Chronometric analysis of the posturo-kinetic programming of voluntary movement

Posturo-kinetic programming was investigated in a simple reaction time paradigm. Standing subjects performed voluntary upper limb elevations under three conditions differing by the importance of their destabilizing effect on the initial balance. Three indexes were considered: the reaction time (RT) corresponding to delay between the response signal (RS) and the onset of the upper limb acceleration at wrist level (Aw); the motor latency (ML) corresponding to delay between RS and onset of the earlier of the two accelerations at shank level (As) which was previously shown to indicate the onset of postural adjustments; and the postural anticipation (PA) corresponding to the delay between the onset of the earliest As and the onset of Aw. The results showed that the RT varied in relation to parameters of the forthcoming movement. RT variations were essentially due to differences in the PA. It appeared that the ML depended on cognitive factors while the PA depended on biomechanical ones. it is concluded that ML corresponds to the "true" reaction time while PA covers the earliest part of the motor response.