Torque Production in HumanMuscles with Voluntary andContractions

Upper and Lower LimbElectrically Stimulated

Two studies are reported in which the elbowflexor and extensor muscle groups and thequadriceps femoris muscle group in fifteen nor­mal female subjects were tested under volun­tary and electrically stimulated conditions. Thetorque produced during a maximum voluntarycontraction (MVC) at each of six pre-determinedjoint angles was compared to the torque pro­duced in maximum tolerated contractions (MTC)by two types of electrical stim,tJlation (conven­tional interferential and hig11 voltage stimula­tion). Results indicated a significant difference(p < 0.01) between the mean torque values pro­duced by the MVC at all angles tested comparedto the MTC. At the most favourable angle forproducing an MTC, a mean torque of between45 and 55% of an MVC for the elbow flexor andextensor muscles, and 65 to 74% for the quad­riceps femoris muscle may be expected fromboth the high voltage and interferential stimu­lators.

GEOFFREY R. STRAUSS

Geoffrey Strauss, M.P.E., is a Lecturer in the Schoolof Physiotherapy at the Western Australian Instituteof Technology

GIOVANNI DE DOMENICO

Giovanni De Domenico, M.Sc., M.C.S.P., Dip. T.P., isa Senior Lecturer in the School of Physiotherapy atthe Western Australian Institute of Technology

This paper stems from an on-gOing research programme on muscle stimulationwhich IS being undertaken In the Centre for Applied Research In ExerCiseSCience and Rehabilitation at the Western Australian Institute of Technology

Electro-motor stimulation (EMS) hasbeen used in the rehabilitation ofmovement disorders for many decades.Earlier views suggesting that suchstimulation was not useful in strength­ening muscle have been challenged bya number of recent studies (Halbachand Straus 1980, Kramer and Mendryk1982, Laughman et at 1983). However,the resurgence of interest in EMS tostrengthen muscle has been accom­panied by minimal investigation into

the optimal electrical and physiologicalparameters needed to achieve these ef­fects. The electrical stimulation para­meters include the shape, frequency,duration and intensity of the pulse usedfor stimulation, and the type, size andplacement of the electrodes. Althougha few studies (Nelson et at 1980, Mor­eno-Aranda and Seireg 1981a, 1981b,Owens and Malone 1983, Kramer et al1984, Walmsley et af 1984) have in­vestigated some of these parameters,

at present no clear conclusions can bedrawn about the optimal stimulationparameters.

The physiological parameters in­clude the maximum level of torque pro­duced by a stimulator, the percentageof torque that should be used in thestimulation training programme, theeffect of angle on torque productionand variations in torque productionwith different stimulators. Anotherconsideration is the training protocol

38 The Australian Journal of PhYSiotherapy. Vol. 32, No.1, 1986

Voluntary and Electrically Induced Torques

Table 1:Summary of knee joint angles used in previous studies of knee extensortorque production.

AUTHORS ANGLE OFKNEEFLEXION(degrees)

Boutelle, Smlth ond Molone (1965) 30McMiken et 01 (1983) 30

Owens ond Molone (1963l 35

Currier, Lehman and lightfoot (1979) 60Nelson et 81 (1980) 60lOlney, Walmsley and Andrew (1983) 60Singer et 01 (1963) 60Currler and Monn (1963) 60loughmon et 01 (1983) 60Kromer et 01 (1964) 60Walmsley, letts and Vooys (1984) 60Selkowltz (1985) 60Mohr et 01 (1985) 60

Romero et 01 (1982) 65

JOhnson, Thurston and Ashcroft (1977) 90Od18 (1982) 90

which includes the frequency,durationand intensity as well as the progressionof training and the individual's re­sponse to training. These parametersrequire vigorous investigation.

Controversy surrounds the relativestrength of contractions developed un­der electrical stimulation compared tovoluntary effort and continues to be asource of much debate (Lloyd et al1986). The work of Kots and Chuilon(1975) suggested that significantlyhigher peak torques could be achievedwith EMS compared to a voluntaryeffort, in the elbow flexor and othermuscle groups. However, these resultshave not been replicated elsewhere; in­deed there is a preponderance of stud­ies on lower limb muscles, particularlythe quadriceps femoris muscle group.Apart from the studies of Ikai, Yabeand Iischii (1967) and Kots and Chui­Ion (1975) very little is known aboutthe effects of stimulation on musclesof the upper limb. In fact no majorstudy of the elbow flexors response toEMS could be found, despite this mus­cle group being one of the groups inwhich Kots and Chuilon reportedgreater EMS induced torques than vol­untary effort.

