Electromyography (EMG) Theory of Operation & Underlying Anatomical and Physiological Issues

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

Simple Overview u Electrical discharge (signals) from muscles recorded with electrodes –Indwelling: needle or fine wire –Surface: mono- or bipolar u Signals are low amplitude voltages at relatively low frequencies ( Hz) of firing –mV –uV

Simple Overview u 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 u Clinical/diagnostic using needle electrodes (usually bipolar) u Research/movement analysis using surface or fine wire electrodes –On/off phenomena –Timing –Signal quantification (integration/area under a curve

Research EMG (cont’d) u Force analysis –EMG ~ force output »Isometric contractions »Isotonic/isokinetic contractions (questionable) u Spectrum/frequency analysis (FFT or DFT) –Fatigue study: slow vs fast twitch motor units u Biofeedback

Underlying Anatomical Issues u Gross level of analysis u Microanatomical level of analysis

The Sarcomere

Underlying Physiological Issues u 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 u 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 u 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) u Acetylcholine (or other neurotransmitters) depolarizes the PSM at the motor endplate –Na + rushes into the cell »Reverses intra-cellular polarity ~ +20mV u Initiates a wave of de- and re-polarizations = AP

Effect of AP u 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 u 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

Motor Units u The functional unit of the neuromuscular system –Terminal axon of motor endplate –Synapse –Post-synaptic membrane of associated muscle fiber u Classification of motor units varies –Physiological analysis –Mechanical/velocity of contraction (twitch) analysis

Burke’s Classification (Physiological) u Type I –Slow twitch oxidative –Fast twitch fatigue resistant u Type IIa –Fast twitch oxidative –Fast twitch fatiguing u Type IIb –Fast twitch glycolytic

Mechanical/Velocity of Contraction (Twitch) Classification u Slow twitch - fatigue resistant –Low conduction velocity –Long twitch contraction time –Low contraction velocity u Well suited for low- level activities at low frequencies ( Hz) –High endurance

Mechanical/Velocity of Contraction (twitch) Classification u Fast twitch - fatiguing –Higher conduction velocity –Shorter twitch contraction time –High contraction velocity u Short duration high- burst activity at intermediate and high frequencies ( Hz) – Low endurance

Muscular Performance u Smoothness of contraction a result of a mix of slow and fast twitch motor units Velocity Slow Fast Muscle Function Precise Well-timed Coarse Impulsive

Motor Unit Recruitment u Slow twitch motor units recruited first –Postural control –Finely graded movements u Fast twitch units recruited last –Rapid, powerful, impulsive movements u EMG can be used to study fatigue by analyzing frequency (e.g., median power frequency) characteristics during spectral analysis

Power Spectrum Frequency (Hz)

Factors That Influence the Signal Information Content of EMG - Table FactorInfluence Neuroactivation- 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 (cont’d) FactorInfluence Electrode-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 Table 1 1 Soderberg, 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.