Latency of anticipatory postural adjustments depends on movement instruction, not on movement velocity

According to literature, Anticipatory Postural Adjustments (APAs) are programmed according to speed of voluntary movement. However, the linkage between APAs and speed has been demonstrated by instructing subjects to voluntarily change movement velocity; therefore, till now it has been impossible to discern whether the key factor determining APA latency is movement instruction or the actual movement speed. Aim of this study is to distinguish between these two factors. We analysed the APA chain that stabilise the arm during a brisk index-finger flexion (Caronni & Cavallari, Exp Brain Res 2009), in two groups of subjects: i) 29 who compose our database and all received a go-fast instruction, but actually performed the movement with different speeds (from 238 to 1180°/s) and ii) 10 new subjects who performed the go-fast instruction at more than 500°/s and were then asked to go-slow at about 50% of their initial speed, thus moving at 300-800°/s. No correlation between APA latency and movement speed was observed when all subjects obeyed the go-fast instruction (P>0.50), while delayed APAs were found in the 10 new subjects when they followed the go-slow instruction (P<0.001). Moreover, at 300-800°/s, APA latency depended on movement instruction: subjects going fast showed earlier APAs than those going slow (P<0.001). These data suggest a stronger role of movement instruction vs. actual movement speed in modifying the timing of postural muscles recruitment with respect to prime mover.