APPLIED SCIENCES
Biodynamics
Quadriceps Activation in Closed and in Open
Kinetic Chain Exercise
ANN-KATRIN STENSDOTTER1,3, PAUL W. HODGES2, REBECCA MELLOR2,
GUNNEVI SUNDELIN1, and CHARLOTTE HÄGER-ROSS1
1
Department of Community Medicine and Rehabilitation, Physiotherapy, Umeå University, Umeå, SWEDEN; 2Department
of Physiotherapy, The University of Queensland, Brisbane, AUSTRALIA; and 3Department of Physiotherapy, School of
Health Education and Social Work, Sør-Trøndelag University College, Trondheim, NORWAY
ABSTRACT
STENSDOTTER, A.-K., P. W. HODGES, R. MELLOR, G. SUNDELIN, and C. HÄGER-ROSS. Quadriceps Activation in Closed and
in Open Kinetic Chain Exercise. Med. Sci. Sports Exerc., Vol. 35, No. 12, pp. 2043–2047, 2003. Purpose: For treatment of various
knee disorders, muscles are trained in open or closed kinetic chain tasks. Coordination between the heads of the quadriceps muscle is
important for stability and optimal joint loading for both the tibiofemoral and the patellofemoral joint. The aim of this study was to
examine whether the quadriceps femoris muscles are activated differently in open versus closed kinetic chain tasks. Methods: Ten
healthy men and women (mean age 28.5 ⫾ 0.7) extended the knees isometrically in open and closed kinetic chain tasks in a reaction
time paradigm using moderate force. Surface electromyography (EMG) recordings were made from four different parts of the
quadriceps muscle. The onset and amplitude of EMG and force data were measured. Results: In closed chain knee extension, the onset
of EMG activity of the four different muscle portions of the quadriceps was more simultaneous than in the open chain. In open chain,
rectus femoris (RF) had the earliest EMG onset while vastus medialis obliquus was activated last (7 ⫾ 13 ms after RF EMG onset)
and with smaller amplitude (40 ⫾ 30% of maximal voluntary contraction (MVC)) than in closed chain (46 ⫾ 43% MVC). Conclusions:
Exercise in closed kinetic chain promotes more balanced initial quadriceps activation than does exercise in open kinetic chain. This
may be of importance in designing training programs aimed toward control of the patellofemoral joint. Key Words: ELECTROMYOGRAPHY, WEIGHT BEARING, COORDINATION, PHYSICAL THERAPY, PATELLOFEMORAL
T
here is a considerable debate regarding the relative
efficacy of open (OKC) and closed kinetic chain
(CKC) exercise for increased strength and control of
the knee muscles. In general, open kinetic chain (OKC)
exercises are single joint movements that are performed in
nonweight bearing with a free distal extremity. In contrast,
CKC exercises are multi-joint movements performed in
weight bearing or simulated weight bearing with a fixed
distal extremity (22). Although clinical trials suggest that
the functional outcome from programs that incorporate
these exercise strategies are similar (11), there is a tendency
toward better results in terms of strength (2) and functional
(28) performance enhancement from CKC exercise. The
basis for selection of each exercise regime is based on the
hypothesis that there are physiological differences between
these strategies and that one strategy may lead to greater
improvements in specific physiological variables.
Several rationales for CKC exercises have been presented. First, CKC has been argued to be more “functional”
as it simulates the role of lower limb muscles in daily
activities (1,6). For instance, rectus femoris (RF) shortens
across the knee and lengthens across the hip in walking and
climbing stairs due to simultaneous knee and hip extension.
Second, it has been argued that proprioceptive feedback
differs between CKC and OKC tasks, perhaps due to compression from body mass in CKC (14) and pressure under
the foot (13). Third, CKC exercise has been suggested to
produce less shear force between the tibiofemoral joint
surfaces as co-contraction of the hamstrings will counteract
the anterior tibial shear force generated by the quadriceps
(16). Thus, from a biomechanical perspective, it is likely
that CKC knee exercise places less strain on the anterior
cruciate ligament (15,16), although the placement of the
body center of mass above the axis of the knee joint determines how the quadriceps and hamstrings co-contract
(18,27). Fourth, the interrelationship between patellofemo-
Address for correspondence: Ann-Katrin Stensdotter, Department of Community Medicine and Rehabilitation, Physiotherapy, Umeå University,
S-901-87 Umeå, Sweden; E-mail: anki.stensdotter@physiother.umu.se.
