Do fast voluntary movements necessitate anticipatory postural adjustments even if equilibrium is unstable?

Anticipatory postural adjustments (APA) were studied in maximum velocity flexion of lower limb from two initial postures, a bipedal stance (Fbu) and unipedal stance (Fuu). In Fbu, the dynamics of center of gravity (CG) and ankle and hip muscle EMG activity showed large APA. In contrast, in Fuu there were no APA, the CG dynamics and the ankle EMG activity started at the same time as the intentional movement while the hip EMG activity started some 30 ms before the thigh flexion. The knee flexion velocity was lower in Fuu than in Fbu (7 rd/s versus 12 rd/s). These results suggest that fast voluntary movements do not require APA when the postural equilibrium is unstable, and that an alternative strategy is used. The absence of APA in Fuu, in contrast to the presence of APA in Fbu, suggests that the postural command and the focal one are time-locked and organized in a parallel process.