M RKVIW E ARTICLE

Critical Appraisal and Hazards of Surface Electromyography

Data Acquisition in Sport and Exercise

Jan Pieter Clarys*', PhD; Aldo Scafoglieri', MSc; Jonathan Tresignie', MSc;

Thomas Reilly^ PhD; Peter Van Roy', PhD

Authors' AfTiliation:

1. Department of ExperimentalAnatomy (EXAN-LK), VdjeUniversiteit Brüssel, Belgium

2. Research Institute for Sportand Exercise Sciences(RISES), Liverpool JohnMoores University, UK

* Corresponding Author;Experimental Anatomy, VrijeUniversiteit Brüssel, Laarbeeklaan103, 1090 Brussels, Belgium

E-mail: jclarys@vub.ac.bc

Received: Mar 30, 2010Accepted: A/ay/7, 2010

Key Words: SEMG; IEMG;Normalization; Electrodelocalization; EMG/force

Abstract

The aim of this critical appraisal and hazards of surfaceelectromyography (SEMG) is to enhance the data acquisitionquality in voluntary but complex movements, sport and exercise inparticular. The methodological and technical registration strategiesdeal with telemetry and online data acquisition, the placement ofthe detection electrodes and the choice of the most adequatenormalization mode.

Findings compared with the literature suggest detection qualitydifferences between registration methods and between water andair data acquisition allowing for output differences up to 30%between registration methods and up to 25% decrease in water,considering identical measures in air and in water. Various hazardsdeal with erroneous choices of muscles or electrode placement andthe continuous confusion created by static normalization fordynamic motion. Peak dynamic intensities ranged from 111% (inarchery) to 283% (in giant slalom) of a static 100% reference. Inaddition, the linear relationship between integrated EMG (IEMG)as a reference for muscle intensity and muscle force is not likely toexist in dynamic conditions since it is muscle - joint angle - andfatigue dependent. Contrary to expectations, the literature shows30% of non linear relations in isometric conditions also.

SEMG in sport and exercise is highly variable and different fromclinical (e.g. neurological) EMG. Choices of electrodes, registrationmethods, muscles, joint angles and normalization techniques maylead to confusing and erroneous or incomparable results.

Asian Journal of Sports Medicine, yol I (No 2), June 2010, Pages: 69-80

INTRODUCTION

The aim of this work (e.g. technical notations) is toprovide more clarity in the variability of surface

electromyography (SEMG) when applied to sport and

exercise. A critical appraisal of this SEMG variationand description of hazards in measurements andchoices should improve the methodological quality ofdata acquisition of complex dynamic voluntary skillsand movements. Background knowledge of trial and

© 2010 by Sports Medicine Research Center, Tehran University of Medical Sciences, All rights reserved.

Clans JP, et al t SM

error, problems and differences of EMG in general andSEMG in particular are the bases of consensus on keyitems that enable exchange, reproduction andcomparison of the electrical activities of complexdynamic and isometric contractions.

Man has shown a perpetual curiosity about theorigins of locomotion in human and other creatures.Amongst the oldest scientific experiments known to us,we must name the detection of electricity and function

The detection of the electrical signal of the humanand animal muscle emanates from long before Galvaniwho took the credit for it''*. Jan Swammerdam alreadyshowed the mechanics of muscular contraction to theduke of Tuscany in 1658'^'. Even if 'Electrology orlocalized electrisation' - the original terminology forEMG - could be the oldest (if not the oldest) bio-scientific detection and measurement technique, it hasremained for very long and until a few decennia ago, arather 'supporting' measurement with limiteddiscriminating qualifications always to be used inconjunction with other measuring methods.

Developments in signal processing, acquisition- andanalysis systems upgraded SEMG into a problemsolving, problem detecting and problem discriminatingscientific discipline.

The reason why EMG needed such a long time toobtain this status is assumed to be related to the factthat EMG took 3 distinct different directions in thecourse of its development; each with variousapproaches and analytic techniques'''''.

