Spike-triggered averaging was used to extract the twitch tensions and contraction times of 144 motor units from the first dorsal interosseous muscle of four subjects for three different directions of static contraction: abduction of the index finger, flexion of the index finger, and adduction of the thumb coupled with flexion of the index finger (hereafter referred to as adduction). Although the twitch tensions were generally largest for the abduction contraction, all units contributed tension to all three directions of contraction. A linear correlation was found for twitch tensions of motor units for the three directions of static contractions. Linear correlations were also found between twitch tension and threshold force of these motor units for each direction, which suggests that an orderly pattern of recruitment, according to increasing twitch size, adequately describes the function of human first dorsal interosseous muscle for all contraction directions. No clear evidence was found for separate groups of motor units in the muscle that were selectively activated for the different tasks. Rank order of recruitment for motor units in the three directions of contraction was correlated, but was not identical. The scatter in our data is discussed in relation to earlier reports of altered motor-unit recruitment during different movements.
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