Electromyographic Analysis of Knee Exercises in Healthy Subjects and in Patients with Knee Pathologies

GARY L. SODERBERG, SCOTT DUESTERHAUS MINOR, KEVIN ARNOLD, THOMAS HENRY, JOYCE KIRCHNER CHATTERSON, DEBRA RIDENOUR POPPE, and CHERYL WALL

Little information exists about the intensity of contraction required from knee and hip musculature during common therapeutic exercises used for patient popula­tions. This study, therefore, was designed to compare electromyographic data obtained from the vastus medialis, rectus femoris, gluteus medius, and biceps femoris muscles during maximally resisted straight-leg-raising (SLR) exercises with EMG data obtained from the same muscles during quadriceps femoris muscle setting (QS) exercises in healthy subjects and in patients with knee pathologies. Of the 30 participants in the study, 16 had a history of knee injury or surgery. All participants performed randomly ordered trials of the SLR and QS exercises while the EMG data were recorded from surface electrodes and normalized to values derived from maximal effort isometric contraction trials. An analysis of variance demonstrated significantly greater activity (p < .05) of the vastus medialis, biceps femoris, and gluteus medius muscles during QS exer­cises than during SLR exercises. The rectus femoris muscle was significantly more active (p < .05) during SLR exercises than during QS exercises. The study demonstrated remarkably different degrees of muscle activation between the SLR and QS exercises, indicating that the exercise selected will affect the therapeutic intention. Key Words: Biomechanics, Eiectromyography, Physical therapy.

In many treatment situations for knee injuries and pathologies, physical ther­apists use either straight-leg-raising (SLR) or quadriceps femoris muscle set­

ting (QS) exercises, or both, for the pur­pose of improving a patient's ability to generate quadriceps femoris muscle ten­sion. Little, however, is known about the effects of these two exercises on the intensity of muscular contraction re­quired. Early work by Pocock led to the remark that relatively large amounts of electromyographic data were recorded from the quadriceps femoris muscles during the QS exercises, presumably be­cause the muscle then was at its shortest length.1 In a 1966 study, Allington et al evaluated EMG recordings from 25 sub­jects who had completed a series of ex­ercises.2 Maximal, manually resisted iso­metric contractions of the quadriceps femoris muscle were performed while the thigh was supported posteriorly. The position of the body was not specified. This type of contraction produced the highest EMG voltages in 67 of 75 tests. Voltages were greatest during the QS exercises in only 8 of 75 trials.

Gough and Ladley followed with a study of EMG recordings from the rec­tus femoris, vastus lateralis, and vastus medialis muscles during SLR and iso­

metric contractions.3 Their data re­vealed that quadriceps femoris muscle activity was greater during isometric contraction in 25 subjects, as opposed to only 6 subjects for the SLR contrac­tion. No details were provided as to how electrical activity from specific muscles related to these exercises.

More recently, Skurja et al reported on 20 subjects who completed both the SLR and QS exercises.4 Measurements included EMG data from the rectus fe­moris and the vastus medialis oblique muscles. Results showed that greater myoelectric activity was produced in the vasti muscles during isolated isometric knee extension than during the SLR exercises. The opposite was true for the EMG data obtained from the rectus fe­moris muscle. These findings were con­firmed further in an extensive study by Soderberg and Cook in a sample of 40 healthy subjects.5 In their work, highly significant (p < .0001) differences ex­isted in the EMG data between the SLR and QS exercises. Of the vastus medialis, biceps femoris, gluteus medius, and rec­tus femoris muscles, only the rectus fe-

Dr. Soderberg is Professor and Director, Physical Therapy Program, College of Medicine, The Uni­versity of Iowa, Iowa City, IA 52242 (USA).

Dr. Minor is Assistant Professor, Department of Physical Therapy, College of Allied Medical Profes­sions, University of Kentucky, Lexington, KY 40536-0084.

Mr. Arnold is a staff physical therapist, Mercy Hospital Medical Center, Sixth and University Ave, Des Moines, IA 50314.

Mr. Henry is a staff physical therapist, Michiana Rehabilitation Institute, Memorial Hospital of South Bend, 615 N Michigan St, South Bend, IN 46601.

Mrs. Chatterson is a staff physical therapist, Hu­mana Hospital-Sunrise, PO Box 14157, Las Vegas, NV 89114.

