Axonal conduction velocity and force of single human motor units

Tungsten microelectrodes of the type used for microneurography have been used to record motor units selectively from the first dorsal interosseous and abductor pollicis brevis muscles of normal subjects and patients who had had complete sections of the ulnar or median nerve. After determining the recruitment threshold and the twitch tension (spike-triggered averaging) of a single unit, its nerve was stimulated at the wrist and the elbow using surface electrodes. By adjusting the position of the surface electrode and the stimulus intensity and by using computerized subtraction of responses just above and below threshold for a given unit, the same motor unit could often be identified in response to stimulation at both sites and its conduction velocity determined. The twitch tension and recruitment threshold of the motor units were closely correlated with the conduction velocity of the motor axons in normal subjects. Preliminary data from patients suggests that this method should be applicable to patients with a number of neuromuscular disorders.