electromyography (emg) theory of operation & underlying anatomical and physiological issues
TRANSCRIPT
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
1.0
-1.0
-0.5
0.0
0.5
40000 500 1000 1500 2000 2500 3000 3500
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
0.0000
0.0010
0.0020
0.0030
0.0040
0.0050
0.0060
0.0070
0.0080
600.00.0 100.0 200.0 300.0 400.0 500.0
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.