The clinical EMG is majorly a diagnostic tool. Thekinesiological EMG (including SEMG) is merely asystem to study function and co-ordination, while the(fundamental) EMG itself, deals with single fiber- andmotor unit action potentials (SFAP and MUAP) andthe related time - frequency domain. Depending on theuser (a physician, an anatomist, an ergonomist, aphysiologist, an engineer, a physiotherapist, aneurologist), one encounters independent improve-ments in registration technology, different approachesof data acquisition and various but specific graphicrepresentations, analytic techniques and signaltreatments software.

All this has resulted into a number of applications inneurology, neurophysiology, neurosurgery, bio-

engineering. Functional Electro Stimulation (FES),orthopaedics, occupational biomechanics and -medicine, rehabilitation and physical therapy, sportmedicine and sport science. This dissemination ofknowledge about the neuromuscular systems both innormal and the disabled was already predicted (and inpart described) by Duchenne de Boulogne (1872)''*'.Electromyography became a diagnostic tool for studiesof muscle weakness, fatigue, pareses, paralysis,conduction velocities and lesions of the motor unit orfor differential neurogenic and myogenic problems.FES developed as a specific rehabilitation tool. Almostin parallel and within the expanding area of EMG, aparticular speciality developed wherein the aim is touse EMG for the study of muscular function and co-ordination of muscles in different movements andpostures.

The application areas of kinesiological EMG andtherewith SEMG can be summarized as follows:studies of normal muscle function during selectedmovements and postures; studies of muscle activity incomplex sports; occupational and rehabilitationmovements; studies of isometric contraction withincreasing tension up to the (relative) maximalvoluntary contraction; evaluation of functionalanatomical muscle activity (validation of classicalanatomical functions); co-ordination andsynchronization studies (kinematic chain); specificityand efficiency studies of training methods; fatiguestudies; the relationship between EMG and force; thehuman-machine interaction; studies on the influence ofmaterial on muscle activity; vocational loading effectstudies related to low back pain and arthrokinematics.Within these various applications, the recording system(e.g. the signal detection, the volume conduction, thesignal amplification, impedance and frequencyresponses, the signal characteristics) and the data-processing system (e.g. rectification, linear envelopeand normalization methods) are going hand in handwith a critical appraisal of choices, limits andpossibilities'^'.Recently Merletti et al. (2009) presented the state ofthe art of the technology of detection and conditioningsystems for SEMG, with a focus on electrode systemtechnology, electrode classification, impedance, noise,transfer function, spatial filtering and on SEMG multi-

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electrode grids and multi channel amplifiers'''.Thisreview, however, majorly related to (intra- andextracellular) SFAP and MUAP (Fig. 1 ), was describedearlier as the fundamental part of kinesiological EMG.The purpose of this work is to consider the activity of awhole muscle (e.g. collection of SFAP's) or functionalmuscle groups including the specific associatedtreatment of the signal, e.g. full wave rectification ofthe raw EMG, averaging into a linear envelop andintegrate (IEMG) to obtain a value of muscularintensity over time (Fig. 2).

Following the criteria of the Intemational Society ofKinesiological Electromyography, SENIAM and theJoumal of Electromyography and Kinesiology, it isrecommended to report the upper cut-off frequency, thelower cut-off frequency and the type of filter used inthe amplifiers. If a direct coupled amplifier is used, theinput impedance and input current should also bereported''''. The type and material of the electrodes, thespace between the contacts, the site and the preparationof the skin should also be documented. With respect tothe processing of data, it is important to mention notonly the use of raw EMG, IEMG, linear envelope,mean rectified EMG (MREMG), together with thesynchronization system and the normalizationtechnique used; such as normalized to maximal

( rilical Appniisal SKMi. l):il:i

voluntary contraction (MVC) or to 50% of the averageof three MVCs, or to the highest peak (per movementor per subject) or to the mean of the subject ensembleaverage.

The linear envelope is the qualitative expression ofthe rectified and eventually averaged signal within awindow choice; independent of its purpose, this linearenvelope can be smoothed. It should equally be clearthat once smoothing is started, integration is no longerpossible and it is unwise to use 'intensity' or 'activitylevel' in this case. Integration refers to the surfaceunder the non-smoothed but rectified signal, to expressthe phenomenon of 'muscular intensity'.

