Chapter 8 - Objectives
- Determine the basis of the muscle fiber action potential and how it propagates along the muscle fiber
- Determine the characteristics of the electromyographic signal
- Understand the basic features of EMG electrodes
- Examine some of the technical issues that alter the characteristics of the EMG signal
- Determine what variables are usually implemented to describe the EMG signal
- Review some examples to illustrate how EMG has been used to understand human movement
Physiology of the Electromyographic Signal
Resting muscle (-90mV) due to various concentrations of Na+, K+, and Cl-
- About 9 to 15mV more positive in slow twitch fibers
- Resting potential can be altered through exercise training
Action potentials (lead to about +30mV) due to Na+ entering the muscle
- Propogation of the muscle fiber action potential
- Action potential travels in opposite directions from the neuromuscular juntion
- Fast-twitch fibers have faster propogations
- Atrophied fibers have slower propogations
- Increases in the length of muscle fibers decrease conduction velocity
- Muscle fiber conduction velocity
- Amplitude is greater in fast-twitch fibers
- Fast-twitch fibers have shorter wavelength
- Larger diameter fibers have greater amplitudes
Increasing muscle force
- Increased recruitment (number of motor units activated) leads to greater force production
- Nervous system controls firing rate
- Increased firing rate leads to greater force production
- Sometimes a doublet is used at the initiation of muscle activation
- Synchronization (more than one motor unit firing simultaneously)
EMG Signal
- EMG signal is the composite sum of all the active motor units
- Cancelation and synchronization
- EMG amplitude increases linearly with increased muscular contraction
- EMG amplitude does not increase linearly with increased muscular force
- Cocontraction
Recording the EMG signal
- Monopolar versus bipolar
- RF and electrical noise is reduced with bipolar
- Common-mode rejection ratio
Surface Electrodes
- When skin is stretched, the recorded EMG drops to about 25mV
- Silver-silver Chloride electrodes help reduce this problem
- Abrading the skin lightly reduces the impedence across the skin surface, reducing the skin stretching problem
- RF signals and electrical noise (Impedence must be minimized)
- Remove dead skin cells and oils
- Increase local blood flow
- Minimize cable distances
- Use sheilded cables
- Braid individual electrode cables together
- Pre-amplified electrodes (higher signal to noise ratio)
- Limitations
- Difficulty in recording activity from deeper muscles (10-20mm depth is range)
- Crosstalk between muscles
- Deeper motor units may be smaller, thus surface EMG measurements may be biased towards large motor units
Fine-Wire Electrodes
- Deeper muscles and a more specific location can be recorded
Needle Electrodes
- Capable of measuring individual motor units
Electrode Placement
- Away from highly tendinous areas
- Avoid the motor point (location where the nerve enters the muscle)
- The endplate zone has the potential for the greatest variability in EMG signal
- Orient electrodes parallel to muscle fibers
Ungrounded EMG
Analyzing and Interpreting the EMG Signal
- Frequency
- Turning Points and Zero-Crossings
- Mean and Median Frequency
- An increase in frequency can mean
- More fast-twitch fibers are active
- A higher firing rate of slow-twitch fibers
- Activation of muscle fibers with higher conduction velocities
- Decreased motor unit synchronization
- Additional activation of synergist muscles
- A decrease in frequency can mean
- Less fast-twitch fibers are active
- Increase in motor unit synchronization
- A decrease in the number of active motor units
- A decrease in motor unit firing rate
- A slowing of conduction velocity
- A change in the intramuscular milieu
- Dynamic contractions
- Signal must remain stationary
Normalization of EMG
- EMG amplitude divided by EMG amplitude during maximal contraction
- Electrical stimulation to produce maximal M-wave amplitude
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