Submitted for publication December 2002.
Accepted for publication July 2003.
0195-9131/03/3512-2043
MEDICINE & SCIENCE IN SPORTS & EXERCISE®
Copyright © 2003 by the American College of Sports Medicine
DOI: 10.1249/01.MSS.0000099107.03704.AE
2043
ral joint forces and contact area differs between the two
tasks. In closed chain tasks, such as squatting, compressive
forces are augmented with increased knee flexion as greater
torque develops as a product of the lengthening lever arm
between the knee joint and the body’s center of mass when
it moves further posterior to the joint axis. However, this
compressive force is distributed by greater contact between
the patella and femur. In contrast, in OKC exercise the joint
stress increases from 90° flexion as the knee extends (5,8) as
a result of the greater torque produced by the lengthening
lever arm when the center of mass of the leg and eventual
load around the ankle moves. Finally, it has been argued that
the coordination of the knee muscles may vary between the
tasks. For instance, electromyographic activity of vastus
medialis has been suggested to be greater in closed chain
tasks than in open chain tasks (7). One study has investigated onset times for the different portions of the quadriceps
in CKC and OKC under different loading conditions and
joint angles but failed to find significant difference (12).
Despite the argument that coordination of the lower limb
muscles may be influenced by closed or open chain tasks,
for the reasons presented above, there is limited direct
evidence of differences in recruitment. The present study
was designed to investigate this question by comparison of
recruitment of muscles in a simple reaction-time knee extension task performed in both OKC and CKC. This task
was selected as it allowed us to control relevant aspects of
the activity. Specifically, we were interested in whether the
onset and initial amplitude of muscle activity of different
portions of the quadriceps would differ between these tasks.
METHOD
Subjects. Ten healthy subjects, three males and seven
females, (mean age 28.5 ⫾ 0.7, mean height 171 cm ⫾8.5,
mean weight 64 kg ⫾15.6) participated in the study. Subjects were excluded if they had a current or previous record
of knee pain, trauma, surgery, or other joint disease or were
involved in competitive sports. The tests were performed in
agreement with the Declaration of Helsinki and informed
written consent was obtained from the subjects. The study
was approved by the institutional research ethic committee.
Electromyography. EMG activity of vastus medialis
obliquus (VMO), vastus lateralis (VL), vastus medialis longus (VML), and RF was recorded with surface electrodes (5
mm disks, Grass, U.S.) placed approximately in parallel
with the muscle fibers over the muscle bellies, based on a
modification of standard proposed by Zipp (30). The distances and angles were measured for optimal electrode
placement (Fig. 1). The skin was carefully prepared by
rubbing with abrasive gel and alcohol. EMG data were
amplified 2000 times, filtered between 20 and 1000 Hz
(Neurolog, UK) and sampled at 2 kHz using Power1401 and
Spike2 software (CED, UK).
Force recordings. Knee extension force was measured
with a strain gauge (Validyne, U.S.). Force data were amplified and sampled at 1 kHz with the EMG data.
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Official Journal of the American College of Sports Medicine
FIGURE 1—Placement of surface EMG electrodes. The angle between
the electrode placement and the long axis of the femur (thin line) and
the approximate distance from the supra patellar border: VMO 4 cm,
VML 15 cm, VL 8 cm, and RF 15 cm. Polar distance for electrodes was
22 mm. The ground electrode was placed over the tibia inferior to
patella.
Procedure. Subjects sat on a firm plinth with the hip
flexed to 90° and knee flexion 30° from full extension.
Ankle joint position was kept at 90°. The pelvis was firmly
strapped to the plinth. This position was used as it represented a mid range position and allowed the joint position to
be kept constant between tasks. Knee extension efforts were
performed as a reaction-time task in two different conditions. For OKC, the strain gauge was connected from the
plinth to a strap around the ankle, approximately 10 cm
proximal to the malleoli and isometric knee extension efforts were made against the resistance of the cable. In the
CKC task, the strain gauge was incorporated into an inelastic belt that passed around the trunk support of the plinth and
under the sole of the foot (Fig. 2). Isometric extension
efforts were performed by pushing the foot into the belt.