Mrs. Poppe is a staff physical therapist, Mary Greeley Medical Center, 117 11th St, Ames, IA 50010.

Miss Wall is a senior physical therapist, Rehabil­itation Unit, Saint Joseph's Hospital, 611 St. Joseph Ave, Marshfield, WI 54449.

Dr. Minor, Mr. Arnold, Mr. Henry, Mrs. Chat­terson, Mrs. Poppe, and Miss Wall were students in the Physical Therapy Program, The University of Iowa, at the time the study was conducted.

This article was submitted September 3, 1986; was with the authors for revision five weeks; and was accepted March 3, 1987. Potential Conflict of Interest: 5.

Volume 67 / Number 11, November 1987 1691

moris muscle was more active during the SLR exercises.

Few studies have included a patient population, and none have specifically evaluated a patient's response to the ex­ercises investigated in this study. Strat­ford established, however, that patients with effused knees diminished myoelec­tric activity at 0 degrees when compared with the 30-degree flexed position.6 A year later, Krebs and associates evalu­ated 30 healthy and 18 postarthrotomy patients during SLR and QS exercises and found highly significant interaction of position (exercises) and limb condi­tion (operated vs nonoperated).7

Although SLR and QS exercises have been used in other studies, they have been used in considerations associated with the effect of the exercise on patellar pain or patellar pain syndromes.8-10

Other studies, among them that of Lieb and Perry,11 have attempted to de­termine the specific role of the vastus medialis muscle in terminal knee exten­sion. For a list of references and a discussion of this issue, the reader is referred to the 1981 work by Duarte-Cintra and Furlani.12

Frequent clinical use of QS and SLR exercises requires that physical thera­pists be knowledgeable about their ef­fects. Previous work on healthy subjects has led us to hypothesize that patients would have greater rectus femoris mus­cle activity during the SLR exercises than during QS exercises. The converse should be true for the vastus medialis, gluteus medius, and biceps femoris mus­cles. Thus, the purpose of this study was to compare EMG data obtained from the vastus medialis, rectus femoris, glu­teus medius, and biceps femoris muscles during maximally resisted SLR exercises with EMG data obtained from the same muscles during QS exercises in healthy subjects and in patients with knee pa­thologies.

METHOD

Subjects Thirty individuals participated in this

study. Fourteen were healthy subjects free from pathology, and 16 were pa­tients (8 men, 8 women) who had a history of knee injury or surgery. All signed informed consent forms for the protocol that had been approved by The University of Iowa Committee on Hu­man Subjects. Descriptive information on the two groups is shown in Table 1. All of the patients, whose status is shown in Table 2, had received or were receiv-

TABLE 1 Descriptive Characteristics of Subjects

Group

Healthy subjects (n = 14) Range

s Patients with knee pathologies

(n = 16) Range

s

Age (yr)

21-26 23.3 1.4

18-72 32.8 18.8

Height (m)

1.6-1.9 (5.2-6.2 ft) 1.7 (5.6 ft) 0.09 (0.3 ft)

1.6-1.9 (5.2-6.2 ft) 1.8 (5.9 ft) 0.1 (0.33 ft)

Weight (kg)

54-91.8 (119-202 lb) 68.6(151 lb) 13.7 (30.2 lb)

57.7-86.4 (127-190 lb) 70.8 (156 lb) 10.0 (22 lb)

TABLE 2 Description of Patient Pathology

Condition

Arthroscopy with meniscectomy of medial and lateral meniscus

Arthroscopy with lateral release Patellectomy with accompanying synovitis Medial collateral ligament sprain with subsequent

immobilization Total knee replacement secondary to degenerative joint

disease Undiagnosed knee pain, stiffness secondary to immobilization Meniscectomy with anterior cruciate ligament deficiency Arthrotomy with lateral release secondary to adhesion Anterior cruciate ligament repair and meniscus tear Intramedullary femoral rod and immobilization Allograft replacement of medial femoral condyle Lateral release and dovetail of tibial tubercle

Number of Patients

1 1 1

1

4 1 1 1 2 1 1 1

ing physical therapy at The University of Iowa Hospitals and Clinics or in the sports physical therapy facility in the athletic department at the time of test­ing. The mean length of time after injury or surgery was six weeks. All patients could generate sufficient quadriceps fe­moris muscle tension to maintain knee extension during the SLR exercises. The right knees of 10 healthy subjects and 9 patients were tested; the left knees of 4 healthy subjects and 7 patients were tested.