The theoretical basis for the resultsclaimed by the Soviet researchers sug­gesting that the maximum toleratedcontraction (MTC) should be capableof reaching levels of torque equal to,or greater than the maximum voluntarycontraction (MVC), have been pro­posed by other authors. The work ofSimonson (1971), Fisher and Jensen(1972) and Pinelli (1974) suggests thatthe number of operative motor unitsin a voluntary contraction is between60 and 90 percent of the maximumnumber of motor units (60 to 70 percent in untrained individuals and up to90 percent in trained individuals). Thisshortfall in force production or forcedeficit has been termed the functionalreserve. The question of whether it ispossible to generate a greater torqueusing EMS than by voluntary efforthas therefore not been definitively an­swered.

Few studies have compared torqueproduced by voluntary and electricallystimulated contractions at a variety ofangles across joint range. For example,in isometric testing and training of thequadriceps femoris muscle group moststudies have chosen a single joint angle;predominantly 60 degrees of knee flex­ion (Table 1). Although this angle mayapproximate the otimal muscle lengthfor maximum torque production, itdoes not necessarily represent the samemuscle length in each subject since theabsolute joint range may be different.

For the reasons mentioned above itwas decided to investigate the torqueproduction in upper and lower limbmuscles with electrically stimulated and

voluntary contractions. Three mainareas were investigated:(1) the response to stimulation of two

large opposing muscle groups inthe upper limb (elbow flexors andextensors) and a large muscle group(quadriceps femoris) in the lowerlimb;

(2) the effect of muscle length (per­centage of joint range) on the levelsof torque produced by EMS com­pared with voluntary isometrictorque production; and

(3) the level of torque produced by twotypes of electrical stimulation,compared to each other and to theMVC.

The Australian Journal of PhYSiotherapy. Vol. 32, No.1, 1986 39

Voluntary and Electrically Induced Torques

Study Number OneMethodsSubjects

Fifteen normal female subjects whowere unused to electrical stimulationparticipated in the study. Each subjecttook part in a familiarization sessionone week prior to the study. Duringthis session each subject was stimulatedto produce a maximum tolerated con­traction (MTC) for both muscle groupsat pre-determined angles in the rangeof motion. In addition, each subjectattempted maximum voluntary con­tractions (MVC) for both musclegroups across the range of motion.

During the experiment, each subjectwas comfortably positioned in supinelYing with the upper arm and shoulderin the anatomical position to allow amovement at the elbow joint througha range of approximately 165 degreeswith the forearm supinated. The non­dominant limb was used for t~ting

purposes and in fourteen subjects thiswas the left arm. Figure 1 illustratesthe experimental situation.

EquipmentRange of movement and torque de­

velopment in the elbow flexors and ex­tensors was measured with the KineticCommunicator (Kin-Com), a roboticdynamometer. The computer associ­ated with the Kin-Com measures andstores values of instantaneous torque(in Newton meters) and angle (in de­grees) at a rate of 100 sampij:s persecond.

Two types of electrical stimulatorwere used, representing the concepts ofconventional interferential (IF) andhigh voltage (HV) Stimulation.

Interferential StimulatorAn Erbe IM-l conventional interfer­

ential stimulator (Electromedizin,GmbH, West Germany), having a car­rier frequency of 5000 Hz, modulatedto a beat frequency varYing from 50to 80 Hz in rapid rotation (approxi­mately 4.0 seconds per complete ro­tation, ie 50 to 80 and back to 50 Hzagain) was used. A pre-modulated out-

put was selected with the currents being'mixed' in the stimulator before deliv­ery to the subject.

High Voltage StimulatorAn Intellect 500 high voltage stim­

ulator manufactured by the Chatta­nooga Corporation of the U.S.A. wasused to provide this type of stimula­tion. A continuous output with a flXedfrequency of approximately 65 Hz wasapplied to the electrodes. The stimulusparameters of both stimulators areshown in Figure 2.

ElectrodesPreliminary investigations indicated

that the same electrode placement wasnecessary for both stimulators, in orderto avoid changing the electrode/skininterface. For this purpose a standardconductive rubber electrode (80 mm by140 mm) was attached to the skin over­lYing the scapular region. The electrodewas placed inside a moistened spongeenvelope measuring 130 mm by 150

r'-------,--_[~ 0 MP UTE R

INTERFERENTIALGENERATOR

SUBJECT

Figure 1: Diagrammatic view of a subject in the experimental positi.on !or testingof the elbow flexor and extensor muscles. The bar across the subject s forearmrepresents the pad attachment of the Kin-Com which registers torque.