Subjects were instructed to respond as quickly as possible
(by either extending the knee or pushing into the belt depending on condition) in response to an auditory stimulus
and to use a moderate effort. Twenty repetitions in sets of 10
were performed for each condition and subjects were allowed 0 –30 s of rest between each repetition and 2–3 min
FIGURE 2—Experimental setup. Subjects where seated with 90° hip
flexion and 30° knee flexion (from full extension). A strap was placed
over the hip. Arrows indicate direction of force applied by subject
against the resistance of the strain gauge.
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of rest between sets of 10. Subjects were encouraged to
relax their quadriceps between each repetition. Experimenters observed EMG activity with high gain to ensure activity
was minimal during the rest period. The order of task
presentation was randomized between sets of OKC and
CKC. Subjects performed a single maximal voluntary contraction for 5 s against manual resistance and with loud
verbal encouragement for each task after completion of 20
repetitions.
Data analysis. The onset of EMG activity of each muscle and the onset of force measure were identified visually
for each trial. To remove observer bias, data were presented
for each individual trial in random order with no reference
to muscles or order of repetition. The time of onset of force
was identified in a similar manner. EMG amplitude was
calculated for the initial 100 ms of the response and normalized to the amplitude recorded during the maximum
voluntary contraction. Data were presented as the difference
between EMG onsets of muscle pairs (VMO:VML, VMO:
VL, VMO:RF, VML:VL, VML:RF, VL:RF), difference between onset of EMG and onset of force, and difference in
peak amplitude.
Statistical analysis. Differences in EMG onset latency
for muscle pairs and EMG amplitude between the open and
closed chain tasks were evaluated with a repeated-measures
ANOVA; two factors, condition (OKC and CKC) and muscle portion (N ⫽ 4). Differences between onset of EMG and
onset of force between the open and closed chain tasks were
evaluated with a repeated-measures ANOVA two factors;
condition (OKC and CKC) and latency (force-muscle portion (N ⫽ 4)). Values where corrected for sphericity (Greenhouse-Geisser). Paired t-tests were used to evaluate specific
differences. The level of probability chosen as statistically
significant was p ⬍ 0.05.
RESULTS
When subjects performed rapid knee extension efforts in
response to an auditory stimulus, there were differences in the
pattern of recruitment of the portions of the quadriceps muscles
between OKC and CKC (condition ⫻ muscle interaction: P ⬍
0.001). The onset of activity was more simultaneous in the
CKC task than in OKC (Fig. 3). Figure 4 illustrates the EMG
onset data expressed relative to the initiation of the force for
each muscle and shows that there was no difference between
muscles for CKC, that is, the onsets of EMG of all muscles
were simultaneous. In contrast, for OKC, there was a difference in latency between EMG onset and onset of force increase
between muscles. The latency was greatest for RF (mean 62 ms
⫾ 20) and shortest for VMO (mean 55 ms ⫾ 22). The data
indicate that for the OKC task the EMG onsets of all muscles
occurred before that of VMO. The relative latency between all
pairs of muscles is significantly different between tasks for all
pairs (RF:VML P ⬍ 0.05, for all the rest P ⬍ 0.001) except
VMO:VL.
Differences in EMG amplitude between tasks were also
identified. The mean amplitude for the normalized EMG
was significantly larger for RF (P ⬍ 0.001) in the OKC task
QUADRICEPS ACTIVITY IN KINETIC CHAIN TASKS
FIGURE 3—Representation of EMG raw data for muscle activity in
CKC and OKC from single subject. Note the more simultaneous onset
of activation in CKC than in OKC and that RF was activated first and
VMO last in OKC. Data are presented with high gain to optimize the
difference of EMG onsets; thus, some data are clipped.
compared with CKC, whereas the mean amplitude for VMO
was significantly larger (P ⬍ 0.05) in the CKC task than in
OKC. Amplitude of activity was not significantly different
between the tasks for other muscles (Fig. 5). The differences
between the EMG amplitudes within a task showed that in
CKC, the VMO amplitude was greater than that of VML
and RF, but less than VL. In the OKC task the VMO EMG
amplitude was less than for VL. RF was less active than VL
in OKC and was in CKC less active than all other muscle
portions. In OKC, VML was least active.
DISCUSSION
The present study shows that there is a difference in time of
onset and amplitude of EMG for the different knee extensors in
open and closed kinetic chain tasks. Most notably, the nearsimultaneous onset of activity of the quadriceps muscles during
closed chain knee extension was not apparent when the task
was performed in open chain. In general, there was agreement between the temporal and spatial EMG parameters.
