Postural response characterization of standing humans to multi-directional, predictable and unpredictable perturbations to the arm.

When the arm of a standing human is perturbed in an unpredictable direction, postural muscles are activated at latencies as short as 50-110ms. While the motion of the body clearly progresses in hand-to-leg sequence, there is no systematic muscle activation sequence from the arm to the leg muscles, suggesting that the activation of the muscles is not likely the result of local stretch reflexes. In fact, the lower limb muscles are activated before the upright posture is significantly disturbed. The short-latency activation amplitude and the activation probability are clearly tuned to the direction of the arm perturbation for both rostral and caudal muscles. The effect of central set on the short-latency response has been investigated by manipulating the predictability of the perturbations. Possible underlying neural mechanisms have been discussed.