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1. Human subjects generated approximately linearly increasing or decreasing voluntary, isometric contractions using the first dorsal interosseus muscle of the hand.
2. Single motor units began firing at 8·4±1·3 impulses/sec (mean ± S.D. of an observation) and increased their firing rate 1·4±0·6 impulses/sec for each change of 100 g in voluntary force. These values were independent of the threshold force for recruiting motor units.
3. At intermediate rates of increasing and decreasing voluntary force (one complete cycle every 10 sec) the firing rate of single motor units varied linearly with force over the entire range of forces studied. However, during slow increases in voluntary force, the firing rate tended to reach a plateau, while during rapid increases an initial train of impulses at a roughly constant rate was observed.
4. The relative importance of recruitment and increased firing rate, as mechanisms for increasing the force of voluntary contraction, was determined. Only at low levels of force is recuitment the major mechanism. Increased firing rate becomes the more important mechanism at intermediate force levels and contributes the large majority of force if the entire physiological range is considered.
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