ACTIVITIES OF SINGLE MOTOR UNITS IN MAN DURING SLIGHT VOLUNTARY EFFORTS

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Abstract
Electrical records were obtained from 1 or a few units of normal human muscle under various conditions of slight voluntary effort. Discrete, slight and brief voluntary efforts may each be accompanied by a single discharge of the motor unit whose activity is being recorded. For quick movements of slightly greater force there may be double discharges of a single recording unit. Rhythmic movements, repeated several times per sec., may each show a single response of a single recording unit. With more intense, quick movements additional motor units are brought into activity. The more intense the movement the more are summated potential spikes to be observed in the initial phase of the response. Sustained movements may be begun slowly with a gradual increase of discharge frequency of the units of lowest threshold and with a gradual accession of additional units. If the movement is begun suddenly there is usually a double discharge of the units of lowest threshold and an approach toward a simultaneity of discharge of the various units. Following this first burst the discharge of the various units quickly becomes quite asynchronous. Cessation of movement may be due either to gradual or to sudden cessation of unit activity. No records have shown rotation or alternation of unit activity in sustained tension efforts provided that the movement pattern has remained unchanged. No evidence supports the statement that there is increased amplitude of the single unit spike with increased muscular tension.

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