In CKC where VMO is activated early (Fig. 3), its am-
FIGURE 4 —Group mean values (SEM) for onsets of activity relative
to onset of force increase. There was no difference in EMG onset time
between muscle portions relative to force in CKC. In OKC, the latency
between onset of activity and onset of force was shorter for VMO than
for all other muscle portions, whereas the latency between onset of
activity in RF relative to onset of force was longer than all others.
Medicine & Science in Sports & Exercise姞
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FIGURE 5—Group mean (SEM) for EMG amplitude normalized to
MVC. VMO had greater amplitude in CKC than OKC. RF showed
greater amplitude in OKC than CKC. * P < 0.05.
plitude was greater compared with OKC, in which its
onset of activity was later. Rectus femoris had greater
EMG amplitude in OKC when it was the first muscle
active compared with a smaller amplitude in CKC where
its EMG onset was later. This may suggest that the initial
relative contribution of muscles with early onset of activity
is larger than for the muscles with later onset of activity.
The differences in EMG onset and amplitude for RF in
the two conditions may be explained by its nature as a
two-joint muscle. In OKC where the force is directed upward, the contribution of RF is increased, presumably as a
result of its dual function as a knee extensor and hip flexor.
In CKC, where the force is directed downward, this is more
akin to hip and knee extension. Indeed the subjects had to be
firmly strapped down during testing conditions, to prevent
extension at the hip in CKC. On the contrary, in OKC there
was less tendency to extend at the hip.
The result from our study shows that CKC provides more
simultaneous activity in the different portions of the quadriceps
muscle than OKC, with earlier onset and greater amplitude of
EMG activity in VMO. Because muscle function has significant impact on the biomechanics of the knee joint, CKC tasks
may provide more optimal loading conditions for the patellofemoral joint due to more central tracking of the patella (20).
A mediolateral muscular imbalance in force production
(3,7,24) and timing (4,26,29) has been suggested by several
authors as important factors contributing to malalignment of
the patella. Malalignment affects the pressure distribution between patella and femur. In vitro and modeling studies of
forces show increased lateral pressure as tension from the
VMO is decreased (20). The main cause for patellofemoral
pain syndrome (PFPS) is believed to be lateral tracking and/or
tilt of patella in the femoral groove. Weakness of the knee
extensors and atrophy of vastus medialis muscle are common
findings (25). Patients with this syndrome also show a decrease
in VMO activity relative to VL. In knee extension the ratio
between VMO and VL activity increases closer to full extension, whereas the ratio in nonsymptomatic subjects remains
steady (3,24). For onset of muscle activity, PFPS patients show
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Official Journal of the American College of Sports Medicine
a delayed onset of activity in VMO relative to VL, when
ascending and descending stairs, by 16 and 19 ms, respectively
(4). In nonsymptomatic subjects, there is no difference in onset
time for VMO and VL in these same tasks. These findings are
supported by other similar studies, however, with smaller time
differences (7,26,29). Degree of decreased reflex response time
in VMO and duration of symptoms have been reported to be
the only factors that significantly predict the outcome of training intervention for this patient group. Shorter reflex time of
VMO predicts a better functional outcome (28).
Clinical implications. Seemingly small time differences (5–10 ms) appear significant for the central nervous
system to coordinate muscle activity for a certain task. Even
with the same joint configuration, the net mechanical effect
of different loading conditions requires the central nervous
system to adjust the strategy accordingly (9). For instance,
recent biomechanical studies have indicated that a delay in
VMO onset of 5 ms has significant consequences for patellofemoral joint mechanics in terms of increased peak and
average lateral contact force (17). In addition increased
relative contribution of VMO force produces a reduction in
lateral patellofemoral joint loading (17). The findings from
the present study, particularly regarding onset and activity
of the VMO may have clinical implications for how to
design training intervention programs for patients suffering
from PFPS. For knee rehabilitation in general, CKC exercises have been promoted in favor over OKC, because CKC
exercises are considered more functional, safe, and effective
(19,21). Exercises designed to remedy muscular imbalances
as described for PFPS should be particularly aimed at VMO.