Preparation To record the myoelectric activity, the

skin of the tested knee was wiped with alcohol-soaked gauze. An electrode assembly* then was attached using double-sided foam adhesive tape. Each assembly, containing circuitry for preamplification with a gain of 35, measured 33 × 17 × 10 mm. Each

*Therapeutics Unlimited, 2835 Friendship St, Iowa City, IA 52240.

assembly held two silver-silver chloride electrodes, which were 8 mm in diame­ter with a 20-mm distance between cen­ters. Conductive gel filled the holes in the tape over the electrode sites. The electrode for the rectus femoris muscle was placed one half of the distance from the anterior superior iliac spine to the superior pole of the patella. The elec­trode for the biceps femoris muscle was located one third of the distance from the ischial tuberosity to the middle of the lateral joint line of the knee. For the gluteus medius muscle, the electrode was located one third of the distance along a line from the midpoint of the iliac crest to the greater trochanter. The electrode for the vastus medialis muscle was applied over the muscle belly found by visually inspecting the thigh while the subject exerted a strong isometric contraction of the quadriceps femoris muscle. In all instances, the electrode assembly was aligned with the direction of the muscle fibers. A common ground electrode was positioned over the right fibular head, and all electrode leads were plugged into main amplifiers.

1692 PHYSICAL THERAPY

RESEARCH Instrumentation

The combined preamplifier and main amplifier system permitted a gain of 100 to 10,000 with a bandwidth of 7 Hz to 6 kHz. After amplification, the EMG signals were full-wave rectified and low-pass filtered (with a cut-off frequency of 8 Hz) to produce a linear envelope. The EMG signals then were cabled to either an Apple IIe computer,† through a Cy­borg Biolab System,‡ or to an IBM PC computer.® The change in the computer was due to an upgrade in data manage­ment capability after nine subjects had been tested. Software for operation of the two computers allowed for use of the same sampling rates and data ma­nipulation, certifying consistency dur­ing all data collection. All EMG data were sampled at a rate of no less than 100 samples per second. Although the EMG preamplifier system minimizes ar­tifact, at least two channels of raw EMG were evaluated continuously for offsets and artifacts on a standard oscilloscope. All four channels were evaluated period­ically for each subject, and at no time did extraneous signals occur.

Protocol

The EMG data were recorded simul­taneously from the four muscles during maximal, manually resisted SLR and QS exercises while the participant as­sumed a semi-Fowler's position on a standard plinth. Because measurement reliability of .72 to .85 had been estab­lished with Pearson product-moment correlation coefficients in an earlier study, the same protocol was adopted for this study.5

Because the EMG data were to be normalized, each participant's maxi­mally evoked EMG recording for each of the specified muscles was selected from three trials of maximal effort iso­metric contraction completed in knee extension, knee flexion, and hip abduc­tion with the knee extended. For this purpose, maximal voluntary contrac­tions of the vastus medialis, rectus fe-moris, and biceps femoris muscles were completed at a clinically determined an­gle of 45 degrees of knee flexion.5 Max­imal EMG responses from the gluteus medius muscle were recorded at an an­

gle of 30 degrees of hip abduction with the knee in extension. Verbal com­mands of "ready," "set," and "push" or "pull" were given to the participants. For maximal isometric contractions and exercise trials, the EMG data were sam­pled for a total of three seconds. The participants were instructed to relax while the data were being stored on a computer disk.

In performing the QS exercises, sub­jects were instructed to press the back of their knee as hard as possible into a small towel that had been placed under the knee. For the SLR exercises, the leg first was elevated above the plinth on the command "set." Manual resistance to active contraction then was applied just proximal and distal to the knee on "push." Three trials of each exercise were completed in a predetermined ran­domized order with rest periods of about 30 seconds to allow data storage.

Design and Data Analysis

The experimental research design was considered to be a two-factor mixed de­sign based on independent groups (ie, healthy subjects vs patients) and re­peated measures according to type of exercise (ie, SLR vs QS). The EMG values from each muscle during the maximal effort extractions and values for each muscle in the three trials for each of the two exercises were used in the data analysis. The percentage of maximal EMG response was calculated for all trials, producing a data set in which all values for each participant were normalized to that participant. The descriptive statistics, including means and standard errors, were computed with a standard statistical package. A linear analysis of variance (ANOVA) was used to test for interaction and to compare data across groups and also across exercises for each of the muscle groups.

