the effect of muscle fatigue on muscle force-couple activation of

8/2/2019 The Effect of Muscle Fatigue on Muscle Force-Couple Activation Of

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Te College at Brockport: State University of New York

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Kinesiology, Sport Studies and Physical Education School of Health & Human Performance

1-1-2001

e Eect of Muscle Fatigue on Muscle Force-Couple Activation of the Shoulder

Timothy J. HenryTe College at Brockport, [email protected]

Sco M. LephartUniversity of Pisburgh

Jorge GiraldoUniversity of Pisburgh

David StoneUniversity of Pisburgh

Freddie H. FuUniversity of Pisburgh

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Recommended CitationHenry, Timothy J.; Lephart, Sco M.; Giraldo, Jorge; Stone, David; and Fu, Freddie H., "e Eect of Muscle Fatigue on MuscleForce-Couple Activation of the Shoulder" (2001).Kinesiology, Sport Studies and Physical Education. Paper 30.hp://digitalcommons.brockport.edu/pes_facpub/30
http://digitalcommons.brockport.edu/http://digitalcommons.brockport.edu/pes_facpubhttp://digitalcommons.brockport.edu/hhphttp://digitalcommons.brockport.edu/hhphttp://digitalcommons.brockport.edu/pes_facpubhttp://digitalcommons.brockport.edu/


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The Effect of Muscle Fatigueon Muscle Force-Couple Activationof the ShoulderTimothy J. Henry, Scott M. Lephart, Jorge Giraldo,David Stone, and Freddie H. FuContext: Muscle fatigue is an important concept in regard to the muscle function ofthe shoulder join t. Its effect on the muscle force couples of the glenohum eral jo in t hasnot been fully iden tified. Objective: Jo examine the effects o f muscle fatigue on muscleforce-couple activation in the normal shoulder. Design: Pretest, posttest. Patients: Tenmale subjects, age 18 -30 years, w ith no previous history of shoulder p roblems. MainOutcome Measures: EMG (area) values were assessed for the anterior and middledeltoid, subscapularis, and infraspinatus muscles during 4 dynamic stabilizing exer-cises before and after muscle fatigue. The exercises examined were a push-up, hori-zontal abduc tion, segmental stabilization, and rotational movement on a slide board.Results: No significant d ifferences were observed for any of the muscles tested. Con-clusions: The results of our study indica te that force-coup le coac tivation of the gleno-humeral joint is not significantly altered after muscle fatigue. Key Words: shoulderrehabilitation, dynamic stabilization, shoulder EMC assessmentHenry TJ, Lephart SM , Giraldo), Stone D, Fu FH. The effect of muscle fatigue on muscle force-couple activa-tion ofth e shoulder. /Spo rt Rehabil. 2001 ; ! 0:246-256. 2001 Human Kinetics Publishers, Inc.

Functional stability in the shoulder depends on a number of factors in-cluding dynamic stabilization.'"^ Dynamic stability is the ability of themuscle force couples surrounding the glenohiomeral joint to stabilize orcenter the head of the hu m eru s on the glenoid surface. These muscle forcecouples are vital to proper kinematics and function in the shoulder com-plex because of the lack of inhere nt static stability p rov ided in the gleno-humeral joint. "* Without proper activation of these muscle force couples,episod es of functional instability can occur. M any stu dies ha ve elucidatedthe effects of irtjury on muscle-firing p at tem s in the up pe r extremity.^' Thesestudies have dem onstrated aberrations in muscle-firing pa ttem s in shoul-


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the most widely accepted definitions are offered by Winter'" and Enoka."Winter'" described muscle fatigue as occurring w he n the muscle fissue can-not support metabolism at the contractile element because of ischemia orlocal depletion of any of the metabolic substrates. Enoka" describes fa-figue as a class of acute effects that impair performance, which includesbo th an increase in the perceived effort necessary to exert a desired forceand an eventual inability to prod uce that force. A combinafion of these 2definifions might be the most appropriate way of defining fafigue.'^

It has been do cum ented th at quadriceps a nd ham string muscle fatigueresulted in an average increase in anterior fibial translation of 32.5%." Inresponse to these findings it has bee n concluded that m uscle fatigue affectsdynamic stability, alters the neuromuscular response, and might play arole in the pathom echanics of injury du ring functional acfivity. Rozzi andLephart'^ investigated the effects of muscle fatigue on knee-joint proprio-cepfion and neuromuscular control and concluded that muscle fatigueappears to affect muscle-firing pattems and might predispose both menand women to increased risk of ligamentous injury.It has been repo rted that muscle fatigue in the upp er extremity resultsin diminished proprioception and kinesthesia.'^"' Because of the dimin-ished afferent input, it is hyp othes ized that fatigue will similarly result inaberrafions in dynamic stabilization and ultimately lead to functional in-stability and predisposition to injury. Without normal activafion pattemsof the muscle force couples, glenohumeral instability might occur.'* Theeffect of fatigue on the dy nam ic stability of the sho ulde r has n ot yet beenelucidated . The objective of this study was to exam ine the effects of m usclefafigue on muscle force-couple acfivation in the normal shoulder.

