electromyography (emg) theory of operation & underlying anatomical and physiological issues

Electromyography (EMG)

Theory of Operation

&

Underlying Anatomical and

Physiological Issues

Theory of Operation

Direct assessment of electrical activity of muscle– Indirectly assesses peripheral nerve continuity

Simple Overview Electrical discharge (signals) from muscles

recorded with electrodes– Indwelling: needle or fine wire– Surface: mono- or bipolar

Signals are low amplitude voltages at relatively low frequencies (75 - 250 Hz) of firing– mV– uV

Simple Overview

Signals are:– Pre-amplified (e.g., gain = 35)– Amplified (e.g., gain = 5000)– Displayed on a monitor or oscilloscope

» Evaluated in real-time

» Stored on HD/tape for subsequent analysis

Types of EMG Analysis

Clinical/diagnostic using needle electrodes (usually bipolar)

Research/movement analysis using surface or fine wire electrodes– On/off phenomena– Timing– Signal quantification (integration/area under a

curve

Research EMG (cont’d)

Force analysis– EMG ~ force output

» Isometric contractions

» Isotonic/isokinetic contractions (questionable)

Spectrum/frequency analysis (FFT or DFT)– Fatigue study: slow vs fast twitch motor units

Biofeedback

Underlying Anatomical Issues

Gross level of analysis Microanatomical level of analysis

The Sarcomere

Underlying Physiological Issues Resting membrane potential

– Potential difference exists across the sarcomere» Intra-cellular fluid has a high [K+]

» Extra-cellular (interstitial) fluid has a high [Na+] and [Cl-]

Net Effect

The (net) effect of concentration gradients, the difference in potential across the sarcolema and active Na+ & K+ pumps results in a potential difference of ~ -80mV (inside of muscle cell relative to outside)

Resting Membrane Potential

System stays in equilibrium (~ -80mV) until an intra- or extra-cellular stimulus is applied– AP causing liberation of Ca+ from the

sarcoplasmic reticulum– Galvanic stimulation

Action Potentials (AP) Acetylcholine (or other neurotransmitters) depolarizes the

PSM at the motor endplate

– Na+ rushes into the cell

» Reverses intra-cellular polarity ~ +20mV Initiates a wave of de- and re-polarizations = AP

Effect of AP

Causes a release of Ca+ from the sarcoplasmic reticulum triggering the molecular interaction of actin and myosin resulting in sacromere (microanatomical level) and gross muscle shorting (macroanatomical level) with resultant tension production

EMG EMG electrically detects AP’s as small voltages

– Records potential difference as a wave of depolarization traverses under one and than the other electrode

– The result is two monophasic waves

Typical EMG Interference Pattern

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Motor Units The functional unit of the neuromuscular system

– Terminal axon of motor endplate

– Synapse

– Post-synaptic membrane of associated muscle fiber Classification of motor units varies

– Physiological analysis

– Mechanical/velocity of contraction (twitch) analysis

Burke’s Classification (Physiological)

Type I– Slow twitch oxidative– Fast twitch fatigue resistant

Type IIa– Fast twitch oxidative– Fast twitch fatiguing

Type IIb– Fast twitch glycolytic

Mechanical/Velocity of Contraction (Twitch) Classification

Slow twitch - fatigue resistant– Low conduction

velocity

– Long twitch contraction time

– Low contraction velocity

Well suited for low-level activities at low frequencies (75 - 125 Hz)– High endurance

Mechanical/Velocity of Contraction (twitch) Classification

Fast twitch - fatiguing– Higher conduction

velocity

– Shorter twitch contraction time

– High contraction velocity

Short duration high-burst activity at intermediate and high frequencies (125 - 250 Hz)– Low endurance

Muscular Performance

Smoothness of contraction a result of a mix of slow and fast twitch motor units

Velocity

Slow Fast

Muscle FunctionPreciseWell-timed

CoarseImpulsive

Motor Unit Recruitment

Slow twitch motor units recruited first– Postural control

– Finely graded movements

Fast twitch units recruited last– Rapid, powerful,

impulsive movements

EMG can be used to study fatigue by analyzing frequency (e.g., median power frequency) characteristics during spectral analysis

Power Spectrum0.0090

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Frequency (Hz)

Factors That Influence the Signal Information Content of EMG - Table 4-11

Factor InfluenceNeuroactivation - firing rate of motor unit AP’s

- no. of motor units recruited

- synchronization of motor units

Muscle fiber physiology - conduction velocity of fibers

Muscle anatomy - orientation & distribution of fibers

- diameter of muscle fibers

- total no. of motor units

Electrode size/orientation - no. of fibers in pickup area

Factors That Influence the Signal Information Content of EMG - Table 4-11 (cont’d)

Factor InfluenceElectrode-electrolyte - type of material and site

interface - electrode impedance decreases with increasing frequency

Bipolar electrode - distance between electrodes

configuration - orientation of electrodes relative to the axis of muscle fibers

Figures and Table1

1Soderberg, G.L. (Ed.) (1992). Selected topics in surface electromyography for use in the occupational setting: Expert perspectives. Washington, D.C.: U.S. Department of Health and Human Services, Public Health Services.