Coactivation of the antagonist muscle does not covary with steadiness in old adults.

The purpose of the study was to determine the association between steadiness and activation of the agonist and antagonist muscles during isometric and anisometric contractions. Young (n = 14) and old (n = 15) adults used the first dorsal interosseus muscle to perform constant-force and constant-load tasks (2.5, 5, 20, 50, and 75% maximum) with the left index finger. Steadiness was quantified as the coefficient of variation of force and the SD of acceleration normalized to the load lifted. The old adults were less steady at most target forces with isometric contractions (2.5, 5, and 50%) and with most loads during the anisometric contractions (2.5, 5, and 20%). Furthermore, the old adults were less steady when performing lengthening contractions (up to 50%) compared with shortening contractions, whereas there was no difference for young adults. The reduced steadiness exhibited by the old adults during these tasks was not associated with differences in the average level of agonist muscle electromyogram or with coactivation of the antagonist muscle.