1. The twitch properties of human thenar motor units were examined in response to intraneural motor-axon stimulation. Force components of thumb abduction and flexion were measured before and after tetanic stimulation. The magnitude, direction, and time derivatives of resultant forces, together with axon conduction velocities, were calculated for each unit. 2. Various indexes of contraction and relaxation rate were measured including contraction time (time from force onset to peak), one-half relaxation time (time from peak force to one-half that value), normalized maximum contraction and normalized maximum relaxation rates (peak positive and negative time derivatives of the force signal normalized to twitch force), and the times at which these maximum rates occurred. 3. For different units, the directions of resultant forces were approximately evenly distributed between thumb abduction and flexion. At the onset of the experiment, initial twitch forces ranged from 3 to 34 mN, contraction times from 35 to 80 ms, and one-half relaxation times from 25 to 108 ms. 4. Resultant twitch forces were positively correlated to normalized maximum relaxation rates, but not to other rate indexes or to conduction velocity. The various contraction rate measures were correlated to each other, but generally not to relaxation rates. 5. After the first test involving tetanic stimulation, the twitches of most units were potentiated and slowed, especially their relaxation phase. However, the extent of these changes varied considerably between units. In general, units with weak initial forces potentiated most, some up to three-fold. These changes in twitch properties were denoted posttetanic twitch potentiation. 6. The ratios between twitch and tetanic forces varied from 0.04 to 0.25 initially, and from 0.08 to 0.34 after potentiation. Twitch and tetanic forces were positively correlated, with a stronger relationship after potentiation (r = 0.79). 7. Axon conduction velocities were not significantly correlated to initial or potentiated twitch forces or tetanic forces. However, weak correlations were found between axon conduction velocities and contraction rate measures.
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