40 The Australian Journal of Physiotherapy. Vol. 32, No.1, 1986

A

Figure 2: Stim.ulus para~eters of ~heconventional Interferentlal and highvoltage stimulators.In (A) the interferential stimulus was amedium frequency sine wave with apulse duration of 100 microseconds a~da carrier frequency of 5000 Hz. The sig­nal was amplitude modulated to a beat­frequency varying from 50 to 80 Hz andback over 4 seconds. .In (B), the high voltaQe pulse had a t~mmonophasic spike with a pulse durationof 100 microseconds at the base. Afixed frequency of apprOXimately 65 Hzwas used.

Voluntary and Electrically Induced Torques

30%

50%

to terminate the current flow at thepoint of maximum tolerance of thestimulation. Thus the experimenter in­itiated the contraction and determinedits rate of increase, but the subjectdetermined the final level of currentflow and therefore the strength of thecontraction.

A single contraction was producedor elicited at each of the six angles forthe six contraction conditions. Eachcontraction was sustained for a mini­mum of two seconds and a maximumof four seconds, with a force of greaterthan 20 Newtons applied directly onthe pad attachment. If this level offorce application was unable to be at­tained, a missing value in the data wasregistered and that subject's resultssubsequently omitted from the analy­sis. One practice trial was given foreach condition at a single joint angle.In order to minimize fatigue, testingconsisted of a single maximum con­traction of stimulation at each of thesix joint angles. Each contraction wasdeveloped and maintained over a periodof four seconds and 10 seconds restallowed between contractions at the sixjoint angles.

Maximum tolerable contraction lev­els were achieved by providing the sub­ject with a switch which allowed them

(i) MVC for the elbow flexors(ii) MVC for the elbow extensors(iii) MTC with interferential stimula-

tion for the elbow flexors(iv) MTC with interferential stimula­

tion for the elbow extensors(v) MTC with high voltage stimula­

tion for the elbow flexors(vi) MTC with high voltage stimula­

tion for the elbow extensors.

occurred were expressed as a percent­age of the active range with 0070 beingfull elbow extension, or the shortestlength of the elbow extensor musclesin the available joint range (Figure 3).(Joint torque for the elbow flexors wasmeasured at the same joint angles, orfrom 30 to 80070 of the flexors rangewhere 0070 was the shortest length ofthe elbow flexors in the available jointrange.)

Torque was produced at each of thesix angles in each of six randomly pre­sented conditons. These conditionswere:

Figure 3: Test ranges for the elbow flexor and extensor muscles. Active rangewas defined as 1000/0 with the fully extended position representing 0% (shortestextensor muscle length).

----------~~~~---------==::=-£~1-- 00/0

Experimental ProtocolEach subject was positioned as pre­

viously described anti the active rangeof motion for the elbow flexors andextensors was then determined. Theweight of the forearm and hand wasdetermined by Kin-Com at an angle of45 degrees from full elbow extension.This enabled all raw data to be con­verted to gravity compensated data inthe report programmes of the Kin­Com.

The pad attachment on the Kin-Comlever arm was positioned at a distanceof 200 nun from the elbow axis ofrotation and rested comfortably on theforearm. The large size of the attach­ment did not allow the forearm to beflexed to an angle of 80070 of rangefrom full extension and the elbow wastherefore not fully flexed. This allowedan experimental range of 20 to 70070 ofeach subject's active range at the el­bow. It was decided that a comparisonof elbow flexor and extensor torquesat particular joint angles was most de­sirable. Therefore the six equally spacedangles at which contractions of boththe elbow flexors and extensors

mm. This electrode sewed as the 're­turn' pathway for stimulation of eitherthe elbow flexors or elbow extensors.Separate stimulating electrodes (one forbiceps, one for triceps, each of size 35nun by 90 mm) were attached directlyover the belly of these muscles. Eachelectrode was also inserted into a mois­tened sponge envelope (70 mm by 135mm). Of the three electrodes attachedto the arm, only two at a time wereused, stimulating either the elbow flexoror elbow extensor muscle groups. Nospecific skin preparation was used otherthan the moistening of the surface fromthe electrode sponges, which had beenimmersed in a hot 5070 Savlon solution.The electrodes were covered by a plas­tic sheet to maintain a warm moistenvironment for stimulation. The plas­tic sheet and electrodes were then heldin place by Nylatex Straps (Chatta­nooga Corporation).