MethodsTen male subjects volun teered for this study . All were healthy youn g adults,age 18-30 years, with no previou s history of shoulder p roblem s. Their me anage was 20.2 3.6 years, mea n heigh t 70.5 2.4 in, and mean w eight 173 8.8 lb . All were recreational athletes who parficipated in some type of ath-lefic activity a rrunimum of 3 times per week.InstrumentationIntramuscular fine-wire electrodes were prepared for electromyographic(EMG) analysis. Consent forms app roved by the univ ersity 's Institufional


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via a 1-1 /2-in 25-gauge hypoderm ic needle. The electrode for the subscapu-laris was inserted via a 3-1 /2-in 22-gauge spinal ne edle. Correct electrodeplacement was confirmed by a manual muscle test as described by Kendall."The wires from each muscle were attached to insulated leads an d tap ed tothe subject's body. A surface-ground electrode w as placed on the clavicleon the involved side. The signals from the leads were tran smitted u sing abattery-operated FM transmitter capable of transmitting up to 4 signalssimultaneously (Noraxon Telemyo System, Noraxon USA, Tucson, Ariz).The battery pack w as held by a research assistant in order to prevent thepack from restricting bodily movem ents.

A maximum m anual muscle test (MVC) was performed for each muscleand used for normalization and quantification of the electrical activitydu ring the exercises. The MVC was taken according to joint-position stan-dards developed by Kend all." The MVC posifion for each muscle wa s con-firmed during pilot studies before the commencement of the current in-vestigation. The EMG signal was filtered by the receiver w ith a ba nd w idthof 16-500 Hz, amplified, and reconverted from analogue to digital data.The signal was then sent to a personal com puter, where the raw EMG datawere sampled at a frequency of 2500 Hz and further analyzed with theNoraxon software. All data analysis was performed on integrated EMG data.The EMG data obtained were normalized by a maxim um voluntary iso-metric contraction during specific manual muscle testing for each muscle.The MVC was m easured for a period of 5 seconds. During normalization,a sam ple of the MVC corresponding to the exact time of the cycle for eachindividual exercise repetifion (milliseconds) was used as 100% MVC foreach exercise. Therefore, the area of the EMG obtained du rin g the MVC forthe exact time for each exercise was set at 100%.

ExercisesFour dynamic stabilizing exercises were performed before and inunedi-ately after a shoulder-fatigue protocol. Each of these exercises has beendocumented by Henry and LepharP as being valid dynamic stabilizingexercises for the shoulder complex. The exercises were as follows: (1) closedkinefic chain shoulder horizontal abductio n/ adducfion m ovem ents on theslide boa rd: Subject begins in a pu sh -up position, proceeds to horizontallyabduct both arms to touch a mark on the slide board, and retums to thestarting posifion (Figure 1); (2) closed kinefic chain shoulder-rotafion move-ments on the slide board: Subject begins in a push-up position, traces a


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Muscle Fatigue and Mu scle Force-Couple Ac tivation 249

Figure 1 Patient performing shoulder horizontal abduction/adduction movementon the slide board .

Figure 2 Patient performing closed kinetic chain rotational movements on the slideboard w ith left-arm rotational movement and right-arm segmental supporting move-ment.mean score was calculated and use d for data analysis. The order of testingfor the exercises was counterbalanced among all subjects. The entirepostfafigue test was com pleted w ithin 3 min of complefion of the fafigueprotocol. For each of the tests, the same invesfigator w as used and the pre-determ ined m arkings on the slide board rem ained constant.


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glenoh um eral joint of the test limb aligned with the axis of rotafion of thedynam om eter and his feet placed on predeterm ined markings on the fioor.The fafigue protocol was performed in a funcfional diagonal patte rn w iththe dominant arm on the Biodex. Each subject began by completing 20maximal repefifions eccentrically at 90 / s. Immediately afterward, subjectsperform ed concentric repefitions a t 120/s unfil the torqu e value of 3 con-secutive repefifions fell below 25% of the inifial peak-torque value for theD2-extension porfion of the diagonal pattern. The number of concentricrepetitions performed in orde r to induce fatigue did no t exceed 50 repefi-fions for any of the subjects.Data ReductionThe area of the M VC for the exact du ratio n of each of the 6 exercises w ascompared with the area of the EMG for each of the 4 muscles. This wascompared with 25% of the MVC for each muscle. This percentage servedas the nvirumum amount of muscle acfivity requ ired in order for that pa r-ticular muscle to provide stabilizafion for the humerus.Paired t tests were conducted in order to determine w hether the fafigueprotocol had a significant effect on the muscle acfivity of the force couples.A paired t test was perform ed for each of the 4 muscles du rin g each of the4 exercises. A preset alpha level of P < .05 w as selected to determine stafis-fical significance.

