electromyography mario lamontagne phd 1. apa 6903 introduction background recording technique ...

ELECTROMYOGRAPHY

Mario Lamontagne PhD

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APA 6903

Introduction Background Recording Technique Analysis of the EMG signal Applications

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Scope of this presentationScope of this presentation

APA 6903

INTRODUCTION

The electromyographic (EMG) signal offers a great source of information to both clinicians and researchers

EMG can be used to detect gait or joints pathologies, to assess a rehabilitation program, to measure the functionality of sport equipment and to implement an effective biofeedback therapy.

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INTRODUCTION

Surface EMG is also widely used in an effort to understand a number of research issues:

• Muscles coordination around a joint• Relationship between muscular force and muscle

electrical activity• Neuromuscular adaptations after joint surgery

following a rehabilitation program.

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BackgroundNervous System

The muscle unit action potential detected by electrodes in the muscle tissue or on the surface of the skin.

Central nervous system (CNS) activity initiates a depolarisation in the motoneuron.

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BACKGROUNDCNS

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BACKGROUNDCNS

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BACKGROUNDCNS

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BACKGROUNDMotoneuron

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BACKGROUNDSYNAPSE

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MOTOR UNIT

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A single axon leading to A single axon leading to a muscle is responsible a muscle is responsible for the innervation of as for the innervation of as few as 3 or as many as few as 3 or as many as 2000 individual muscle 2000 individual muscle fibres. fibres.

A neuron and the A neuron and the muscle fibres are muscle fibres are referred to as motor unitreferred to as motor unit

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MOTOR UNIT

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One -motoneuron plus all the muscle fibers it enervates

Innervation ratio varies with number of fibers per motor unit (large leg muscles have many fibers per motoneuron for stronger responses, facial and eye muscles have few fibers and therefore permit finer movements but weaker contractions)

All-or-none rule – once a motoneuron fires all its muscle fibers must fire

Graded muscle responses occur because of orderly recruitment of motor units, i.e., lowest threshold motor units fire first followed by next lowest threshold. Highest threshold and strongest motor units fire last.

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MU ACTION POTENTIAL

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When an action potential reaches the muscle at localized motor points (AKA innervation points) sarcoplasmic reticulum and t-tubule system carries the message to all parts of the muscle fiber

A rapid electrochemical wave of depolarization travels from the motor point causing the muscle to contract

Followed by a slower wave of repolarization and a brief refractory period when it cannot contract

The wave of depolarization can be sensed by an electrode and is called the electromyogram (EMG). The repolarization wave is too small to detect.

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MU ACTION POTENTIAL

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A neuron and the muscle A neuron and the muscle fibers are referred to as motor fibers are referred to as motor unit (MU)unit (MU)

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The nerve impulse is transmitted in a nerve axon as schematically shown down below

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+ -

A B

MU ACTION POTENTIAL

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MU ACTION POTENTIAL

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A dipole is moving along a volume conductor. A differential amplifier records the difference between the potentials at point A and B on the conductor.

+ -A B

+ -

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MU ACTION POTENTIAL

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+ -A B

The dipole is moving along the conductor. The potential A is getting more negative.

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MU ACTION POTENTIAL

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More the dipole is moving between the potentials more the signal is positive

+ -A B

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MU ACTION POTENTIAL

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+ -A B

Finally, the connector B registers the positive end of the dipole and the connector A is returning to zero. The result of the amplification becomes negative

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MU ACTION POTENTIAL

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+ -A B

The triphasic curve has some similarity with an action potential which passes through a nerve axon.

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MU ACTION POTENTIAL

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Number of MU varies with the type and Number of MU varies with the type and function of muscles.function of muscles.

