromyogram to the force, length, velocity, and contraction rate
of the cineplastic human biceps. Am J Phys Med 1981;60:1–19.
[25] Eloranta V, Komi PV. Function of the quadriceps femoris muscle
under the full range of forces and differing contraction velocities
of concentric work. Electromyogr Clin Neurophysiol 1981; 21:
419–31.
[26] Perry J, Easterday CS, Antonelli DJ. Surface versus intramuscular
electrodes for electromyography of superficial and deep muscles.
Phys Ther 1981;61:7–15.
[27] Zuniga EN, Truong XT, Simons DG. Effect of skin electrode
position on averaed electromyographic potentials. Arch Phys Med
Rehab 1970;51:264–75.
[28] Basmajian JV, Stecko GA. A new bipolar electrode for electro-
myography. J Appl Physiol 1962;129:371–80.
[29] Kendall F, McCreary E. Muscle testing and function. Baltimore,
MD: Williams & Wilkins; 1993.
[30] Gossman DA, Rose SJ. Relationship between muscle length and
torque production in hamstring. Phys Ther 1983;63:768.
[31] Gossman RA, DeLitto A, Katholi CR, Rose SJ. Relationship of
torque and inherent hamstring muscle length. Phys Ther 1984;
64:716.
[32] Hornsby TM, Nicholson GG, Gossman MR, Culpepper M. Effect
of inherent muscle length on isometric plantar flexion torque in
healthy women. Phys Ther 1987;67:1191–7.
[33] Houtz SJ, LebowMJ, Beyer FR. Effect of posture on strength of the
knee flexor and extensor muscles. J Appl Physiol 1957;11:475–80.
[34] Walmsley RP, Yang JF. Measurement of maximum isometric
knee flexor moment. Physiother Canada 1980;32:83–6.
[35] Bohannon RW. Decreased isometric knee flexion torque with hip
extension in hemiparetic patients. Phys Ther 1986;66:521–3.
[36] Huxley AF. The activation of striated muscle and its mechanical
response. Proc Royal Soc Lond Biol 1971;178:1–27.
[37] Huxley AF. Muscle structure and theories of contraction. Prog
Biophys Chem 1957;7:255–318.
[38] Gordon AM, Huxley AF, Julian FJ. The length–tension diagram
of single vertebrate striated muscle fiber. J Physiol (London)
1964;171:28–30.
[39] Smidt GL. Biomechanical analysis of knee flexion and extension.
J Biomech 1973;6:79–92.
[40] Bender JA, Kaplan HM. The multiple angle testing method of
the evaluation of muscle strength. J Bone Joint Surg 1963;45-A:
135–40.
[41] Murray MP, Baldwin JM, Gardner GM, Sepic SB, Downs WJ.
Maximum isometric knee flexor and extensor muscle contractions:
normal patterns of torque versus time. Phys Ther 1977;57:637–43.
[42] Mendler HM. Knee extensor and flexor force following injury.
Phys Ther 1967;47:35–45.
[43] Wickiewicz TL, Roy RR, Powell PL, Perrine JJ, Edgerton VR.
Muscle architecture and force–velocity relationships in humans.
J Appl Physiol 1984;57:435–43.
[44] Fukunaga T, Ichinose Y, Ito M, Kawakami Y, Fukashiro S.
Determination of fascicle length and pennation in a contracting
human muscle in vivo. J Appl Physiol 1997;82:354–8.
[45] Komi PV, Buskirk ER. Reproducibility of electromyographic
measurements with inserted wire electrodes and surface electrodes.
Electromyography 1970;10:357–67.
[46] Rosentswieg J, Hinson MM. Comparison of isometric, isotonic
and isokinetic exercises by electromyography.Arch Phys Med
Rehab 1972;53:249–52.
[47] Hallen LG, Lindahl O. Muscle function in knee extension. Acta
Orthop Scand 1967;38:434–44.
[48] Lieb FJ, Perry J. Quadriceps function: an electromyographic
study under isometric conditions. J Bone Joint Surg 1971;53-A:
749–58.
[49] Pocock GS. Electromyographic study of the quadriceps during
resistive exercises. Phys Ther 1963;43:427–34.
[50] Salzman A, Torburn L, Perry J. Contribution of rectus femoris
and vasti to knee extension. An electromyographic study. Clin
Orthop 1993;290:236–43.
[51] Haffajee D, Moritz U, Svantesson G. Isometric knee extension
strength as a function of joint angle, muscle length and motor unit
activity. Acta Orthop Scand 1972;43:138–47.
[52] Peres G, Maton B. Validity of the muscle equivalent concept in
human muscular isometric contractions at different elbow angles.
Electromyogr Clin Neurophysiol 1987;27:135–43.
[53] Soderberg GL, Minor SD, Arnold K, et al. Electromyographic
analysis of knee exercises in healthy subjects and in patients with
knee pathologies. Phys Ther 1987;67:1691–6.
[54] Williams PE, Goldspink G. Changes in sarcomere length and
physiological properties in immobilized muscle. J Anat 1978;
127:459–68.
[55] Tardieu C, Tabary JC, Tardieu G, Tabary C. Adaptation of
sarcomere numbers to the length imposed on the muscle. Adv
Physiol Sci 1980;24:99–113.
[56] Tabary JC, Tardieu C, Tardieu G, Tabary C. Functional
adaptation of sascomere number of normal cat muscle. J Physiol
1976;72:277–91.
[57] Clarkson HM. Musculoskeletal assessment joint range of motion
and manual muscle strength. 2nd ed Philadelphia: Lippincott
Williams & Wilkins; 2000.
O. Mohamed et al. / Clinical Biomechanics 17 (2002) 569–579 579
	Relationship between wire EMG activity, muscle length, and torque of the hamstrings
	Introduction
	Methods
	Subjects
	Instrumentation
	Procedure
	Subject preparation and electrode insertions
	Manual muscle testing
	Dynamometer testing
	Test sequence
	Target torque
	Data analysis
	EMG data processing and quantification
	Statistical analysis
	Results
	Subject description
	Repeated trials
	Effect of inherent hamstring length on EMG and torque
	Effect of change of hip positions on maximum isometric knee flexion torque
	Effect of change of knee positions on maximum isometric knee flexion torque
	Effect of the most lengthened position on knee flexion torque
	Effect of change of hip and knee position on EMG activity of the two-joint hamstrings
	Effect of change of hip and knee positions on EMG activity of the other three knee flexors
	EMG Activity at target torque
	Discussion
	Effect of inherent muscle length on isometric knee flexor torque
	Effect of change of hip angle on isometric torque
	Effect of change of knee angle on isometric torque
	Effect of hip and knee angle on EMG activity of the knee flexors
	EMG activity at target torque
	General relationship of maximum flexion torque to EMG
	References