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1. The contractile properties of human motor units from the first dorsal interosseus muscle of the hand were studied during voluntary isometric contractions using recently developed techniques.
2. The twitch tensions produced by motor units varied widely from about 0·1-10 g. The twitch tension of a motor unit varied nearly linearly as a function of the level of voluntary force at which it was recruited over the entire range of forces studied (0-2 kg).
3. The number of additional motor units recruited during a given increment in force declined sharply at high levels of voluntary force. This suggests that even though the high threshold units generate more tension, the contribution of recruitment to increases in voluntary force declines at higher force levels.
4. Contraction times for these motor units varied from 30 to 100 msec. Over 80% had contraction times less than 70 msec, and might be classed as fast twitch motor units. The larger motor units, which were recruited at higher threshold forces, tended to have shorter contraction times than the smaller units.
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