MusclesMuscles Number of muscleNumber of muscle’’s fibers/Neurons fibers/NeuronPlatysmusPlatysmus 2525

Long Digital FlexorLong Digital Flexor 9595

Tibialis AnteriorTibialis Anterior 609609

GastrocnemiusGastrocnemius 17751775

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MU ACTION POTENTIAL

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Motor Unit RecruitmentMotor Unit Recruitment

Once an action potential reaches a muscle fiber, it Once an action potential reaches a muscle fiber, it propagates proximally and distally. This is called propagates proximally and distally. This is called motor action potential (MAP).motor action potential (MAP).

A motor unit action potential (MUAP) is A motor unit action potential (MUAP) is spatiotemporal summation of MAPs for an entire spatiotemporal summation of MAPs for an entire MU.MU.

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MU ACTION POTENTIAL

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An EMG signal is the algebraic summation of An EMG signal is the algebraic summation of many repetitive sequences of MUAPs for all many repetitive sequences of MUAPs for all active motor units in the vicinity of the recording active motor units in the vicinity of the recording electrodeselectrodes

MUAP1

MUAP2

MUAP3

MUAP4

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MU ACTION POTENTIAL

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MU 1

MU 2

MU 3

MU 4

Muscle tension

MU 1

MU 2MU 3MU 4

MU RecruitmentMU RecruitmentThe order of MU The order of MU recruitment is according recruitment is according to their sizes. The to their sizes. The smaller ones are active smaller ones are active first and the bigger ones first and the bigger ones are active last.are active last.

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MU ACTION POTENTIAL

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MUAP vs. Force MUAP vs. Force

– For a voluntary contraction, muscleFor a voluntary contraction, muscle’’s force s force depends on the number of MU and the depends on the number of MU and the frequency of activationfrequency of activation

– MuscleMuscle’’s force is proportional of the cross-s force is proportional of the cross-sectional area of the active muscle fibers.sectional area of the active muscle fibers.

– Muscle force during isometric action is Muscle force during isometric action is around 30 N/cmaround 30 N/cm22

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Recording Techniques

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A wide variety of electrodes are available to measure the electrical muscle output

• microelectrode and needle electrode (not practical for microelectrode and needle electrode (not practical for movement studies)movement studies)

• Surface electrodes (SE) and Intramuscular wire Surface electrodes (SE) and Intramuscular wire electrodes (IWE) are commonly used in movement electrodes (IWE) are commonly used in movement studiesstudies

The differential preamplifier increases the amplitude of the difference signal between each of detecting electrode and the common ground. The advantage of the differential preamplifier is to improve the signal-to-noise ratio of the measurement.

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Differential amplifier

Leads

Electrodes

Ground electrode

Cable

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EMG Signal Detection Summary• Bipolar electrodes (active electrode rather than

passive electrodes

• Distance between electrodes 10 to 20 mm apart

• Bandwidth of 20-500 Hz

• CMRR greater than 100 dB

• Noise less than 2mV

• Electrode located on the midline of the muscle belly

ability of a differential amplifier to perform accurate subtractions (attenuate common mode noise)usually measured in decibels (y=20 log10x)EMG amplifiers should be >80 dB (i.e., S/N of 10000:1, the difference between two identical 1 V sine waves would be 0.1 mV)most modern EMG amplifiers are >100 dB

ability of a differential amplifier to perform accurate subtractions (attenuate common mode noise)usually measured in decibels (y=20 log10x)EMG amplifiers should be >80 dB (i.e., S/N of 10000:1, the difference between two identical 1 V sine waves would be 0.1 mV)most modern EMG amplifiers are >100 dB

dynamic range is the linear amplification range of an electrical device

typical A/D computers use either +/–10 V or +/–5 V amplifiers usually have +/–10 V or more, oscilloscopes and

multimeters (+/–200 V or more) tape or minidisk recorders have +/–1.25 V EMG signals must be amplified usually 1000x or more but not too

high to cause amplifier “saturation” (signal overload) if too low, numerical resolution will comprised (too few

significant digits, from 12 bit to 8 bit or less)