RESULTS

Descriptive data, including means and standard errors, are shown in Fig­ures 1 and 2. The ANOVAs completed for each muscle are shown in Tables 3 through 6. Nonsignificant interactions and nonsignificant diagnosis main ef­fects resulted for all four muscles eval­uated. Significant exercise main effects are shown in each table. Figure 3 graph­ically represents the differences when the healthy subjects and patients are combined into one group.

DISCUSSION The means and standard errors for

the respective muscle groups and exer­cises shown in Figures 1 and 2 graphi­cally depict the differences in muscle activity found in this study. No statisti­cal tests for differences between muscles could be completed because of limita­tions inherent in across muscle and channel comparisons of EMG data. No between-group (diagnosis) differences were demonstrated. The exercises, of primary interest in this study, produced obvious differences. The rectus femoris and vastus medialis muscles displayed opposite patterns of activity, with the vastus medialis, gluteus medius, and bi­ceps femoris muscles demonstrating much greater activity during the QS ex­ercises than during the SLR exercises (Fig. 3).

Current data and data from the pre­vious study of Soderberg and Cook, al­though showing some difference in the mean values, agree well (Fig. 4).5 Some variance among groups of participants was anticipated, even with the testing of essentially equal proportions of right and left legs. Other factors such as the application of resistive loads by different investigators also may have been re­sponsible. Review of the data shows that the quadriceps femoris muscles pro­duced lower percentages of maximal EMG values in this study, whereas the other two muscles studied showed in­creased levels of activity (Fig. 4). In gen­eral, no systematic trend was apparent. Further review of data across trials in­dicated no orderly patterns associated with the trial in which participants reached the maximal EMG value.

The magnitude of the mean values demonstrated clear differences in the muscles used and the level of activity produced in the different contractions (Fig. 3). The hypothesis, therefore, that EMG data obtained from SLR and QS exercises would be similar in healthy subjects and in patients with knee pa­thologies is accepted. We could not compare our data to other studies be­cause no similar calculations or com­parisons of exercises have been reported. Even Skurja et al, who reported similar results for the vastus medialis and rectus femoris muscles, did not provide any numerical data.4

The results of this study agree with Pocock's findings of relatively large lev­els of EMG activity recorded during QS exercises.1 Although recognizing that a shorter muscle length would explain greater EMG activity, we believe that

† Apple Computer, Inc, 20525 Mariani Ave, Cu­pertino, CA 95014.

‡ Cyborg Corp, 343 Western Ave, Boston, MA 02135.

§ International Business Machines Corp, PO Box 1328-S, Boca Raton, FL 33432.

Volume 67 / Number 11, November 1987 1693

our results could not have been influ­enced significantly because the partici­pants were asked only to clear their heel from the supporting surface during the SLR exercises. Although we recognize that the precise relationship of EMG activity to tension still is debated, EMG analysis is considered to be an appropri­ate measure for assessing the relative intensity of muscle activity produced during exercises of interest to the physical therapist. Our results also are consistent with the work of Gough and Ladley in that their "static" exercise consistently produced greater EMG ac­tivity than the SLR exercises.3 Their results should be considered with cau­tion, however, because they allowed subjects to complete the last few degrees of knee extension with a 3-in|| block behind the knee.

In response to our previous work with healthy subjects, some clinicians sug­gested that the results could be affected by preceding the SLR exercises with the QS exercises. To address this issue, with particular interest in how the vastus me-dialis and rectus femoris muscles would respond, we requested 11 of the healthy subjects and 10 of the patients with knee pathologies to complete an additional series of exercises. Each participant completed three more contractions, spe­cifically performing the QS exercises be­fore the SLR exercises. We termed the first phase of this exercise (ie, perform­ance of QS exercises) the combined be­fore (COB) phase, and the second phase (ie, performance of SLR exercises) was termed the combined after (COA) phase.

