Fluctuations in motor output during steady contractions are weakly related across contraction types and between hands.

The presence of differences in motor unit activity across contraction types and between hands suggests that the magnitude of fluctuations in motor output is only weakly related when these conditions are compared. Twenty right-handed young (24.1 +/- 5.3 years) and old (72.5 +/- 4.9 years) adults performed three levels (</=40% of maximal force) of isometric and anisometric contractions with the first dorsal interosseous muscle of each hand. The fluctuations in motor output were quantified by the coefficient of variation for force during isometric contractions, and as the standard deviations of acceleration and position during anisometric contractions. There was no effect of age on fluctuations in motor output. The magnitude of the fluctuations in motor output was weakly related across contraction types (r(2) < 0.325) and between hands (r(2) < 0.262). Furthermore, the standard deviations of acceleration and position during shortening and lengthening contractions were largely unrelated to one another. Because the activity in a motor unit population differed across contraction types and hands during steady contractions, the central nervous system likely employed distinct strategies to accomplish these different tasks.