• electrode pairs in parallel with fibres• midway between motor point and myotendonous

junction (belly of muscle)• approximately 2 cm apart, better if electrodes are

fixed together to reduce relative movements• leads should be immobilized to skin• ground electrode placed over electrically neutral area

usual bone• N.B. there should be only one ground electrode per

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FREQUENCY RESPONSE

•frequency responses of amplifier and recording systems must match frequency spectrum of the EMG signal•since “raw” surface EMGs have a frequency spectrum from 20 to 500 Hz, amplifiers and recording system must have same frequency response or wider•since relative movements of electrodes cause low frequency “artifacts,” high-pass filtering is necessary (10 to 20 Hz cutoff)•Since surface EMG signals only have frequencies as high as 500 Hz, low-pass filtering is desirable (500 to 1000 Hz cutoff)•therefore use a “band-pass filter” (20 to 500 Hz)

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impedance is the combination of electrical resistance and capacitance

all devices must have a high input impedance to prevent “loading” of the input signal

if loading occurs the signal strength is reduced

typically amplifiers have a 1 M input resistance, EMG amplifiers need 10 M or greater

10 G amplifiers need no skin preparation

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Dry skin provides insulation from static electricity, 9V battery discharge etc.

unprepared skin resistance can be 2 M or greater except when wet or “sweaty”

if using electrodes with < 1 G input resistances, skin resistance should be reduced to < 100 k

Vinput = [ Rinput/(Rinput + Rskin) ] VEMG

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telemetry has less encumbrance and permits greater movement space

radio telemetry can be affected by interference and external radio sources

radio telemetry may have limited range due to legislation (e.g., IC, FCC)

cable telemetry (e.g., Delsys) can reduce interference from electrical sources

telemetry more expensive than directly wired systems

telemetry has limited bandwidth (more channels reduces frequency bandwidth)

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Analysis of the EMG signal

RAWRAW

OnsetOnsetPeakPeak

In the time domain:• the root-mean squared (RMS)

value or also called Linear Envelop)

• the average rectified value

• Both are appropriate and provide useful measurements of the signal amplitude

• Muscle onset (time)

• Peak amplitude of RMS

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EMG: In the time domain

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same as taking the absolute value of the raw signal

mainly used as an intermediate step before another process (e.g., averaging, linear envelope and integration)

can be done electronically and in real-time

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Averaged EMGsimple to computecan be done in real-timeaveraged EMG is a “moving average” of a full-wave rectified EMGmust select an appropriate “window width” that changes with sampling rateeasy for determining levels of contraction

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Linear Envelope EMGrequires two-step process: full-wave rectification followed by low-pass filter (4-10 Hz cutoff)can be done electronically (but adds a delay)reduces frequency content of EMG and thus lowers sampling rates (e.g., 100 Hz) and memory storageeasy to interpret and to detect onset of activitycan be ensemble-averaged to obtain patternsdifficult to detect artifactsuseful as a control (myoelectric) signal

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Ensemble-Averaged EMGusually applied to cyclic activities and linear envelope EMGsrequires means for identifying start of cycle or start and end of activity

• foot switches or force platforms can be used for gait studies

• microswitches, optoelectric or electromagnetic sensors for other activities

• can also use a threshold detector of a kinematic or EMG channel

each “cycle” of activity must be time normalized

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EMG: In the time domainEnsemble-Averaged EMGamplitude normalization is often done

• to maximal voluntary contraction (MVC)• to submaximal isometric contraction• to EMG of a functional activity

mean and standard deviations for each proportion of cycle are computedmean and s.d. or 95% confidence interval may be presented to show representative contraction during activity cycleeasier to make comparisons among subjects“grand” ensemble-averages (average of averages) for comparisons among several experimental conditions

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Integrated EMG (iEMG)important for quantitative EMG relationships (EMG vs. force, EMG vs. work)best measure of the total muscular effortuseful for quantifying activity for ergonomic researchvarious methods:

• mathematical integration (area under absolute values of EMG time series)

• root-mean-square (RMS) times duration is similar but does not require taking absolute values