Telemetric, on-line and remote on-lineregistration approaches

Over the years, the improvement of devices forregistering the EMG signal and the evolution ofmethodological approaches to both EMG dataacquisition and computerized pattem analysis havebeen valuable for bio-engineering, occupational- andsport medicine, physiotherapy, sports biomechanics,and also eventually for trainers and coaches. Since theend of the 1960s there has been a development in

Motor Unit Action Potential (MUAP)

Single Fiber Action Potentiai (SFAP)

Fig. 1 : Wheti a skeletal muscle ftber is activated by a MUAP, a wave of electricaldepolarization, referred to as an SFAP, travels along the surface of the ftber

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Ctarvs JP, cl al

Raw EMG

t » J üti Mil i t üi. Rectified EMG

/*\ Unear envelop

Integrated EMG

RMEMG= linear envelope

Fig. 2: The basic EMG signals used in dynamic and complex sport, exercise and occupational activities

miniaturized telemetric devices for monitoringcomplex human movements remotely. Especially forkinesiological purposes, the telemetric devices haverecently been changed from two-channel registrationsto eight or more-channel systems.

There are obvious and numerous advantages totelemetric measurement of muscular activity, althoughsome difficulties may be encountered in fieldcircumstances. For example, it is difficult to link morethan two or three transmitters in parallel due to thelimited free radio wave possibilities at present, inparticular if working on location. Secondly, since thebeginning of the research employing telemetric EMGbreaks in transmission, atmospheric conditions, staticor other disturbances have never been truly controllableand this has not changed today'"*l

In order to measure muscle activity in complexsports movements in the laboratory or field studies,different features are taken into consideration:

i. The EMG data acquisition system with itselectrodes should allow total freedom of movementfor the subject - in other words, movementswithout additional resistance.

ii. The set-up should allow adaptation to thecharacteristics of the field and movementcircumstances, in different situations. It should beapplicable to activities in sports such as swimming,skiing, archery, cycling and so on, and to theworking environment of health-care professionals.It should accommodate long-term activity and

movement over large distances and allowcontinuous measurements (up to tape limits),

iii. It should be possible to monitor at least six to

twelve muscles simultaneously,iv. The combined registration and data acquisition set-

up should be user-friendly.

A multichannel FM recorder, 'active' electrodes, aregulation-amplification unit and differentsynchronization modes need to be integrated into onesystem with different possibilities in order to allowsuch a combination. This integration was done in botha 'conventional on-line' and 'remote' configuration(Fig.3)f"l

In practical terms, a conventional on-line systemwill connect the (active pre-amplified) electrodes of theathlete with a wire to the (additional) amplification anddata acquisition system. This methodological choice issuggested for all movements within a limited areaand/or with limited displacement of the athlete (e.g.shooting sports, treadmill running, cycling on roles,etc.) . Most sports, however, need distance (ski,rowing, cycling, track running, etc.) .

Two methodological solutions are at hand: (i) Atelemetric system (with amplifiers on the subject) withradio wave characteristics (e.g. transmitter on thesubject-receiver at distance) or with a multiplexsystem, (ii) A remote (on-line) system. Electrodes areamplified on the subject and signals are registered onminiaturized recording systems. The whole system ison-line and carried by the subject. A distance control

\ol l,No2,June2t)lt)

MC'rillcpl Appraisal .SKMG Data Acquisition in .Sport

system serves to start and stop the registration systemremotely (Fig. 3). ,

elemetricAcquisition

conventionalAcquisition

Fig. 3: SEMG data acquisition systems to be usedin sport, exercise and occupation

The method of choice is the remotely control on-line system carried by the athlete because of its higherreliability. Measurements (on the M. biceps and M.brachioradialis) with one pair of electrodes per muscle,identical amplification between muscle and registrationunit taken simultaneously with a telemetric and aremote (on-line) system on subjects (N=9) executingidentical isometric and dynamic functions indicatedloss of electricity (IEMG) with a telemetric

,[12.13]

Electrodes, water and air data acquisition

The choice is easily made among the most commonlyused sensor (e.g. passive surface electrode Ag/AgCL -0lOmm), the active pre-amplified surface electrodes,the intra muscular needle and wire electrodes. Theactive electrode being the most reliable for complexdynamic sport and exercise movement' '"I

The advantage of the active electrode over theclassic passive electrode is decline of erroneousregistrations. This feature has become important since

we have found that, despite thorough precautions(different taping and plastic varnish for protecting theelectrodes), water does decrease the detectableelectrical output of human muscle. In other words, animaginary identical intensity will produce moreelectricity in air than in water. The implications forusing EMG in aquatic environments have beenreviewed elsewhere''^•'^'.