The Australian Journal of PhYSiotherapy. Vol 32, No.1, 1986 41

Voluntary and Electrically Induced Torques

The maximum value (gravity com­pensated) of the torques at eacn"angle,for the elbow flexors and extensors forall contraction conditions, was re­corded from the numeric list of datadisplayed on the computer screen. Themaximum torques were then enteredinto a datafile for statistical analysis(descriptive statistics, 3-way analysis ofvariance (ANOVA) with repeatedmeasures) (Winer 1971).

ResultsTable 2 and Figures 4, 5 and 6 show

the mean torque produced at each jointangle under the three conditions stud­ied for the elbow flexors and extensors.Table 2 and Figure 4 give the meanvalues for maximum torque, while Fig­ures 5 and 6 express the mean valuesof the maximum torque for the twotypes of stimulation relative to the

torque developed by the elbow flexorsand extensors during the voluntaryeffort condition. It is clear from thesefigures that the MTC under both formsof electrical stimulation was consider­ably less than that achieved voluntarilyfor both the elbow flexors and exten­sors.

Analysis of variance was conductedon the 11 subjects with complete data~

(Table 3). The three factors were mus­cle group, contraction condition andangle (muscle length).

The results may be summarized asfollows:

There was no 3-way interaction be­tween factors, and no 2-way interac­tion between angle and contractioncondition. However there was:(1) a significant interaction between

muscle group and angle (p < 0.01).That is, the torque for the elbowflexors increases with angle while

the torque produced by the elbowextensors decreases with angle. Ateach angle, the torque developedby the elbow flexor muscle groupwas significantly greater than thetorque produced by the elbow ex­tensor muscle group, whether pro­duced under voluntary or stimu­lation conditions (see Figures 4, 5and 6).

(2) a significant interaction betweenmuscle group and the contractionconditions (p < 0.01). That is, ateach angle there were significantdifferences between the level oftorque produced under voluntaryand stimulated conditions. Thelargest difference occurred betweenthe voluntary and electrical stimu­lation conditions (p < 0.01). Therewere no significant differences be­tween the interferential and high­voltage conditions.

Table 2:Summary table of the elbow flexor and extensor torques at each joint angle for the three contractionconditions.

ANGLE (0% is defined as full elbow extension)20% 30% 40% 50% 60% 70%

ELBOW FLEXORS

MVC Mean 24.74 28.86 32.47 33.31 32.59 31.23S.D. 4.30 5.28 4.26 3.69 4.02 3.35S.E. 1.30 1.59 1.28 1.11 1.21 1.01

MTC from IF Mean 9.16 12.03 13.72 15.73 14.11 13.52S.D. 3.33 3.65 3.68 2.69 3.01 2.66S.E. 1.00 1.10 1.11 0.81 0.91 0.80

MTC from HV Mean 8.36 11.82 12.98 13.53 13.27 14.34S.D. 3.60 3.20 4.26 4.99 4.77 3.96S.E. 1.09 0.96 1.28 1.50 1.44 1.19

ELBOW EXTENSORS

MVC Mean 19.43 17.66 18.00 18.06 17.94 16.86S.D. 2.37 3.04 2.74 2.95 2.52 2.41S.E. 0.72 0.92 0.82 0.89 0.76 0.73

MTC from IF Mean 11.84 10.80 10.67 10.61 8.90 8.34S.D. 3.09 2.81 3.27 3.17 2.46 2.57S.E. 0.93 0.85 0.99 0.95 0.74 0.77

MTC from HV Mean 11.05 10.75 10.37 10.00 8.88 7.39S.D. 2.91 3.19 3.08 2.72 2.52 2.19S.E. 0.88 0.96 0.93 0.82 0.76 0.66

42 The Australian Journal of Physiotherapy. Vol. 32, No.1, 1986

Voluntary and Electrically Induced Torques

Table 3:Analysis of variance SUmmary table.

SOURCE OF SS MS F SIG

CONTRACT ION 2 334193250 167096625CONDITION (CC) 20 27150000 1357500 123091 p <001

MUSCLE 1 80869500 80869500GROUP (NG) 10 17535750 1753575 46 117 P <P 01

ANGLE (A) 5 7162750 143255050 6935000 138700 10328 P <001

(CC) by (MG) 2 48041250 2402062520 12447500 622375 38595 P <001

(CC) by (A) 10 1348250 134825100 7418500 74185 1817 NS

(MG) by (A) 5 20733500 414670050 5668500 113370 36577 p <001

(CC) by (NG) 10 1247750 124775by (A) 100 6116750 61 167 2040 NS

Table 4:Summary table of the knee extensor torques at each muscle length forthe three contraction conditions.