Because the design for this segment of the study was similar to that of the main study, this data set was analyzed using an ANOVA. Results showed that this smaller test subgroup demonstrated be-tween-exercise differences consistent with the results for the total groups of subjects. The ANOVA results revealed that the rectus femoris and vastus me-dialis muscles produced a significant in­teraction effect that precluded formal comparisons for main effects. Informal review of the data, however, showed that for both the rectus femoris and vastus medialis muscles the EMG activity was markedly increased when the COB phase was compared with either the QS exercises or the SLR exercises. For the rectus femoris muscle, the mean value for the COB phase was 125% of the maximal EMG values. The vastus me­dialis muscle increased to 161% of the

QSH

QSP

SLRH

SLRP

SE

Fig. 1. Percentage of maximal EMG activity for the rectus femoris and vastus medialis muscles for each type of exercise (QS = quadriceps femoris muscle setting, SLR = straight-leg-raising) by group (H = healthy subjects, P = patients with knee pathologies). SE is the standard error, pooled across exercise within groups.

QSH

QSP

SLRH

SLRP

SE

Fig. 2. Percentage of maximal EMG activity for the biceps femoris and gluteus medius muscles for each type of exercise (QS = quadriceps femoris muscle setting, SLR = straight-leg-raising) by group (H = healthy subjects, P = patients with knee pathologies). SE is the standard error, pooled across exercise within groups.

TABLE 3 Analysis of Variance Summary for Rectus Femoris Muscle

Source Group Subject Exercise Group x exercise

df 1

28 1 1

SS 2225.2

68648.5 24630.2 4059.4

F 1.59 1.76

17.65 2.91

P NS NS

.0002 NS

|| 1 in - 2.54 cm.

1694 PHYSICAL THERAPY

RESEARCH

QSHP

SLRHP

Fig. 3. Graphic representation of the main effects, by muscle, shown by an analysis of variance. QSHP is quadriceps femoris muscle setting exercise data for healthy subjects and patients with knee pathologies combined. SLRHP is straight-leg-raising exercise data combined across groups.

QSB

QSC

SLRB

SLRC

Fig. 4. Comparison of the results of the current study with data from a previous study1 (QS = quadriceps femoris muscle setting, SLR = straight-leg-raising, B = previous study, C = current study).

TABLE 4 Analysis of Variance Summary for Vastus Medialis Muscle

Source Group Subject Exercise Group x exercise

df 1

28 1 1

SS 426.0

26024.5 22963.0

961.6

F 0.47 1.02

25.13 1.05

P NS NS

.0001 NS

mean value produced during the maxi­mal effort trials. When compared with the values shown in Figure 3, this differ­ence is dramatic, particularly because the only difference between the QS ex­ercises and the COB phase was that the subjects' hands were placed on the an­terior aspect of the leg so that resistance could be offered for the SLR component of the exercise that would produce the data for the COA phase. An explanation can be offered by the data of Haberichter et al who, studying the effects of sus­tained muscle pressure on the H-reflex, found that slight pressure to a muscle weakly facilitated its motoneuron excit­ability.13 This possibility of facilitatory effects of muscle pressure on these ex­ercises is certainly of interest and poten­tially of great clinical import. No other explanations are readily apparent. Eval­uation of the COA means 227% and 143% of maximal EMG values for the rectus femoris and vastus medialis mus­cles, respectively, compared with COB means of 125% and 161% of maximal EMG values show the expected direc­tion in that the EMG activity of the rectus femoris muscle increased during the SLR phase, whereas the vastus medialis muscle decreased in EMG activity.

The ANOVA, including the com­bined exercises of COB and COA for the biceps femoris and gluteus medius muscles, produced nonsignificant inter­actions, thus allowing post hoc compar­isons. Of the eight possible comparisons of the SLR and QS exercises to the COB and COA exercises, only the QS-COA comparison produced a nonsignificant correlation coefficient. Review of the mean values showed that for both mus­cles the EMG activity increased from between 17% and 55% (Fig. 3) to 72% and 111 % of the maximal values. This increased activity is probably of clinical significance, assuming that one of the objectives of using the QS exercises be­fore the SLR exercises is to facilitate the contraction of the biceps femoris and gluteus medialis muscles. Although we found a demonstrable effect of this tech­nique on the activity produced in these muscles, we can offer no specific expla­nation as to the causative factors.