Recently, there has been a renewed interest incomparing the reproducibility of EMG underimmersion and on land, mostly in a EMG/forcerelation. Pöyhönen et al. (1999) observed that MVCforce output and muscle intensity significantlydecreased during immersion compared to the samemotion on land''^'. This significant signal decreaseranged between 11 and 35%, depending on the muscle

Rainoldi et al. (2004), Veneziano et al. (2006) andMasumoto and Mercer (2008) experimented withwaterproofing electrodes and obtained correspondingresults. Maximal effective waterproofing can decreasethe water and on land differences to zero'''''**''''.

Both in water and on land, in particular for dynamicregistrations, the placement of the detection and thereference (ground) electrodes are of great importance.They do not correspond to the S EN 1AMrecommendations which focuses merely on amplitudeand frequency characteristics, timing properties andMUAP width and shape^""'. Dynamic SEMG in sportand exercise is looking at a much wider picture ofwhole muscle contraction and relaxation (e.g. inter-muscular and inter-individual synchronization).Therefore, localizing the detection electrode accordingto standard osteologic reference points is to be avoided.In a complex sport and exercise activity combiningconcentric, eccentric and co-contractions, the musclebelly will travel continuously up and down through thecontraction/elongation axis and thus under the skin. Ifthe detection electrode is located differently from themidpoint of the contracted muscle, chances will beconsiderable and at some levels of the displacement,the detection electrode loses the muscle under the skin(Fig. 4).

At the same time, not all muscles are suitable forSEMG. If the muscular surface is less than lOcm ,̂possibility of losing the muscle to be detected under the

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ClarvsJP.ctalM

Surface electrodes

Dynamic/ballistic motion

M. sartoriusM.gracilis

M. teres major

M. extensor carpi radialls brevis

M. semi membranosus

M. extensor digitorum longus

(th* uppM pan of the) M. erector spina«

CROSS-TALK

Fig. 4: If the muscle belly surface is too small and/or of the detection electrode is placed on a different locationthan the midpoint of the contracted muscle in dynamic situations, the investigator will have to deal with cross-talk

electrode, increases significantly leading almostautomatically to cross-talk (Fig. 4).

Cross-talk is the detection of myo-electric activityof neighboring muscles instead of, or in combinationwith the originally studied muscle. Not all cross-talkwill lead to erroneous registrations. If the neighboringmuscles are active as a function group within thecomplex movement, the summation of motor unitcontribution can alter the intensity, not the function.Cross-talk is to be considered as a side effect inherentto SEMG^-". ;

In general, it is agreed that electrodes should beplaced on the muscle from which a good and stableSEMG signal can be recorded. The SENIAMrecommendations for that matter, take into account thelocation of motor points, muscle tendons andosteologic references'""I These issues becomeirrelevant if the midpoint of the contracted muscle is

Critical appraisal limitations and hazardswith SEMG

Most activities in sport and exercise settings involvecomplex movement patterns that are often complicatedby external forces, impacts and the equipment usedduring the movement. An electromyogram (or itsdérivâtes) is the expression of the dynamic

involvement of specific muscles within a determinedrange of that movement. Mostly SEMG is used toinvestigate the activity of a series of muscles, seldomjust one or two. The choice of these muscles is basedeither on practical knowledge of the skill or on basicanatomy. Thorough knowledge of both sport andkinesiology are essential. It is important to note that theselection of muscles for SEMG measurements requirescareful considerations. Some of these choices can leadto erroneous registration, sometimes without beingnoticed by peer reviewers'''^