ANGLE (OX 1S defined as full knee extens1on)

20:1 I 30:1 I 40:1 I 50:1 I 60:1 I 70~

KNEE EXTENSOR TORQUES

MVC MEAN 892 1163 1429 159 1 1625 1549SO 220 235 255 220 273 309S.E 57 6 1 66 57 70 80

MTC from IF MEAN 592 823 1057 1057 1046 835SD 19.7 25 1 360 438 563 457SE 5 1 65 93 113 145 118

MTC from HV MEAN 477 72 1 934 889 764 563SD 87 193 317 388 438 399SE 22 50 82 100 113 103

KNEE EXTENSOR MTC TORQUES AS A PERCENTAGE OF MVC TORQUES

IF MTC 664 708 740 664 644 539HV MTC 535 620 653 559 470 364

Study Number TwoMethodsSubjects

The same fifteen female subjects whoparticipated in the fITst study were thesubjects for the second study con­ducted one month later.

Test PositionThe subjects were seated and

strapped into position so that the non­dominant limb adopted the positionindicated in Figure 7. A hip angle ofapproximately 105 degrees wasachieved by reclining the subject at 45degrees to the vertical and elevating thethigh to 30 degrees from the horizon­tal. For further stability during testing,the pelvis was restrained by two web­bing straps.

Once the subject had been appro­priately positioned, the active range ofmotion for the knee extensors was de­termined. The weight of the leg andfoot was determined by Kin-Com atan angle of 45 degrees from full kneeextension. This enabled all raw data tobe converted to gravity compensateddata in the report programs of the Kin­Com.

The pad attachment on the Kin-Comlever arm was positioned comfortablyon the leg a distance of 270 mm fromthe knee joint axis of rotation. To beconsistent with the previous study, theexperimental range was defined as sixequally spaced angles in the active rangeof knee flexion. These angles were 20,30, 40, 50, 60, and 70070 of active kneeflexion where 0070 is the shortest lengthof the knee extensors or full knee ex­tension. Figure 8 illustrates the rangeof motion tested. One contraction wasproduced or elicited at each of the sixangles for the three contraction con­ditions.

Each contraction was sustained, witha force of greater than 20 Newtonsapplied by the leg directly on the padattachment of the Kin-Com, for a min­imum of two seconds and a maximumof four seconds. In all subjects thisminimum level of force application wasable to be attained.

The Australian Journal of Physiotherapy. Vol. 32, No.1, 1986 43

Voluntary and Electrically Induced Torques

Figure 4: Mean values for elbow flexor and extensor muscle torque production,at each angle tested.

Table 5:Analysis of variance summary table.

SOURCE DF 55 MS F SIG

CONTRACTION 2 2791236 1395618000CONDITION 28 1097684 39203000 35600 p <001

ANGLE 5 1247232 24944640670 612812 8754457 28494 P <001

CONTRACT ION 10 306104 30610400CONDITION BV ANGLE 140 696016 4971543 6 157 p <001

30 40 50 60 70% OF ELBO'w' EXTENSOR RANGE

II ELBOW FLEXORS

II ELBOw' EXTENSORS

the voluntary effort condition. It isclear from Table 4 and Figures 10 and11 that the MTC under both forms ofelectrical stimulation was considerablyless than that achieved voluntarily.

A two factor ANOVA was con­ducted on the 15 subjects (Table 5).The two factors were contraction con­dition and angle (muscle length).

The results showed that there wasa significant 2-way interaction be­tween angle and contraction condition(p < 0.01). This means that the angleat which maximum torque was devel­oped was different for the contractionconditions, with maximum torque

40

30

MEAN t'1'v'(: 20TORQUE (Nrn)

10

020

ResultsTable 4 and Figures 10 and 11 show

the mean torque produced at each jointangle under the three contraction oon­ditions studied for the knee extensors.Table 4 presents the mean absolute val­ues for maximum torque and also ex­presses the mean values of the electri­cally stimulated contraction conditionsrelative to the torque developed during

computer screen. The maximumtorques were then entered into a da­tafHe for statistical analysis (descriptivestatistics, 2-way analysis of variancewith repeated measures) (Winer 1971).

The maximum value (gravity com­pensated) of the torques at each angle,for the knee extensors for all contrac­tion conditions, was recorded from thenumeric list of data displayed on the

EquipmentThe equipment used in this study was

identical to that used in the previousexperiment.

