Are simultaneous postural adjustments (SPA) programmed as a function of pointing velocity?

This paper deals with the influence of velocity on the postural adjustments that occur during the course of a voluntary movement, that is to say, simultaneous postural adjustments (SPA). To this aim, a pointing task performed at different velocities (V) was considered. Upper limb kinematics and body kinetics were recorded. Using a 2-DOF model, the body was divided into two parts: the right upper limb (termed the "focal" chain) and the rest of the body (termed the "postural" chain). This model allowed us to calculate the kinetics of both subsystems (-F x and [Formula: see text]), with one corresponding to the resultant action on the shoulder (AoSh: -F x) and the other to the resultant reaction of the shoulder (RoSh: [Formula: see text]). The influence of pointing velocity on peak amplitudes and durations was evaluated, as was their instantaneous relationship ("Lissajous ellipse"). The results showed that RoSh and AoSh display similar diphasic profiles, whose amplitude and duration vary with movement velocity. In addition, RoSh is in phase advance of AoSh, the advance being all the shorter as the focal movement velocity becomes faster. Finally, SPA appears to play a dual role, which includes a propulsive action during upper limb acceleration and body stabilization during deceleration. These new findings strengthen the hypothesis that the postural chain is programmed according to task velocity in the same way as the focal chain and that both are coping in order to make the task more efficient.