1983; 63:915-921.PHYS THER. D P Currier and Ralph MannStimulation in Healthy IndividualsMuscular Strength Development by Electrical

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Muscular Strength Development by Electrical Stimulation in Healthy Individuals

D. P. CURRIER and RALPH MANN

The effects of electrical stimulation and its comparison with various resistance training modes were investigated in 34 subjects who trained their dominant leg isometrically three times a week for five weeks. Group E (n=8) used maximum voluntary isometric contractions only, Group S (n=8) received only electrical stimulation, and Group ES (n=9) trained using a concurrent combination of isometric exercise and electrical stimulation. In addition, Group C (n=9) , a control group, did no experimental training. The expected gain in torque was shown in the isometric strength scores of the three experimental groups; how­ever, there was little change in the isometric strength of the control group and in the isokinetic (100, 200, and 300° /sec) results of any of the four groups. After adjustments were made for pretest strength results, a one-way analysis of covariance indicated that the posttest isometric strength scores of the three experimental groups differed significantly from those of the control group. No other posttest differences were found among any of the other training modes.

Key Words: Electric stimulation, Muscular strength.

The use of electrical stimulation in clinical practice and in sports medicine has received increasing atten­tion in the literature. Several investigators have ex­amined strength response to electrical stimulation during repeated sessions and reported increased strength of skeletal muscle,1"3 improved muscular contraction among patients after knee surgery,4, 5 in­creased girth measurements with training,1"4 and re­duced atrophy, that is, prevention of a fall in succinate dehydrogenase (SDH) activity during immobiliza­tion.5, 6 In contrast, some studies using electrical stim­ulation in repeated sessions have not supported the claims of increased muscular strength7, 8 or increased girth measurements.2, 3

Examination of the literature reveals certain com­mon characteristics among studies of electrical stim­ulation for augmentation of muscular strength. Al­though many investigators have used electrical stimuli

to produce tetanic muscular contractions, the rates have ranged from 33 to 1,000 pulses per second (pps).1-3, 5-7 The study using the fastest (1,000 pps)7

rate was unsuccessful in augmenting muscular strength, whereas favorable results were reported in studies using rates of 33,6 50,3 60,2 65,1 and 2005 pps. Various waveforms—half waves3 and faradic,1 surg­ing (trapezoid),4 and rectangular6 waves—have all been used successfully. Finally, the intensities of stim­uli used may have varied considerably. Several inves­tigators reported using current intensities within their subjects' tolerance1-3, 5; however, the actual intensities were often not cited or the units of measurement varied (eg, 60 mA1 and 100 V5), making a common level of current intensity difficult to identify.

Most of the above studies appear to be the result of renewed interest in electrical stimulation following claims attributed to the Russian investigator, Kots.1, 9, 10 His claims concerning the benefits of elec­trical stimulation include relieving pain in injured areas, increasing blood flow to local areas, strengthen­ing muscle, increasing muscle size, and facilitating muscle contraction and function. Most of the reported studies designed to produce these results, however, have been performed without benefit of an electrical stimulator similar to that used by Kots.

This study, therefore, was prompted by the follow­ing concerns: 1) reports citing favorable effects from

Dr. Currier is Professor, Department of Physical Therapy, HP500, University of Kentucky Medical Center, Lexington, KY 40536 (USA).

Dr. Mann is Associate Professor, Department of Health, Physical Education, and Recreation, Seaton Building, University of Kentucky, Lexington, KY 40506.

This investigation was partially funded by the Arthritis Founda­tion.

This article was submitted January 25, 1982; was with the authors for revision 22 weeks; and was accepted for publication January 6, 1983.

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TABLE 1 Demographic Data for Subjects

Group

Control Exercise Electrical stimulation Exercise and electri­

cal stimulation All subjects

F

4 4 3

4 15

Sex

M

5 4 5

5 19

Height (cm)

X

170.2 175.8 169.9

175.5 173.0

s

10.2 7.3

12.0

9.5 9.7

Weight (kg)

X

67.4 73.8 68.1

71.2 70.1

s

11.2 9.4

18.0

8.6 11.9

Age (yr)

X

25.2 22.5 21.4

21.7 22.5

s

3.6 4.2 2.1

1.3 3.3

electrical stimulation are based on clinical observa­tion rather than on studies with rigid controls; 2) electrical stimulation has been inadequately com­pared with other approaches of increasing muscular strength and girth; 3) studies have attempted unsuc­cessfully to augment strength using electrical stimuli at 25 pps11 or 1,000 pps7; and 4) electrical stimulators of the prototype used by Kots1 have been unavailable to confirm claims of others who modeled their treat­ment or research approaches after those of Kots. The purposes of this study were to determine the effects of electrical stimulation with specific characteristics and to compare different resistance training modes (using isometric exercise, electrical stimulation, iso­metric exercise-electrical stimulation, and no training) on increasing muscular strength and limb girth.