Various researchers have placed surface electrodeson the M. sartorius, the M. gracilis and the M. teresmajor. Measuring the SEMG of these muscles understatic conditions creates little or no problems, but undercomplex dynamic conditions, the sartorius and gracilismuscles disappear from under the electrodes as doesthe M. teres major, especially during arm motion above90° abduction. It is therefore uncertain which muscleshave contributed to the EMG patterns presented. Otherresearch groups have selected for their studies the M.extensor carpi radialis brevis. This muscle has a verysmall superficial 'strip' accessible under the skin. TheEMGs of this muscle are dubious and may give moreinformation about the M. extensor digitorum. The sameproblem arises during measurement of the M.semimembranosus (under the M. semitendinosus);although the superficial muscle belly parts may seemsufficient, the combination of displacement of the

Vol I, No 2, June 2010

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superficial M. semitendinosus with a lack of functionalsurface again gives different information from whatwas expected (e.g. the cross-talk phenomenon).

The use of wire electrodes does not necessarily havethis problem, although measuring the M. subscapularisin this way - especially during front crawl inswimming and during golf swing movements, forexample, - is questionable. The point of view of theanatomist who is confronted with these situations in thedissection room and palpation classes should not bediscounted. The competent anatomist will not select theM. sacrospinalis, but instead chooses the M. erectorspinae for measurement. One group, however, reportedmeasuring the SEMG of the M. tibialis posterior duringskiing. Clarys (2000) assumed that this was a printing

The reference or ground electrode is another matter.It is a common knowledge that the reference electrodeis placed on electrical inactive tissue (e.g. bone). Theidea is to minimize (common mode) disturbances. Theliterature suggests a series of preferred locations, e.g.the wrist bones, the crista iliaca, the tibia, the processusspinosus of C6-C7 and the sternum. For complexdynamic movements the sternum and the os frontalisare the choices of preference.

EUCTRODE SITE

Good skin preparation for the better fixation of theelectrodes consists of shaving, sandpapering andcleaning. But it is only expected to have indirect effectson the SEMG signal quality. Proper skin preparation isnecessary to reduce skin (and subcutaneous tissue)impedance. Skin impedance phenomena are commonknowledge but too little interest is given to the relatedhuman Body Composition (BC), the variation of skinthickness and subcutaneous adipose patterning'"''.Figure 5 clearly indicates the extension of thisvariability but no direct relation has been studiedbetween SEMG and BC at different locations, yet.

Normalizationreference

the choice of the better

No two EMG profiles that result in the same action areever identical, because of the change in the actualmotor units controlling the activity. Due to this knownvariability of the EMG signal, not only betweensubjects but also between different trials, differentnormalization techniques have been developed toreduce variability. In general, the EMG of maximumeffort or the highest EMG value has been selected as

REGION

SubscapularTricepsBIctpsForearmPectoral chestAxMlarv chestMld-axlllary lineSupra-splnaeVentral thighMedial thighDorsal thighSupra-patellaMedial calf

Total mean

FEMAU

Uft

3.472.020.981.202.681.952.721.871.861.632.181.821.57

1.99

Right

3.412.251.001.202.641.802.721.952.021.562.121.971.57

1.98

MALE

Left

4.062.421.551.502.472.702.872.042.22l J l2.412.251.71

2.30

Right

4.112.611.601.522.442.823.112.542.421.752.442.351.78

2.42

Fig. 5: Mean skin thickness per region, left and right, males and females and adipose tissue (AT)depth at a selection of classic electrode locations. (Data from a cadaveric study'"'')

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the normalizing factor. In the main, the subject is askedto perform a MVC of the muscle (groups) beingstudied. This amplitude, either raw or rectified, overtime is then used as a reference value (e.g. 100%). Theuse of the MVC reference is very popular and isacceptable in all static applications. For all dynamicactivities the use of an isometric reference isquestionable. Several investigators have recentlyreported EMG values in dynamic activities thatexceeded the maximal isometric effort (Table 1).

Therefore, other normalization techniques havebeen developed in kinesiological EMG especially forsport exercise and occupation, e.g. normalization to thehighest peak activity in dynamic conditions, to meanintegrated EMG (ensemble average), to EMG per unitof measured force (net moment), and so on.