Comment on the effects of knee ef­fusion and surgery is necessary because both have been shown to be factors in the amount of EMG activity produced in postsurgical conditions. The impor­tance of these factors was demonstrated in a recent study by Stratford in which severely effused knees produced a de-

Volume 67 / Number 11, November 1987 1695

crease in EMG activity at 0 degrees of knee flexion when compared with 30 degrees of knee flexion.6 That none of the patients in our study had an acute or markedly effused knee makes any potential effect of these factors on our results unlikely. Clinicians should be aware of these factors and probably should heed the advice of Stratford who states that in these cases maximal iso­metric contractions of the quadriceps femoris muscle should be completed at 30 degrees of knee flexion.6

In another recent report, Krebs et al provide a review of the literature and new data to support the hypothesis that limbs that have undergone surgical pro­cedures are likely to demonstrate loss in ability to recruit motor units, thus af­fecting the EMG response.7 Their work demonstrated that the motor unit activ­ity to joint angle relationship is normal preoperatively, and diminishes postop­eratively, before returning to normal three weeks or more after surgery. Again, because none of our patients were tested immediately after surgery, the results of this study probably were not influenced. Clinicians, however, should note the recommendation of Krebs et al that the postarthrotomy quadriceps femoris muscles should be exercised with the knee slightly flexed to

TABLE 5 Analysis of Variance Summary for Gluteus Medius Muscle

Source Group Subject Exercise Group x exercise

df 1

28 1 1

SS 130.2

31325.0 3151.8

23.8

F 0.30 2.55 7.19 0.05

P NS .007 .01 NS

TABLE 6 Analysis of Variance Summary for Biceps Femoris Muscle

Source Group Subject Exercise Group x exercise

df 1

28 1 1

SS 962.8

61061.0 13852.4

16.3

F 2.05 4.65

29.54 0.03

P NS

.0001

.0001 NS

generate, simultaneously, maximum mechanical tension and motor unit ac­tivity.

CONCLUSION

This study clearly shows that the SLR and QS exercises, as commonly used by clinicians, have different effects on the level of muscular activity derived from the rectus femoris, vastus medialis, glu­teus medius, and biceps femoris mus­cles. The results show that exercise se­lection will influence the degree of

muscular activity and assumably the in­tensity of the contraction required for the specified therapeutic purpose.

Acknowledgments. We thank Jan-ine Wolbers for her assistance with data collection and Tom Cook for his assist­ance with computer programming. We also express appreciation to the physical therapists in the Musculoskeletal Divi­sion of the Department of Physical Therapy, The University of Iowa Hos­pitals and Clinics, for arranging for pa­tient participation.

REFERENCES

1. Pocock GS: Electromyographic study of the quadriceps during resistive exercise. Phys Ther 43:427-434, 1963

2. Allington RO, Baxter ML, Koepke GH, et al: Strengthening techniques of the quadriceps muscles: An electromyographic evaluation. Phys Ther 46:1173-1176,1966

3. Gough JV, Ladley G: An investigation into the effectiveness of various forms of quadriceps extension. Physiotherapy 57:356-361, 1971

4. Skurja M Jr, Perry J, Gronley J, et al: Quadri­ceps action in straight leg raise versus isolated knee extension (EMG and tension study). Ab­stract. Phys Ther 60:582, 1980

5. Soderberg GL, Cook TM: An electromyo­graphic analysis of quadriceps femoris muscle

setting and straight leg raising. Phys Ther 63:1434-1438,1983

6. Stratford P: Electromyography of the quadri­ceps femoris muscles in subjects with normal knees and acutely effused knees. Phys Ther 62:279-283, 1982

7. Krebs DE, Staples WH, Cuttica D, et al: Knee joint angle: Its relationship to quadriceps fe­moris activity in normal and postarthrotomy limbs. Arch Phys Med Rehabil 64:441-447, 1983

8. Wild JJ, Franklin TD, Woods GW: Patellar pain and quadriceps rehabilitation: An EMG study. Am J Sports Med 10:12-15,1982

9. Moller BN, Krebs B, Tidemand-Dal C, et al: Isometric contractions in the patellofemoral

pain syndrome: An electromyographic study. Arch Orthop Trauma Surg 105:24-27,1986

10. Antich TJ, Brewster CE: Modification of quad­riceps femoris muscle exercises during knee rehabilitation. Phys Ther 66:1246-1250,1986

11. Lieb FJ, Perry J: Quadriceps function: An elec­tromyographic study under isometric condi­tions. J Bone Joint Surg [Am] 53:749-758, 1971

12. Duarte-Cintra A, Furiani J: Electromyographic study of the quadriceps femoris in man. Elec-tromyogr Clin Neurophysiol 21:539-554, 1981

13. Haberichter PA, Mueksch AE, Rohrberg MG, et al: Muscle pressure effects on motoneuron excitability. Abstract. Phys Ther 65:723,1985

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