• electronically (see next page)

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Other Techniques auto-correlation (correlate signal with

itself shifted in time, gives signal characteristics)

cross-correlation (correlate signal with another EMG signal, tests for crosstalk)

zero-crossings (the more crossings the greater the level of recruitment)

peak counting (number of peaks above a threshold)

single motor unit detection double differential amplifier (velocity of

propagation)

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ECG Crosstalk ECG crosstalk occurs when recording

near the heart (ECG has higher voltages then EMG)

EEG crosstalk when near scalp (rare) difficult to resolve

• use right side of body (away from heart)• move electrodes as far away from heart as

possible• “signal averaging” (average many trials)• indwelling electrodes

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Muscle Crosstalk

one muscle’s EMG is picked up by another muscle’s electrodes

can be reduced by careful electrode positioning

can be determined by cross-correlation difficult to distinguish crosstalk from

synergistic contractions biarticular muscles have “extra” bursts of

activity compared to monoarticular muscles (if so crosstalk is not a problem)

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In the Frequency domain:• Spectral Density

–Median Frequency–Mean Frequency

• Wavelet

This represents the frequency contents of EMG signal.

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Frequency Spectrumuseful for determining onset of muscle fatiguemean or median frequency of spectrum in unfatigued muscle is usually between 50-80 Hzas fatigue progresses fast-twitch fibres drop out, shifting frequency spectrum to left (lowering mean and median frequencies)mean frequency is less variable and therefore is better than medianuseful for detecting neural abnormalities

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Interpretation of the EMG signal

EMG is a tool not without its hidden weaknesses

These problems have the potential to mask any benefit obtained from the recorded information.

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Anecdotal Demonstration

Adrian R. M. Upton conducted an anecdotal demonstration of the difficulty of documenting brain death by placing EEG electrodes in an upside-down bowl of lime Jell-O (reported in The New York Times, March 6, 1976, p. 50).

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Interpretation of EMG

As with EEG traces, the interpretation of the recorded EMG should be conducted with care.

However, with proper use, the surface electromyogram is a powerful and effective tool for both clinical evaluation and research.

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Applications in Orthopaedics

Most of the applications of sEMG and imEMG are based on:• Muscle activation and timing• Muscle contraction profile• Muscle strength of contraction • Muscle fatigue.

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Recent technological development in sEMG moved research from the laboratory to the field applications.

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APPLICATIONS IN SPORT MEDICINE

Objective: Examine the neuromuscular response to

functional knee bracing relative to anterior tibial translations.

Design: During randomized brace conditions,

electromyographic data with simultaneous skeletal tibiofemoral kinematics and GRF were recorded from four ACL deficient subjects to investigate the effect of the functional brace during activity.

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Ramsey, D. K., Lamontagne, M., Wretenberg, P., Valentin, A., Engström, B., & Németh, G. (2003). Electromyographic and biomechanics analysis of anterior cruciate ligament deficiency and functional knee bracing. Clin Biomech (Bristol, Avon). 2003 Jan;18(1):28-34

Muscle Activation and Timing1

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Methods: Kinematic and kinetic measure-ments

were synchronously recorded with the EMG signal. The EMG data from the RF, S, BF, and LG were integrated for each subject in three separate time periods: 250 ms preceding foot-strike and two consecutive 125 ms time intervals following foot-strike.

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Results: With brace, ST activity

significantly decreased 17% prior to footstrike

whereas BF significantly decreased 44% during A2, (P<0.05).

RF activity significantly increased 21% in A2 (P<0.05).

No consistent reductions in anterior translations were evident.

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Conclusion: Joint stability may result from proprioceptive feedback

rather than the mechanical stabilising effect of the brace. As a result of bracing, we observed decreased S and BF activity but increased RF activity. We suggest increased afferent input from knee proprioceptors and brace-skin-bone interface modifies EMG activity.