In an extensive review of sport specific andergonomie studies using EMG, the missing informationmostly concerned the issue of normalization. In themajority of both sport and occupational studies inwhich a normalization technique is mentioned, theMVC technique has been used. This approach,however, is unreliable in all dynamic situations forseveral reasons'''':i. Different maxima may be observed within the same

subject repeating at different occasions, the same'maximal' but isometric effort

ii. Different maxima are observed at different anglesof movement, both in eccentric and concentricmovement modes

iii. Additionally, the question of linearity may arisewhen the values measured during isotonic dynamic-ballistic-complex sports movements (or heavylifting tasks) exceed the 100% MVC. For example,Clarys et al. (1983) found dynamic percentages inswimming up to 160%, while Jobe et al. (1984)reported up to 226% of MVC in baseball pitchingand Hintermeister et al. (1997) even 283% of MVCreference in a giant slalom (Table l)f-'*-'- -''̂ .It seems reasonable to suggest that a statically

obtained EMG, such as MVC, cannot be an appropriatereference for dynamic EMG. In other words, mostproblems disappear when the proper normalizationtechniques are used. The data must be standardized forthe cycle time and the amplitude. Several types ofamplitude normalization are in use. All of them workand are valid, but only if applied correctly. Theimportant thing is to choose a normalization technique,apply it rigorously, compare the reported results withyour clinical estimate of the patient's or athlete'sperformance and adjust either your perception ofperformance or the EMG technique to achieveagreement between the EMG and actual performance(Table 2).

Table 1: Dynamic SEMG intensities compared to 100% static MVC in different sports studies and atdifferent velocities in swimming'''

ExamplesM. Triceps Brachii (% of 100% MVC)

Maximum X total(N=l) (N=60)

Lewillie, 1973

Clarys et al., 1983

Jobe et al., 1984

Clarys et al., 1990

HoseaetaL, 1990

Clarys et al., 1994

Hintermeister et al., 1997

Back StrokeBreast Stroke

Dolphin

CrawlSculling

Baseballpitching

Archery (90m)

Golf swing

Special slalom

Giant slalom

142%

120%

160%

138%195%

226%

111%

115%

180%

283%

CrawlSlowNormalSprint

Breast StrokeSlowNormalSprint

Back StrokeSlowNormalSprint

DolphinSlowNormalSprint

104114124

5696120

106114142

102114160

8696110

206080

728096

8080104

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M(ritfcal Apprüisiil S}-!M(> DiiCii Acquisition in Sport

Table 2: Frequently used normalization techniques in sport, exercise and occupation

For SI AI'IC piirnosfs

MVC (pre-post test) max. effortAverage of three 50% MVCEMG amplitude per unit isometric moment of ForceRelative Voluntary Contraction (RVC) average ofthree isometric contractions overtime (20see) with aknown load (10 to 20 kg range)

For DYNAMIC piirpiisos

Highest peak (amplitude) overtime (sec) withina movement series (within the same subject)The highest activity (IEMG) of differentensemble averages

Kinesiological reasoning and basic biomechanicstell us that statically obtained EMG cannot create areference for dynamic-, ballistic- and complex EMG. Itwould be physiologically and biologically incorrect.

Activity level in terms of IEMG and force,strength, torque i

In isometric work tbe IEMG signal - being theexpression of muscular intensity - is by many assumedto be proportional to the force generated by the muscleaction. Muscle intensity however, even isometric is notalways linearly related to the force, if such relationdoes exist!? A scanning of 31 references dealing withthe relation between IEMG (mV) and force (N or kg)reveal a majority, indeed of linear relationships

between IEMG and force. But a considerablepercentage of mixed (e.g. not perfectly linear) and ofnon-linear relationships were found (Fig. 6).

The mixed group is or can be most confusing, e.g. ifEMG data are not supported by other variables. Thedegree of linearity is influenced by the muscle type andits function'"^' or can be changing within the sameisometric contraction from a linear to a non-linearrelation over time (Fig. 7). Raw EMG and the linearenvelop (RMEMG) can stay unchanged, but force willdecrease over time. If we want to maintain that force,although one cannot hold it, the EMG will increase'".Under dynamic conditions, the relationship is not sosimple due to the changing force-torque characteristicsat different phases of movement. In very fast, ballistic-type motions, the EMG signal demonstrates alternatingbursts of activity in both agonist and antagonist