METHOD

Subjects

The 34 subjects completing the study were healthy volunteers. All were informed of the experimental protocol and risks and gave written consent before participating. Five subjects withdrew from the study because of scheduling conflicts or disinterest. Two subjects withdrew because of pain; one developed a patellar tendinitis of the test-training leg, and the other strained his back. Subjects ranged in age from 18 to 32 years, in height from 157.5 to 189.9 cm (62.0-74.8 in), and in weight from 46.8 to 88.9 kg (103.2-196.0 lb) (Tab. 1). All but one subject identified the right lower extremity as dominant.

Practice and Pretest

Before being tested, all subjects participated in two practice sessions to acquaint them with specific testing procedures to be used. Exercises used during the practice sessions consisted of repeated 15-second maximum voluntary isometric contractions (MVC) of the dominant knee extensor muscles, followed by 50-second rest periods, and isokinetic exercises at rates of 100, 200, and 300°/sec. Three separate repetitions

were performed by each subject, under each condi­tion, during two practice sessions held on consecutive days. No measurements were taken during these prac­tice sessions.

During the pretest, each subject's weight (kg), height (cm), upper leg girth measurement (cm), and maximum knee moment of force (torque Nm) were obtained. The girth measurement of the subjects' dominant upper leg was taken at a level midway between the greater trochanter of the femur and the proximal border of the patella to accommodate for the various heights of the subjects. Knee torque was tested by means of a Cybex® II dynamometer.* Maximum torque was tested in a static position (o°/ sec) with the hip and dominant knee positioned in 60 degrees of flexion. Maximum dynamic torque was also tested at arbitrarily chosen knee extension rates of 100°/sec, 200°/sec, and 300%ec. Subjects were seated on an adjustable chair with the involved thigh and ankle stabilized by web straps; they held their backs firmly against the backrest (60° of flexion) by their effort to extend their knees and to grasp the handgrips mounted on the sides of the test chair. The axis of rotation of the Cybex® II dynamometer was aligned with the anatomical axis of rotation of the knee. The lever arm of the apparatus was adjusted to accommodate the length of the subject's lower leg.

Torque was assessed during the pretest by deter­mining all subjects' MVC at 0°/sec and maximum isokinetic contractions at rates of 100°/sec, 200°/sec, and 300°/sec. All subjects attempted three maximum contractions at each of the four rates; the largest torque score recorded at each rate was used in the analysis.

In an attempt to standardize motivation of the subjects during pretest, the examiner repeatedly of­fered verbal encouragement (shouting) while all sub­jects were trying to develop maximum voluntary mus­cle contractions. Torque was recorded by a dual channel recorder during each of the three voluntary contractions at rates of 0°/sec, 100°/sec, 200°/sec, and 300°/sec.

* Cybex Division of Lumex Inc, 2100 Smithtown Ave, Ronkon-koma, NY 11779.

PHYSICAL THERAPY 9 1 6

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RESEARCH

TABLE 2 Means and Standard Deviations of Girth Measurements

Group

Control Exercise Electrical stimulation Exercise and electri­

cal stimulation All subjects

Pretest (cm)

X

52.27 52.74 50.85

52.86 52.22

s

2.66 2.58 4.70

2.00 3.06

Posttest (cm)

X

52.47 52.68 52.00

53.24 52.61

s

2.81 2.60 5.38

2.90 3.42

Difference (cm)a

+0.20 - 0 . 0 6 - 1 . 1 5

+0.38 +0.39

aNS.