Experimental ProtocolEach subject was randomly assigned

to the three contraction conditions andtested at each angle of knee flexion.The three conditions were:(i) maximum voluntary isometric

contraction,(ii) MTC with interferential stimula­

tion,(iii) MTC with high voltage stimula­

tion.

Electrode ConfigurationPreliminary studies indicated that the

conventional electrodes supplied witheach of the two stimulators did notallow a strong comfortable muscle con­traction. In order to overcome thisproblem two special electrodes were de­veloped. The system adopted allowedmaximum stimulation of the musclewhile remaining comfortable for thesubject. Details of the electrodes areshown in Figure 9.

Each electrode consisted of an ex­ternal sponge envelope of the size andshape indicated. The envelope con­tained a metal foil electrode approximately one centimetre smaller than thesponge jacket. Each sponge envelopewas soaked in a 5070 Savlon solutiqnand hot water. No specific skin prep­aration was used other than the use ofthe warm sponge envelopes on the skin.The electrode was covered by a thicklayer of plastic in ordpr to keep thearea warm and moist. Each electrodewas strapped on to the thigh usingNylex Straps (Chattanooga Corpora­tion) in the position indicated in Figure9B. This enabled the greater part ofthe muscle to be affected by the stimu­lation.

44 The Australian Journal of PhYSiotherapy. Vol. 32, No.1, 1986

Voluntary and Electrically Induced Torques

Figure 6: Mean torque values for maximum tolerable contractions produced byhigh voltage (HV) stimulation of the elbow flexors (EF's) and extensors (EE's).Results are expressed as a percentage of the MVC at each test angle.

Figure 5: Mean torque values for m~lmum tolerable contractions produced byinterterential (IF) stimulation of the elbow flexors (EF's) and extensors (EE's).Results are expressed as a percentage of the MVC at each test angle.

DiscussionIn the introduction to this paper,

three areas for investigation were de­scribed. This discussion will focus onthe experimental results as they relateto these three areas.

The response to stimulation of upperand lower limb muscles

Both studies have shown signifi­cantly greater torque production in thevoluntary condition compared with thetwo modes of electrical stimulation.These findings do not support the re­sults of Kots and Chuilon (1975) butare in agreement with results from otherinvestigators (Kramer et af 1984,Walmsley et af 1984).

Several arguments may be advancedto account for the observed findingthat EMS produces lower levels oftorque than voluntary effort. Some ofthese arguments are physiological andothers mechanical. With respect totorque production in the voluntarycondition, the influence of other mus­cles acting as stabilizers cannot be ig­nored. Muscles working to stabilize theshoulder act to increase the effectiveelbow flexion and extension torquesthrough better fIXation of the proximalattachments of these muscles. Furthercontributions to the MVC torque pro­duction during elbow flexion and ex­tension come from the wrist flexorsand extensors, respectively. In the caseof the quadriceps femoris muscle, kneeextension was facilitated by the activityof a variety of hip and trunk muscles.

achieved at angles closer to full kneeextension for the stimulated contrac­tions (40070 of active range) and furtherinto flexion in the voluntary condition(60070 of range). At each angle therewere significant differences between thelevel of torque produced under vol­untary and stimulated conditions. Thelargest difference occurred between thevoluntary and electrical stimulationconditions (p < 0.01). There were nosignificant differences between the in­terferential and high-voltage condi­tions.

II IF FOR EF'S

III IF FOR EE'S

• HV FOR EF'S

.. HV FOR EE'S

30 40 50 60 70% OF ELBO'w' EXTENSOR RANGE

30 40 50 60 70% OF ELBOW EXTENSOF~ RANGE

70

60

50MEAN

PERCENT A(,E 40OF VOLUNTARY

TORQUE (%) 30

20

10

020

70

60

50t"1EAN

PERCENT AGE 40OF

VOLUNTARY 30TnRClllF (ql;)

20

10

020

The Australian Journal of Physiotherapy. Vol. 32, No.1, 1986 45

Voluntary and Electrically Induced Torques

20%

of its location, size and ease of stimu­lation. There are, however, some dis­advantages in using this muscle group.In order to produce high torque values,the posture and stabilization of thesubject is of considerable importance(Hart et at 1984). Although the quad­riceps group is the prime knee extensor,contamination from other muscles suchas the sartorius is still possible.

The overall effect of the problemswith the MVC and MTC is that theMVC torque is probably artificially in­flated while the MTC torque is lowerthan might otherwise be the case. If itwere possible to correct these prob­lems, it may well be that the differencebetween the MVC and the MTC be­come less distinct.

