A novel viewpoint on anticipatory postural adjustments during gait initiation

Anticipatory Postural Adjustments (APAs) are coordinated muscular activities aiming to counteract the postural perturbations produced by the voluntary movements. Many studies about gait initiation indicate as APAs those activities that precede heel-off of the leading foot, choosing the heel-off as the onset of voluntary movement. However, since initiating gait means pushing forward the body rather than lifting the foot, the onset of gait should be the displacement of the Centre of Mass (CoM), which starts with the backward shift of the Center of Pressure (CoP). Therefore, leg muscles driving such shift are prime movers and APAs should be searched for in trunk muscles. In order to test this hypothesis, we analyzed gait initiation in 15 right-footed healthy subjects. The CoP position and the electromyographic activities were measured with a force plate and surface probes, respectively. On the right side, Rectus and Obliquous Abdominis were active in 11 and 13 subjects, respectively, starting in average 33 and 54 ms before the CoP shift; Erector Spinae at L2 and T3 levels were instead inhibited (9 and 7 subjects, 104 and 120 ms). On the contralateral side, the same muscles showed excitatory APAs (Rectus and Obliquous Abdominis in 11 and 12 subjects, 27 and 82 ms; Erector Spinae in 10 and 7 subjects, 75 and 32 ms). Present results provide a novel framework for distinguishing postural from voluntary actions, which may be relevant for gait disorders diagnosis and rehabilitation.