Training and Posttraining Measurement

After the pretest, subjects were assigned randomly to one of four groups. All groups trained isometri-cally, with each subject positioned as in the isometric pretest. Group C, the control group (n=9), received no experimental training after the practice and pretest sessions. Group E, the exercise group (n=8), per­formed a series of MVC of the knee extensor muscles during each training session. Group S, the electrical stimulation group (n=8), received a series of electrical stimuli that produced involuntary muscle contraction of the knee extensor muscles during each training session. The contraction sequence for Group S was totally controlled by the timing cycle of the stimula­tor, and the subjects were instructed not to voluntarily contract muscles being electrically stimulated. Group ES, the isometric exercise-electrical stimulation group (n=9), performed the same exercise routine as Group E but received electrical stimulation concurrently with each voluntary muscle contraction. All training groups performed 10 contractions of 15 seconds, fol­lowed by 50 seconds of rest, and completed a total of 15 sessions (3/wk for five weeks). These subjects were instructed to begin each training session with an intensity (load) level of at least 60 percent of their pretest MVC score. This starting level was used in an effort to ensure muscle adaptations from resistance training.12 Torque developed by each muscle contrac­tion during all training sessions was recorded. Sub­jects were requested not to alter their normal daily routine during the study.

An Electrostim 180-2†was used to provide the desired electrical stimuli. The stimulator produced individual 0.1 msec sinewaves at a frequency of 2,500 Hz delivered with 10 msec "on" and "off' bursts of current modulated with a rate of 50 pps. Tetanic isometric muscular contractions lasted for 15 seconds, during which time the current was surged so that peak intensity was reached after 5 seconds and sus­tained for 10 seconds. At the end of the 15-second current stimulating period, the intensity abruptly de­

clined and was followed by a 50-second rest before the next stimulating period (isometric producing rep­etition). Intensity was set by the subjects before the start of each training session to produce a torque of 60 percent or more of their pretest MVC score.12 In each case, the level of torque was verified by the examiner. The stimulus intensities ranged from 26 to 86 mA ( . = 45.8 mA) for Group ES and from 37 to 86 mA ( = 55.3 mA) for Group S.

One stimulating electrode was placed proximally over the femoral nerve inferior to the inguinal liga­ment, and the other electrode was placed distally over the quadriceps femoris muscle (5-7 cm proximal to the upper border of the patella). Circular (13 cm in diameter), flexible conductive rubber electrodes were used, and a moist sponge was placed between the skin and electrode according to the manufacturer's instruc­tions. Groups E, ES, and S preceded each training session with five submaximal muscle contractions (voluntary—E, and/or electrically stimulated—ES, S) as in practice sessions.

After the experimental groups completed training, all subjects, including Group C, were measured for upper leg girth and tested for static and dynamic torque as they were in the pretest. Means and stan­dard deviations were calculated for subject character­istics and performance variables. To compare the relative posttest girth and strength among groups, a one-way analysis of covariance (ANCOVA)13 was used for each contraction rate. In each case, the covariate was the pretest value. A .05 probability level was used for all tests of statistical significance.

RESULTS

Table 2 gives the means and standard deviations for pretest and posttest girth measurements. Analysis revealed that there were no statistically significant differences in the posttest girths among the groups as a result of the training.

Descriptive statistics for pretest and posttest torque scores are given in Table 3. The results indicated the expected increase in the level of torque developed by the experimental groups in the static position over time. Furthermore, ANCOVA indicated that after

† Micromed Instruments, 4996 Place de la Savane, Montreal, Canada H4B 1R6.

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TABLE 3 Means, Standard Deviations, and Changes of Group Torque (Nm) at Various Test Speeds (°/sec)

Group

Control Pretest Posttest

Change

Exercise Pretest Posttest

Change

Electrical stimula­tion

Pretest Posttest Change

Exercise and elec­trical stimulation

Pretest Posttest

Change

0

X

377.3 383.2

s

96.3 91.6

+ 5.9

342.6 446.8

112.6 161.4

+ 104.2

372.2 433.9

126.1 116.4

+ 61.7

368.4 455.0

102.5 141.2

+ 86.6

100

X

203.4 208.2

s

59.7 55.1

+ 4.8

190.2 202.4

67.3 71.2

+ 12.2

194.9 217.0

70.6 74.6

+ 22.1

213.0 214.7

75.2 71.1

+ 1.7

200

X

134.2 135.8

S

39.6 35.8

+ 1.6

135.1 138.3

51.4 54.2

+ 3.2

126.5 138.1

50.9 46.9

+ 11.6

142.2 143.6

51.3 51.2

+ 1.4

300

X

84.1 86.2

s

27.8 25.7

+ 2.1

84.3 88.8

29.5 33.4

+ 4.5

79.0 89.3

34.7 30.3

+ 10.3

93.3 94.3

37.1 34.3

+ 1.0

adjustments were made for pretest torque values, the static training effects were different among the groups during the posttest (Tab. 4). As shown in Table 5, post hoc analyses (least significant difference test) using the adjusted least squares means for the posttest static strength results revealed that the experimental groups differed significantly from the control group. No significant differences, however, were found be­tween the three experimental groups at the conclusion of training. In the related dynamic torque tests, there was no evidence of strength changes over time. In addition, no significant changes between pretest and posttest results were found for any of the other three rates of contraction (100, 200, and 300°/sec).

