Contraction time and voluntary discharge properties of individual short toe extensor motor units in man.

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Abstract
The contraction time and the voluntary discharge properties of 45 short toe extensor motor units were studied in man. The contraction time of the individual motor unit was studied by using selective electrical nerve stimulation or by averaging the increase in force related to its electromyographic (EMG) potential in tonic voluntary contraction. Both methods showed a range of contraction times from 40-90 ms. The discharge properties of the individual motor unit were studied with EMG techniques, permitting the identification of its potentials during maximum voluntary effort. A motor unit which could be driven continuously and had a mininum rate of about 10/s and a maximum rate of about 30/s had a conctraction time between 60 and 90 ms. A motor unit which could not be driven continuously and had a minimum rate of about 20/s and a maximum rate above 40/s had a contraction time between 40 and 55 ms. A motor unit with intermediate voluntary discharge properties had an intermediate contraction time. Each motor unit fires at its fusion frequency in voluntary contraction; the voluntary discharge frequency range of a motor unit can be used as an indication of its contraction time.