Linear Mined Non-linear

Fig. 6: Distribution (%) of IEMG - Force relations in isometric conditions

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ClarvsJP.etal

• Bkeps (N=61)• Deltoid (N=76)• FDI (N=43)

Force

I

3020 60 80 r 90 120 ISO

TIME(s)

40

FORCE{% OF MAXIMAL VOLUNTARY CONTRACTION)

Fig. 7: Linearity can deviate depend on muselé fiinetioti (left)'"'', and linearity of the EMG/force relationship canchange over time (right)'' ' '

muscles. In such movements the EMG is triphasic inactivity'"'''. A first burst of EMG activity is evident inthe agonist, followed by signals from the antagonist.The first burst of activity from the agonist representsthe propulsive force that initiates limb motion whereasthe first activity in the antagonistconcludes withdeceleration of the limb. The second phase of activityin the agonist determines the final positioning of thelimb.

During eccentric muscle actions, the tensiongenerated can exceed that during isometric actionwhich in turn is greater than during a concentric actionwhen the same muscle shortens in length. Despite thehigh EMG activity during an eccentric action, theenergy expended may be quite low since fewer motorunits may be recruited. It is during eccentric muscleactions that muscle damage may occur. Leakage ofcreatine kinase through the muscle membrane into theblood is evident following eccentric work and theensuing soreness peaks 48-72 hours later. Thephenomenon of 'delayed onset muscle soreness' mayalso be linked to an inflammatory response due tomicro-trauma within the muscle and its connectivetissue. The damage occurs due to the forces involved instretching the muscles which are resisted by thecontractile components'^"'''.

Eccentric actions are dominant in weight loweringactivities compared to the predominance of concentric(and isometric actions) in lifting activities.

The electromyographic activity can be seen as aneuromuscular response to match the biomechanicalrequirements. The EMG signal information can be usedin different ways depending on the question at hand. Abasic question is whether we are interested in forcesand torques (biomechanics) or muscle activation(physiology). Both approaches have sport andoccupational medical relevance. However, aninvestigator should decide which approach is the mostsuitable for the actual question at issue since the chosenapproach is closely linked to the choice of calibrationstrategy and interpretation of results'"'*'.

If we are interested in forces and torques, we haveto establish a calibration curve between SEMG, IEMGin particular and force or torque'"". This relationshipinvolves several error sources which too often havebeen overlooked. With this biomechanical approach,fatigue effects are considered as confounders since theyalter the EMG-force relationship.In addition, every joint angle within a dynamiccomplex muscle activity will influence the EMG/forcerelation making strength assumption from EMGdata almost impossible and biologically irrelevant

(Fig. sf'\ I :

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M( rilical AppraKul SKM<i Data \a|iiisilicin in Sp<irl

1.0 n

0.6

0.00.0 0.2 0.4 0.6

FORCE

0.8 1.0

Flg. 8: The joint angle during sport and exercise willseriously influence the EMG/force relationship

CONCLUSION

SEMG in sport, exercise and occupation is a clearsubtopic of kinesiological EMG and its experimentalapproach including signal treatment is different fromSFAP and MUAP studies (e.g. intra and extracellular).Kinesiological EMG, e.g. as a functional andco-ordination tool, is different from clinical ornetirological EMG which is merely a diagnostic tool.

Sport and exercise need data acquisition systemsthat allow total freedom of motion. The on-line incombination with a remote registration system

accommodates best long term and distance or surfacedemanding movements.

Water and telemetry may be at the origin of signaldecrease (loss) while muscle displacement under thedetection electrode will create cross-talk or willmeasure other activities than assumed. The localizationof the electrode on the midpoint of the contractedmuscle which has to be superior to lOcm^ willeliminate a maximum of cross-talk.As to the better normalization technique in sport andexercise, MVC references are perfect for all staticcircumstances, but must be avoided in dynamicsituations.

IEMG being the intensity signal is not per se similarto force, strength and torque e.g. ^V is not the same asN or kg.

Skin impedance is a well known signal treatmentfactor, but the variability of skin thickness andsubcutaneous layer patterning deserves more study.

ACKNOWLEDGMENTS

The authors wish to thank all participants andresearchers for their cooperation in the past.

Conflict of interests: The authors disclose no conflictsof interest.

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