ResultsNo significant differences were noted for any of the variables assessed be-tween the pretreatment and p osttreatmen t EMG values. The pretreatm entand po sttreatment mean scores and SDs are presented in Figures 3 through 6.


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O

Figure 4 Mean EMG data for segmental supporting exercise on slide board.

Figure 5 Mean EMG data for rotational movement on slide board.


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DiscussionThe present findings raise many issues with regard to the funcfion of thedynamic stabilizers in the shoulder following fafigue. The results appearto indicate that the dynamic stabilizers function postfafigue similarly tothe way they do in their normal, prefatigue state. The magnitude of thecoacfivafion during the functional acfivifies was not altered significantlyafter the fafigue protocol. This is an innportant finding with reg ard to dy -nam ic stabilizafion of the gleno hum eral joint. The subjects in this invesfi-gafion had normal shoulders, and the prefafigue muscle acfivity duringth e 4 functional acfivifies was used as baseline muscle acfivity for dyn am icstabilizafion. The fact that no significant differences were noted confirmsthe ability of the muscle force couples of the glenoh um eral joint to performnorm ally in the fafigued state.None of the 4 muscles invesfigated in our study exhibited any signifi-cant difference in acfivafion levels (area of EMG) from the prefafigue to thepostfafigue state. It has been previously established that, compared withresting m uscle, fafigue changes the EMG signal.'"' '^''^ Those auth ors re-port that m uscular fafigue appe ars to result in a reducfion in mem braneconducfion velocity, while amplitude remains constant. This decrease inconducfion increases the w idth of the muscle signal and, therefore, increasesthe area und er the musc le signal curve. This is interpre ted as an increase inthe m ean area of the muscle contracfion.'"'^

This finding is not su pp orte d by the results of our invesfigafion, whichrevealed no increases in EMG area. Similarly, Hultm an an d Sjohohn^ foundtha t after a muscle-fafigue protoco l, EMG acfivity quick ly re turne d to nor-mal levels, whereas force remained at a reduced level. This finding couldcertainly be considered consistent wi th the lack of significant changes no tedin the EMG acfivity in our study.The effect of fafigue on upper extremity dynanuc stabilizafion has notbeen thoroughly invesfigated previously. One recent study, however, thatwas performed on the shoulder indicated no significant decrease in dy-namic stabilizafion of the glenohumeral joint following muscle fafigue.'This study was performed with a l-arm push-up in order to assess dy-namic stabilizafion. The study performed by Myers^ assessed both pro-priocepfion and neuromuscular control. The findings indicated a signifi-cant decrease in propriocepfive awareness but no significant alterafion inneurom uscular control. Our results concur with the findings of Myers etal,^ w ho s tate that it is difficult to explain the lack of a significant decrease


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muscle-firing pattems and EMG acfivity of the stabilizing musculature.^'From the p revio us research we can conclude that joint afference, or pro p-riocepfion, is significantly altered after muscle fafigue.Other studies have established a relafionship between fafigue and di-minished joint-posifion sensibility and kinesthesia in both the lower andthe up pe r extremity.'* Recent research sug gests that muscle fafigue wors-ens or impairs joint-posifion sensibility, in both the upper and the lowerextremity.'*' ' * A lthough joint-posifion sense was altered , the resea rch re -vealed that joint kinesthesia was not significantly altered after m uscle fa-figue. These results were obtained in the quadriceps, hamstring, elbow,and shoulder. The deficits no ted in joint-posifion sense might hav e a directlink to alterafions that have been noted in muscle acfivity after fafigue.With regard to neuromuscular control and dynamic stabilizafion of theshoulder, it is very well docum ented that there is a direct relafionship be-twe en muscle fafigue and dim inished propriocepfion in the shoulder.''^ Inaddifion, Wickiewicz* has dem ons trated that once rotator-cuff muscles arefafigued, the humeral head migrates superiorly during acfivifies that in-clude arm elevafion.The lack of diminished neuromuscular control in our study might berelated to the natu re of the exercises. The exercises used in this study hav e

been previously established as valid dynamic stabilizing exercises byHeruy.^" Each of these acfivifies provided muscle coacfivafion of the dy-namic force couples around the glenohumeral joint. The mechanism bywhich these exercises induce coacfivafion is based on the characterisfics ofclosed kinefic chain exercise. These characterisfics inc lude com press ion ofthe humeral head into the glenoid fossa and stimulafion of the arficularmechan oreceptors. The sfimulafion of the arficular mechanorecep tors elic-its a coacfivafion response of the dynamic force couples. With regard toour findings, it is plausible to conclude that the joint compression p rov idedby these dynamic stabilizing exercises is sufficient to stimulate musclecoacfivafion, even in the fafigued state. Thus, there were no significantdecreases in neu rom uscular control postfafigue. This explanafion might fitthe findings of Myers et al,' as well. Their stud y also em ployed an acfivityinvolving axial loading and joint compression of the shoulder.