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Gender Difference for a cut motion Male and Female elite football players Control speed Cue given at 1.2m from the FP

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See EMG DataSee EMG Data

CC1122

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We investigated possible differences in muscle fatigue and recovery of knee flexor and extensor muscles in patients with a deficient anterior cruciate ligament compared with patients with a normal anterior cruciate ligament.

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Muscle Fatigue1

Surface EMG can be used as muscle fatigue indicator

Tho, K., Németh, G., Lamontagne, M., & Eriksson, E. (1997). Electromyographic Analysis of Muscle Fatigue in Anterior Cruciate Ligament Deficient Knees. Clinical Orthopaedics & Related Research(340), 142-151.

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SEMG of 15 patients with ACL deficiency was measured while the muscles were under 80% of MVC for 60 s and remeasured after 1, 2, 3, and 5 minutes of rest

Knee joint was at 45 degrees of flexion.

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Muscle Fatigue1

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Findings showed that:• First 60 s of contraction

> all muscles recorded significantly decreased MPF

> an increase in LEEMG amplitude. • Rate of decrease of MPF was significantly greater in

the injured quadriceps and normal hamstrings. • All muscles recovered to the initial MPF level after

1 min of rest but two muscles in the injured and normal limb recorded an overshoot of mean power frequency during the recovery phase.

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Muscle Fatigue1

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The findings confirmed • the fatigue state in all the muscles, suggest

recruitment of more Type II fibers as the muscle fatigue

• show the physiological adaptation of the quadriceps and hamstrings to ACL deficiency.

• dissociation between low intramuscular pH and mean power frequency during the recovery phase.

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Muscle Fatigue1

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We investigated the possible influence of wearing functional knee braces on various factors of muscle fatigue.

• Measured parameters were; MVC, Peak Velocity (PK), power and number of repetition to muscle fatigue during isokinetic exercise, and also muscle fatigue during 50s isometric contraction

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Muscle Fatigue2

Lamontagne, M. & Sabagh-Yazdi, F. (1999). The Influence of Functional Knee Braces on Muscle Fatigue. Paper presented at the XVIth of the International Society of Biomechanics, Calgary, Canada.

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Two groups of healthy and ACL-deficient knee joint subjects with an average age of 28.8 years and 26,6 years respectively volunteered to this study.

All tests were performed on an isokinetic device (Kin-Com 500H) while the EMG signal was collected at 1000 Hz for six muscles (RF), (VL), (VM), (G), (MH) and (LH).

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Muscle Fatigue2

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Analysis of EMG data revealed that• no significant differences were obtained for the EMG

amplitude or the integral of the linear envelope EMG between the groups and conditions

• During the 50s isometric exercise at 80% MVC, the fatigue state is represented by decline of MF value of EMG signal greater than 10 Hz

• Muscle fatigue state was obtained in all muscles

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Muscle Fatigue2

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• Percentage of decline of MF in the Gastrocnemius was significantly different between the groups (p<0.05).

• Percentage of decline of median frequency in VM and G of ACL group and VL and G of healthy group was found statistically different (p<0.05) between conditions.

• the outcomes showed a high correlation between the subjective perception of fatigue and percentage of decline of the MF (r = 0.64) for VL and RF muscles during the brace condition.

• All other muscles showed very low correlation.

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Muscle Fatigue2

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CONCLUSION

Factors like signal reliability, muscle synergy, mechanisms of proprioception, muscle fatigue mechanisms have been a great deal of interest in movement studies but these topics certainly need more research in order to understand muscle function and adaptation for ordinary people and athletes.

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Lamontagne, M. (2000). Electromyography in sport medicine (Chapter 4). In Rehabilitation of Sports Injuries (Ed. G. Puddu, A. Giombini, A. Selvanetti ), Springer-Verlag, Berlin, Heidelberg, New York

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Partly funded by:

Natural Sciences and Engineering Council of CanadaandLet People Move

Partly funded by:

Natural Sciences and Engineering Council of CanadaandLet People Move

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