An electromyogram (EMG) records the collective action potentials of muscle fibers. By stimulating a nerve and measuring the resulting motor response via needle or surface electrodes, clinicians can diagnose the location and severity of neuromuscular diseases like muscular dystrophy or ALS.
The recording is evaluated based on three fundamental properties:
Amplitude (measures the number of functional muscle fibers and motor units activated; decreases if fibers are damaged).
Duration (measures the speed of nerve conduction and synchronized fiber activation; affected by distal latency and neuromuscular transmission).
Waveform (measures the overall integrity and "shape" of the motor response; used to differentiate neurological disorders from primary muscle tissue diseases).
Key Factors Influencing Results
Stimulation: Supramaximal stimulation is required to ensure a maximal motor response.
Conduction Velocity: Depends on the integrity of the myelin sheath. Higher velocities are typically recorded from more proximal nerve stimulation.
Temperature: Cold temperatures artificially increase amplitude while slowing conduction velocity.
| Property | Myogenic (e.g., Muscular Dystrophy) | Neurogenic (e.g., ALS/Nerve Damage) |
| Amplitude | Decreased (Fewer functional fibers per unit) | Variable (Can be high due to reinnervation) |
| Duration | Shortened (Loss of muscle fibers) | Increased (Slowed conduction or reinnervation) |
| Waveform | Polyphasic/Small (Fragmented signals) | Large/Complex (Giant potentials) |
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