The effect of muscle lengtb (percentageof joint range) on tbe levels of torqueproduced by EMS compared to vol­untary effort

The results from both studies showa significant angle effect for all musclegroups tested, and highlight the im­portance of muscle length. The abso­lute angle of joint motion or range isnot the important factor, but rather,the effective muscle length in thosemuscles contracting or responding toelectrical stimulation. With respect tothe knee extensor muscles, previousstudies have measured knee extensorforce or torque at a variety of jointangles (Table 1). These angles in somecases appear to be arbitrarily chosen.There is an obvious logic in choosinga specific absolute angle approximatingthe angle at which peak torque is pro­duced (Smidt 1973, Lindh 1979). Forexample, an angle of 60 degrees fromfull knee extension does not necessarilyimply the same muscle length (relativeto the resting muscle length) for eachperson. For a maximum voluntary iso­metric contraction this angle has beenshown to be around 60 to 70 degreesof knee flexion (Knapik et af 1983).Others have chosen to measure a peaktorque in an isokinetic contraction(Godfrey et at 1979, Halbach andStraus 1980).

40%

50%

often a contraction of the wrist exten­sors, resulting from stimulation of theradial nerve. The net result of this un­controlled muscle activity could havebeen a decrease in the effective elbowflexion or extension torque.

Many muscle groups of the bodypresent the experimenter with consid­erable difficulties in terms of the iso­lation of both MVC and MTC to thedesired muscles. It is perhaps largelyfor these reasons that the quadricepsfemoris muscle has been used by mostauthors to study the effects of electricalstimulation. This group is the only ef­fective extensor of the knee and offersthe experimenter advantages because

60%

70%100%

Figure 7: A diagrammatic view of a subject seated in the experimental positionfor testing of the quadriceps femoris muscle group.

//

In other words, the maximal voluntarycontraction torque is not produced bya single muscle group. If it was possibleto isolate a maximum contraction to asingle muscle or group, the resultingtorque production would be apprecia­bly less than observed in the usual formof MVC.

In stimulating the elbow flexor andextensor muscles, it proved extremelydifficult to localize the contraction tothe desired muscles. When stimulatingthe elbow flexors, current overflow tothe median and ulnar nerves sometimesresulted in unpredictable activity in thewrist flexors. Similarly, when stimu­lating the elbow extensors there was

Figure 8: Test range for the quadriceps femoris muscle group.

46 The Australian Journal of Physiotherapy. Vol. 32, No.1, 1986

Voluntary and Electrically Induced Torques

Figure 9: Electrode placement for the stimulation of the quadriceps femorismuscle group. Each metal electrode was approximately one centimetre smallerthan the dimensions given above for the sponge envelopes. Moistened envelopeswere positioned on the thigh as indicated in B (De Domenico and Strauss 1985).

contributed to the lower absolutetorque levels from this stimulator.However, these differences were notsignificant. There were no differencesbetween the perceived comfort ofstimulation produced by the two stim­ulators.

There are other reasons which mayaccount for the lower torque valuesgenerated in an MTC. It is possiblethat the type of electrical signal usedto stimulate the MTC was inappro­priate, since the optimal stimulatingparameters have not been defined.Other types of stimulation may wellproduce much larger muscle torques.

During normal physiological musclecontraction, the smaller slow-twitch(ST or Type I) units are recruited firstand their activity corresponds to lowlevels of tension production. Theseunits are fatigue resistant and capableof maintaining low levels of tensionfor long periods. As more tension isrequired, the fatigue resistant fast­twitch (Fra or Type Ila) fibers aregradually recruited. High tension levelsare produced with the addition of hightension-producing, fast fatiguable, fasttwitch (FIb or Type lIb) units. Thushigh muscle tension is produced bylarge fast fatiguing motor unit activitybuilt upon a background of tensionproduced by smaller units (Burke 1980).

The manner in which the intensityof the stimulus is controlled may affectthe recruitment pattern of motor units.Solomonow et al (1983) suggest thatthe control of stimulus intensity inorder to modulate tension may not beparticularly effective. The larger motorunits have a lower threshold to elec­trical stimulation (petrofsky 1978, Pe­trofsky and Phillips 1981) and it isreasonable to assume that these motorunits would be activated first by theapplication of an electrical stimulus ofhigh intensity.