The mean percentage of training intensity (TI) was determined by the formula TI = T t / M V C 1 × 1 0 0 , where T t is mean weekly torque, and MVC1is maxi­mum voluntary isometric contraction at pretest. Mean weekly torque (T t) is the mean of all torque scores for a particular group recorded each training session over the five weeks (10 repetitions ×3 d/wk × 5 weeks = mean of 150 scores). The TI was 119.1 percent for Group E, 88.4 for Group ES, and 66.7 for Group S. The TI by each week of training is illus­trated in Figure 1.

Figure 2 represents mean torque developed during a specific repetition (in order of occurrence) for all 15 training sessions. The torque decrement (fatigue curves) over a continuum of repetitions for Group E was 10 percent, for Group S was 21 percent (24% between the 2nd and 10th contraction), and for Group ES was 20 percent.

Most subjects in the training groups reported mus­cular soreness after the pretest, and some subjects receiving electrical stimulation experienced discom­fort concurrent with each contraction throughout the training program. The muscular soreness and discom­fort experienced by subjects during this study will be discussed in a separate communication.

DISCUSSION

When posttest mean girth measurements were ad­justed for pretest influence by ANCOVA, no signifi­cant difference was found. This finding is in agree­ment with that of Godfrey and co-workers2 and of Halback3 but disagrees with the reports of others.1'4

Muscular hypertrophy from isometric exercise has been reported to be less than that from dynamic resistance training in healthy subjects.14 Because the electrical stimulation used on all subjects in the above

TABLE 4 Analysis of Covariance of Adjusted Posttest Static Strength Using Pretest Static Strength as the Covariate

Source

Group Error TOTAL

df

3 29 32

SS

26796.20 39921.93 66718.13

MS

8932.07 1376.62

F

6.49 —

P

<.01 —

PHYSICAL THERAPY 9 1 8

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RESEARCH

TABLE 5 Probability of Differences Between Least Squares Means of the Adjusted Posttest Static Torque Values

Group

Control (C) Exercise (E) Exercise and electri­

cal stimulation (ES) Electrical stimulation

(S)

Posttest torque adjusted mean (Nm)

273.65 348.09 333.38

314.62

Probability value for least significant difference test

C

.0003a

.0019

.0307

E

.4198

.0830

ES

.3092

S

a Example: the control group differed from the exercise group at the .0003 level.

studies produced isometric contractions (at varied limb positions), further study is needed to determine the effectiveness of this treatment mode on muscular size in healthy subjects and in subjects having atro­phy. Electrical stimulation has been reported to be beneficial in the treatment of patients with disuse atrophy of muscle,4 patients who have undergone surgery of the knee,4 ,5and patients with chondro­malacia patellae.1

The subjects in each experimental group increased their mean static torque after 15 training sessions. The gains were sufficient to make the posttest strength results of these groups significantly greater than those of the control group. Results for the torque gains in the experimental groups using conventional exercise (Groups E and ES) were expected. The gains in isometric torque developed by Group S were impres­sive. This latter finding reinforces earlier claims that such a passive approach to training will augment strength in normally innervated muscle.1"5 Electrical stimulation of the type used in this study may become a useful clinical tool for preventing undesirable effects of immobilization.

Group E trained with the greatest mean intensity (119.1% of MVC 1 and achieved the largest net torque

gain (104.2 Nm), whereas the lowest mean training intensity (66.7%) and lowest net torque gain (61.7 Nm) were attributed to Group ES. Although Group S trained with the lowest intensity among the exper­imental groups in this study, it is evident that electri­cal stimulation without the voluntary contraction ef­forts of the subject can improve the muscle's capabil­ity of increasing strength. Also, the fact that Group ES did not produce mean torque scores above those of Group E confirms a previous report.11

Stimulus pulse rates affect the tension developed in normal muscle,1 5 , 1 6nd various muscles may have varied responses to the frequency of the stimulation.15