Our study shows in healthy subjects that CKC promotes
more simultaneous quadriceps activity and earlier onset and
greater amplitude in EMG activity for VMO than does
OKC. To what extent this also applies to PFPS needs to be
investigated. We compared OKC and CKC tasks under
isometric conditions in identical positions, seated with the
hip in 90° and 30° knee flexion from full extension, with
moderate force exertion. However, activation patterns may
be different for OKC and CKC as other biomechanical
conditions apply for dynamic conditions with different joint
angles and loading conditions. Evaluation of CKC training
intervention has showed that for patients with patellofemoral pain, more selective VMO activation can be obtained in
closed kinetic chain exercises at 60° knee flexion (23).
Hodges and Richardson (10) reported greater VMO activity
in CKC, which could be further augmented by additional hip
adduction. Even though CKC in PFPS may elicit earlier and
greater VMO activity than OKC exercises, this may not
guarantee a normalization of VMO activity in other activities. It also remains to be investigated whether and to what
extent an eventual normalization of VMO activity in an
exercise condition has a carry over effect to daily activity
with improvement of physical function and reduced pain.
This project has been funded by the National Health and Medical
Research Council of Australia, Sør-Trøndelag University College,
Trondheim, Norway, Trygg Hansa’s Research Foundation, Sweden,
Faculty of Medicine and Odontology, Umeå University, Sweden, and
The Swedish Research Council (no. 220-3-02).
http://www.acsm-msse.org
REFERENCES
1. AUGUSTSSON, J., and R. THOMEE. Ability of closed and open kinetic
chain tests of muscular strength to assess functional performance.
Scand. J. Med. Sci. Sports 10:164 –168, 2000.
2. AUGUSTSSON, J., A. ESKO, R. THOMEÉ, et al. Weight training of the
thigh muscles using closed versus open kinetic chain exercises: a
comparison of performance enhancement. J. Orthop. Sports Phys.
Ther. 27:3– 8, 1998.
3. BOUCHER, J. P., M. A. KING, R. LEFEBVRE, and A. REPIN. Quadriceps femoris muscle activity in patello-femoral pain syndrome.
Am. J. Sports Med. 20:527–532, 1992.
4. COWAN, S. M., K. L. BENNELL, P. W. HODGES, K. M. CROSSLEY, and
J. MCCONNELL. Delayed onset of electromyographic activity of
vastus medialis obliquus relative to vastus lateralis in stair ascent,
and stair descent. Am. J. Sports Med. 29:167–174, 2001.
5. ESCAMILLA, R. F., G. S. FLEISIG, N. ZHENG, S. W. BARRENTINE, K. E.
WILK, and J. R. ANDREWS. Biomechanics of the knee during closed
kinetic chain and open kinetic chain exercises. Med. Sci. Sports
Exerc. 30:556 –569, 1998.
6. FITZGERALD, G. K. Open versus closed kinetic chain exercise:
issues in rehabilitation after anterior cruciate ligament reconstructive surgery. Phys. Ther. 77:1747–1754, 1997.
7. GRABINER, M. D., T. J. KOH, and J. T. ANRDISH. Decreased excitation of vastus medialis oblique and vastus lateralis in patellofemoral pain. Eur. J. Exp. Musculoskel. Res. 1:33–39, 1992.
8. GRELSHAMMER, R. P., W. W., COLEMAN, and V. C. MOW. Anatomy
and mechanics of the patellofemoral joint. Sports Med. Arthroscopy Rev. 2:178 –188, 1994.
9. HäGER-ROSS, C., K. J. COLE, and R. S. JOHANSSON. Grip-force
responses to unanticipated object loading: load direction reveals
body-and gravity-referenced intrinsic task variables. Exp. Brain.
Res. 110:142–150, 1996.
10. HODGES, P. W., and C. A. RICHARDSON. The influence of isometric
hip adduction on quadriceps femoris activity. Scand. J. Rehabil.
Med. 25:57– 62, 1993.
11. HOOPER, D. M., M. C. MORRISSEY, W. DRECHSLER, D. MORRISSEY,
and J. KING. Open and closed kinetic chain exercises in the early
period after anterior cruciate ligament reconstruction: improvements in level walking, stair ascent, and stair descent. Am. J.
Sports Med. 29:167–174, 2001.
12. KARST, G. M., and G. M. WILLETT. Onset timing of electromyographic activity in the vastus medialis oblique and vastus lateralis
muscles in subjects with and without patellofemoral pain syndrome. Phys. Ther. 75:813– 823, 1995.
13. KAVOUNOUDIAS, A., R. ROLL, and J. P. ROLL. The plantar sole is a
dynamometric map for human balance control. Neuroreport
9:3247–3252, 1998.