In the experiments reported here, thestimulus intensity was increased rapidlyto maximum tolerance within a twosecond period and then held constantfor a further two seconds. Such a pro­tocol may elicit an altered recruitment

B

Torque production by two types ofelectrical stimulator compared to eachother and the MVC

Both studies showed that there wasno significant difference in the level oftorque production between the typesof electrical stimulation. Some subjectswere able to tolerate the full output ofthe high voltage stimulator when elic­iting an MTC from the quadriceps fe­moris muscle group and this may have

140 mm

A

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~---+---~ 1 20 mm

200 mnl

170 mm

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There is some justification for meas­uring the MVC or MTC in the appro­priate limb position which produces thegreatest torque. This position need notnecessarily be the same for measuringthe MVC and the MTC. The MTCshould then be compared with theangle where the MVC is maximal andnot at an angle where the MVC is lowerin absolute terms than its maximumcapability.

120 mm

The Australian Journal of Physiotherapy. Vol. 32, No.1, 1986 47

Voluntary and Electrically Induced Torques

Figure 10: Mean values for voluntary and electrically stimulated torques(IF = interferential, HV = high voltage) from the quadriceps femoris musclegroup.

Figure 11: Mean torque values for maximum tolerable contractions in the quad­riceps femoris muscle group. Results are expressed as a percentage of the MVCat each joint angle tested.

pattern with consequent reduction inthe MTC, perhaps because of the de­layed tension production by the slow­twitch units and earlier fatigue by thefast-twitch units.

Solomonow et oJ (1983) suggested analternate method of controlling muscletension produced by EMS. They ad­vocate the use of two different typesof stimulation to the appropriate mo­tor nerve. One signal produces stimu­lation of all motor nerves and thusproduces high tensions of tetanic mus­cle contraction; the force of the musclecontraction is then modulated by theapplication of a second 'high' fre­quency signal (500 Hz) to the samenerve at a more distal location. Thismethod produced effective control overmuscle tension in cat muscles and mayhave some implications for the stimu­lation of human muscles.

Furthermore, over-the-skin stimula­tion (TENS) may not be the bestmethod for achieving high muscle ten­sions. Direct stimulation of caninemuscles has produced torque levels ap­proximately seven times higher thansurface stimulation (Moreno-Arandaand Seireg 1981b). Similar results mightbe possible in human subjects.

While experiments comparing torqueproduction in an MTC are unlikely toshow that EMS can produce highermuscle torque than can be producedvoluntarily, they may be of value inworking towards defining optimalstimulation parameters. While thespecific stimulus characteristics of theequipment are extremely important, theindividual responses to stimulationmust also be recognized. This study,and many others, show a considerablevariation in torque production fromelectrical stimulation. It may thereforenot be appropriate to define a singleset of stimulus parameters. It is pos­sible, indeed highly likely, that eachindividual has slightly different optimalstimulation parameters.

At the present time little is knownabout the optimal stimulating para­meters as is evidenced by the greatvariety of stimulation types used by

~-

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III '-HC FPOf1 HII'

• IF FOR KE'S

.. HV fOR KE'S

30 40 50 bO 70%~~~NEE EXTH60R RANC,E

30 40 50 60 70% OF KNEE EXTENSOR OF RANGE

80

70

60

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VOLUNTAF~'y'

TORQUE (%)30

20

10

020

180

160

140

120

100f1EAN TO~'QUE lJ~m)

80

60

40

20

(I

20

48 The Australian Journal of PhySIotherapy. Vol. 32, No 11 1986

Voluntary and Electrically Induced Torques

researchers. In most cases the choiceof stimulus parameters has been basedmore on the availability of the stu­mulator and less on an objective anal­ysis of the stimulus characteristics. Thisarea should be the prime focus forresearch into the wider topic of theclinical use of EMS.

In summary, the results from the twoexperiments reported here show a sig­nificant difference between the MVCproduced in the elbow flexors and ex­tensors, and the MTC produced byinterferential or high voltage stimula­tion. Similar differences were seen withthe quadriceps femoris muscle group.Interferential and high voltage stimu­lation were equally effective in prod­ucting muscle contraction in the mus­cles tested. Given the experimentalprotocol described in the text, the meantorque generated by the two types ofstimulation, were in the region of 45to 55070 of the MVC for the ..elbowmuscles and 65 to 74070 of the MVCfor the quadriceps femoris musclegroup. Several important factors mayhave combined to produce an MTCwhich was significantly less than thecorresponding MVC. Further researchis urgently needed, particularly directedtowards defining the range of optimalstimulation parameters.Acknowledgements

The authors wish to thank all of thesubjects who participated in the twostudies. Special thanks are extended toJoy Reymond for invaluable assistancewith the statistical analysis.

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The Australian Journal of PhySiotherapy. Vol 32, No.1, 1986 49