The findings of our study are very importan t with regard to the shoul-der joint. The in\portance of dynamic stabilizafion in the shoulder is welldocumented,''^' '''^*' ' and the fact tha t ne uro muscular control a pp ears to berelafively unaffected by fafigue is a step in the right direcfion for under-stand ing sho ulder funcfion. The results of ou r study, performed on norm al


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to sustained or isometric-type contracfions was measured. The EMG as-sessment in the present stud y wa s measu red du ring a series of funcfionaltasks, more specifically, dyn am ic stabilizing exercises. The use of indw ell-ing electrodes should also be considered a possible limitafion. With ind-welling electrodes, the muscle activity recorded might only be re presenta-five of a small porfion of a large musc le. This is not a concern in the presentstudy, however, because of the consistency of ou r resu lts.In order to proper ly interpre t our resu lts, the fafigue protocol shou ld bethoroughly exam ined. The protocol w as induc ed o n the shoulder muscu-lature by havin g subjects perform concentric an d eccentric exercises on theBiodex Isokinefic Dynamometer. Isokinetic dynamometers are a popularmethod of inducing muscle fafigue because of their ability to quanfifymuscle-force production.'* The fatigue protocol was performed in an openkinefic cha in posifion, whereas the test exercises were performed in a closedkinefic chain posifion. Previous fafigue protocols for both the upper andlower extremifies employed the open-chain fafigue p rotocol because of theability of the isokinetic dynamometer to provide objecfive evidence ofmuscle fatigue .''* The degree of muscle fatigue w as quantified by the sub -jects' ability to generate torque and was easily measured on the Biodexsystem. This method has been previously reported by Rozzi'* and app earsto follow the tradifional definifion of fafigue as the inability to generateforce. Our subjects com pleted concentric repefifions in a diagona l p atternuntil they w ere unable to generate 25% of their peak torque for 3 consecu-five repefitions. Although the isokinefic strength values produced duringthe diagonal patte rn represented a significant decrease in peak torque, wehav e no conclusive method of confirming that the 4 target muscles experi-enced substantial fatigue. This should be considered a limitation of thestudy. The fafigue protocol employed is, however, similar to other refer-enced protocols in other refereed studies ,''* and we believe that our p roto-col produ ced mu scle fatigue in the shoulder musculature.

ConclusionThe results of our study indicate that EMG acfivity of the muscle forcecouples of the glenohum eral joint is not significantly altered after musclefafigue. Force-couple EMG acfivity w as assessed du rin g funcfional dynam icstabilizafion exercises for the upper extremity. Although propriocepfionhas been dem onstrated to be altered after muscle fatigue in the up pe r andlower extremifies, neuromuscular control of the efferent pathway has not


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Although this study begins the process of understanding the effect offafigue on neuromuscular control and dynamic stabilization, further re-search is warranted in this area. The present study assessed the area ofEMG during funcfional exercises. Further study in this area should assessmuscle-firing pattems during these stabilizafion acfivifies. Informafion onthe muscle-firing pattems, in concert with informafion gathered here, mightprovide a more thorough understanding of the efferent pathway aftermuscle fafigue.

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fatigue on the relationship of arm dominance to shoulder proprioception. /Orthop Sports Phys Ther. 1996;23(6):348-352.18. Lattanzio PJ, Petrella RJ, Spourle JR, Fowle PJ. Effects of fatigue on knee pro-prioception. Clin J Sport Med. 1997;7:22-27.19. Kendall FP, McCrary EK, Provance PG. Muscles: Testing and Function. Balti-more, Md: WilUams & Wilkins; 1993.20. Heruy TJ, Lephart SM, Stone D, Giraldo JL, Fu FH. An electromyographic analy-sis of dyn anu c stabilizing exercises for the shoulder. / Athletic Train. 1998;33:S14.21. Gersten JW, Cenkovich GC, Jones GD. Harmo nic analysis of norma l an d ab-normal electromyograms. Am J Phys Med. 1965;44:235-240.22. Johansson S, Larsson LE, Ortengren R. An autom ated m ethod for the frequencyanjilysis of myoelectdc signals evaluated by an investigation of the spectralchanges following strong sustained contractions. Med Biol Eng. 1970;8:257-264.23 . Hultman E, Sjoholm H. Electromyogram, force, and relaxation dine duringand after continuous electrical stimulation of human skeletal muscle in situ. /

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