14. KIEFER, G., L. FORWELL, J. KRAMER, and T. L. BIRMINGHAM. Comparison of sitting and standing protocols for testing knee proprioception. Physiother. Can. 30 –34. 1998.
15. KVIST, J., and J. GILLQUIST. Sagittal plane knee translation and
electromyographic activity during closed and open kinetic chain
exercises in anterior cruciate ligament-deficient patients and control subjects. Am. J. Sports Med. 29:72– 82, 2001.
16. LUTZ, G. F., R. A. PALMITIER, K. N. AN, and E. Y. S. CHAO.
Comparison of tibiofemoral joint forces during open-kinetic-chain
QUADRICEPS ACTIVITY IN KINETIC CHAIN TASKS
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
and closed-kinetic-chain exercises. J. Bone Joint Surg. Ser. A
75:732–739, 1993.
NEPTUNE, R. R., I. C. WRIGHT, and A. J. VAN DEN BOGERT. The
influence of orthotic devices and vastus medialis strength and
timing on patellofemoral loads during running. Clin. Biomech.
15:611– 618, 2000.
NINOS, J. C., J. J. IRRGANG, R. BURDETT, and J. R. WEISS. Electromyographic analysis of the squat performed in self selected lower
extremity neutral rotation and 30° of lower extremity turn-out
from the self selected neutral position. J. Orthop. Sports Phys.
Ther. 25:307–315, 1997.
RIVERA, J. E. Open versus closed kinetic chain rehabilitation of the
lower extremity: a functional and biomechanical analysis.
J. Sports Rehabil. 3:154 –167, 1994.
SAKAI, N., Z. P. LUO, J. A. RAND, and K. N. AN. The influence of
weakness in the vastus medialis oblique muscle on the patellofemoral joint: an in vitro biomechanical study. Clin. Biomech.
(Bristol, Avon) 15:335–339, 2000.
SNYDER-MACKLER, L. Scientific rationale and physiological basis
for the use of closed kinetic chain exercise in the lower extremity.
J. Sports Rehabil. 5:2–12, 1996.
STEINDLER A. Kinesiology of the Human Body under Normal and
Pathological Conditions. Springfield, IL: Charles C Thomas,
1977, p. 63.
TANG, S. F., C. K. CHEN, R. HSU, S. W. CHOU, W. H. HONG, and
H. L. LEW. Vastus medialis obliquus and vastus lateralis activity in
open and closed kinetic chain exercises in patients with patellofemoral pain syndrome: an electromyographic study. Arch.
Phys. Med. Rehabil. 82:1441–1445, 2001.
TASKIRAN, E., Z. DINEDURGA, A. YAGIZ, B. ULUDAG, C. ERTEKIN,
and V. LöK. Effect of the vastus medialis obliquus on the patellofemoral joint. Knee Surg. Sports Traumatol. Arthrosc. 6:173–
180, 1998.
THOMEE, R. P., J. AUGUSTSSON, and J. KARLSSON. Patellofemoral
pain syndrome: a review of current issues. Sports Med. 28:245–
262, 1999.
VOIGHT, M., and D. WEIDER. Comparative reflex response times of
the vastus medialis and vastus lateralis in normal subjects and
subjects with extensor mechanism dysfunction. Am. J. Sports Med.
10:131–137, 1991.
WILK, K. E., R. F. ESCAMILLA, G. S. FLEISIG, S. W. BARRENTINE,
J. R. ANDREWS, and M. L. BOYD. A comparison of tibiofemoral
joint forces and electromyographic activity during open and closed
kinetic chain exercises. Am. J. Sports Med. 24:518 –527, 1996.
WITVROUW, E., R. LYSENS, J. BELLEMANS, K. PEERS, and G. VANDERSTRAETEN. Open versus closed kinetic chain exercises for patellofemoral pain: a prospective, randomized study. Am. J. Sports
Med. 28:687– 694, 2000.
WITVROUW, E., C. SNEYERS, R. LYSENS, J. VICTOR, and M. BELLEMANS. Reflex response times of vastus medialis oblique and vastus
lateralis in normal subjects with patellofemoral syndrome. J. Orthop. Sports Phys. Ther. 24:160 –165, 1996.
ZIPP, P. Recommendations for the standardization of lead position
in surface electromyography. Eur. J. Appl. Physiol. 50:41–54,
1982.
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