Instruction-dependent muscle activation patterns within a two-joint synergy: separating mechanics from neurophysiology.

The following issue was addressed in the present study. Can instruction modify the involvement of different muscles when mechanical conditions and characteristics of a motor task remain unchanged? Subjects (N = 9) produced force by pressing their right hands against a fixed pad and were asked, in different trials, to predominantly use muscles that control either their elbows or their wrists. Force characteristics remained the same. In some trials, the setup was unexpectedly released so that a flexion movement occurred. Instruction changes induced changes in the muscle activation patterns and in preprogrammed reactions seen in unexpectedly released trials. The authors conclude that there may be different neurophysiological solutions to a specific mechanical task and that patterns of muscle activation may reflect features of both the explicit motor task and the subject's intention that may have no obvious mechanical correlate.