Muscle strength of the quadriceps femoris muscle appears to be augmented by electrical stimulation when tetanic contractions are produced at rates be­tween 33 and 350 pps.1 - 3 , 5 , 6 , 1 5 The highest pulse rate of stimuli to which muscle and nerve are able to respond depends on the length of their refractory period.17 The refractory period may vary according to the muscle or nerve, so tissue having recovery times of 1 to 2 msec will not produce optimum tension in response to stimuli of 500 to 1,000 pps.18 This condi­tion may also be Wedensky inhibition, a decrease in muscular tension caused by electrical stimulation ex-\img\

Fig. 1. Mean training efforts of groups for the five-week period. Means are expressed as a percentage of pretest maximum voluntary isometric contraction (MVIC1). \img\

Fig. 2. Mean torque (fatigue) patterns showing loss of force with repetitions during training. Each point repre­sents the mean of group subjects during a particular repetition over the five-week period of training.

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ceeding certain rates.19 Thus, a rate of 1,000 pps when applied to various muscles of the upper extremity and shoulder girdle7 may be too rapid to augment strength, whereas 50 pps (used in this study) seems to be acceptable for increasing torque produced by the quadriceps femoris muscle. Sugai et al studied the adductor pollicis muscle and found that complete fusion of tetanus occurred between 40 and 50 pps in 80 percent of his subjects; rates below 40 pps, how­ever, caused reduced tension because of incomplete fusion of tetanus.16 The functional characteristics of the quadriceps femoris muscle are similar to those of the adductor pollicis muscle when electrically stimu­lated by currents with similar pulse frequencies and durations.20 Electrical stimulators having rates of 50 pps are available to physical therapists for clinical use; however, unless the stimulator has a variable frequency capability, augmentation of torque for muscles other than the quadriceps femoris and ad­ductor pollicis muscles may not occur. This limitation warrants further study.

That no torque gains were recorded at the dynamic contraction rates after isometric resistance training further supports the concept of training specificity. Torque gains were recorded by the experimental groups only at the rate and knee angle at which training was conducted, 0°/sec and 60 degrees of flexion, respectively. This finding is in agreement with the specificity concept for conventional isometric contractions through voluntary effort21 but has not been reported for muscle training by electrical stim­ulation.

The results indicate that intensity increased in a linear fashion for each group during the training program (Fig. 1). Examination of the slope of the lines indicates that the rate of change of intensity is relatively similar for each group. In addition to re­flecting training intensities, the plotted mean values also represent the weekly gains in torque. Therefore, recorded mean training torques were confounded by the intensity of training stimuli and the increase of strength with each week of training. The two effects are difficult to separate and therefore were labeled as percentage of training intensity (TI) because subjects were encouraged to train during each session with an intensity at least 60 percent of their pretest MVC. Motivation through verbal encouragement was con­sidered to have played an important role in influenc­ing the training progress, especially when the routine was somewhat unpleasant for Groups S and ES.

Fatigue is expected to follow exercise sessions, par­ticularly if the exercises are strenuous. In the present study, the exercise group exerting a mean of 119.1 percent of their initial MVC during training produced a pattern of 10 percent torque loss between the 1st and 10th repetition (Fig. 2). An even greater force decrement (fatigue) occurred among subjects receiv­

ing electrical stimulation (Groups S and ES). The electrical stimulation group demonstrated the greatest decrement of torque developed between the 2nd and 10th repetition among the three training groups. Al­though this decrement is difficult to explain, the authors suspect that some subjects, after setting the current intensity to a level that would produce 60 percent of MVC1, found the stimulus unpleasant enough to cause them to cocontract their knee flexor muscles as a diversion from the electrical stimuli. This cocontraction of knee extensor and flexor muscles may have altered the torque for one repetition, but by the second repetition, subjects became tolerant of the stimuli. The pattern for Group S follows a con­sistent mean torque loss between the 2nd and 10th training repetitions. Because the timing interval of 15 seconds current output and 50 seconds rest was fixed by operational controls of the electrical generator, no further adjustments were made once the subject's current intensity was established before the first train­ing repetition. Further study is needed to determine whether a force decrement would occur if the current intensity was adjusted after each drop in measured torque.

The study was designed to train subjects using a current intensity sufficient to develop 60 percent of pretest MVC. Obviously the 60 percent intensity of MVC1 only occurred in the first few repetitions of training. Figure 1 shows, however, that on the whole the mean training intensity per week resulted in a value between 57 percent (week 1) and 75 percent (week 5) of MVC1 for Group S. Because of torque gains occurring during the five-week training period, the percentage of MVC1 at any one session may have been less than 60 percent of concurrent MVC, which was not remeasured until termination of the program. Training intensity adjustments for the graduated torque gains that occurred during the five-week pe­riod were not made during the study.

