APAs mean that the onset of postural changes occurs prior to the onset of the postural disturbance due to the movement. They also mean that a feed-forward postural control is associated with the movement control that prevents the occurrence of movement-related posture and equilibrium disturbances.
Specifically, the control of equi- librium during leg movements is interesting to analyse because the moving limb is involved in body support. Movements of the leg change the support conditions and entail a shift of the centre of gravity position prior to movement onset [3].