CONCLUSION

Results of this study support the following conclu­sions: 1) high intensity electrical stimulation of 2,500 Hz modulated at 50 pps does augment torque when subjects train with isometric contractions under con­ditions of this experiment; 2) no increase in muscle size (girth measurements) is produced after five weeks of training by isometric exercise, electrical stimula­tion, or isometric exercise-electrical stimulation; 3) resistance training methods used in this study pro­duced torque gains, but no statistical differences in torque were found among the three modes; and 4) no torque gains were noted when dynamic rates of con­traction (100°/sec, 200°/sec, and 300°/sec) were tested after static resistance training.

PHYSICAL THERAPY 920

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RESEARCH

REFERENCES

1. Johnson DH, Thurston P, Ashcroft PJ: The Russian technique of faradism in the treatment of chondromalacia patellae. Physiotherapy Canada 29:266-268, 1977

2. Godfrey CM, Jayawardena H, Quance TA, et al: Comparison of electrostimulation and isometric exercise in strengthening the quadriceps muscle. Physiotherapy Canada 31:265-267, 1979

3. Halback JW: Comparison of electro-myo-stimulation to iso­kinetic training in increasing power of the knee extensor mechanism. Journal of Orthopaedic and Sports Physical Therapy 2:20-24, 1980

4. Williams JGP, Street M: Sequential faradism in quadriceps rehabilitation. Physiotherapy 62:252-254, 1976

5. Ericksson E, Haggmark T: Comparison of isometric muscle training and electrical stimulation supplementing isometric muscle training in the recovery after major knee ligament surgery. Am J Sports Med 7:169-171, 1979

6. Munsat TL, McNeal D, Walters R: Effects of nerve stimulation on human muscle. Arch Neurol 33:608-617, 1976

7. Massey BH, Nelson RC, Sharkey BC, et al: Effects of high frequency electrical stimulation on the size and strength of skeletal muscle. J Sports Med Phys Fitness 5:136-144, 1965

8. Ericksson E: Sports injuries of the knee ligaments: Their diagnosis, treatment, rehabilitation, and prevention. Med Sci Sports 8:133-144, 1976

9. Babkin D, Timtsenko N (eds): Notes from Dr. Kots' (USSR) lectures and laboratory periods, Canadian-Soviet exchange symposium on electrostimulation of skeletal muscles. Con­cordia University, Montreal, Quebec, Canada, December 6 -15, 1977

10. Cummings G: Physiological basis of electrical stimulation in skeletal muscle. Certified Athletic Trainers Association Jour­nal 3:7-12, 1980

11. Currier DP, Lehman J, Lightfoot P: Electrical stimulation in exercise of the quadriceps femoris muscle. Phys Ther 59:1508-1512, 1979

12. Knuttgen HG (ed): Neuromuscular Mechanisms for Thera­peutic and Conditioning Exercise. Baltimore, MD, University Park Press, 1976, p 107

13. Dayton CM: The Design of Educational Experiments. New York, NY, McGraw-Hill Book Co, 1970, pp 22 -63 , 304-341

14. Rasch PH, Morehouse LE: Effect of static and dynamic exercises on muscular strength and hypertrophy. J Appl Physiol 11:29-34, 1957

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16. Sugai N, Warsley R, Payne JP: Tetanic force development of adductor pollicis muscle in anesthetized man. J Appl Physiol 39:714-717, 1975

17. Brobeck JR (ed): Best & Taylor's Physiological Basis of Medical Practice, ed 10. Baltimore, MD, The Williams & Wilkins Co, 1979, pp 44 -77

18. Kovacs R: Electrotherapy and Light Therapy, ed 6. Philadel­phia, PA, Lea & Febiger, pp 98-100, 1950

19. Bures J, Petran M, Zachar J: Electrophysiological Methods in Biological Research, ed 3. New York, NY, Academic Press Inc. 1967, pp 412-414

20. Edwards RHT, Young A, Hosking GP, et al: Human skeletal muscle function: Description of tests and normal values. Clin Sci Mol Med 52:283-290, 1977

21 . de Vries HA: Physiology of Exercise: For Physical Education and Athletics, ed 3. Dubuque, IA, Wm C Brown, 1980, p 392

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