Effects of amplitude and predictability of perturbations to the arm on anticipatory and reactionary muscle responses to maintain balance.

Disturbances to balance arising from forces applied to the upper limb have received relatively little attention compared to disturbances arising from support surface perturbations. In this study we applied fast ramp perturbations to the hand in anterior, posterior, medial and lateral directions. The effects of perturbation predictability and amplitude on the postural response of upper limb, trunk and lower limb muscles were investigated. Perturbations were applied either in blocks of constant amplitude and direction (predictable) or with direction and amplitude varying randomly (random) from trial to trial. The spatial-temporal patterns of anticipatory muscle activation under the predictable condition and the reactionary responses following the perturbation under both conditions were similarly organized. The size of the response increased systematically with the perturbation magnitude for both anticipatory and reactionary changes in muscle activation. However, the slope of the relation between perturbation amplitude and the magnitude of the change in muscle activation was greater when perturbations were predictable than when they were randomly selected. The timing of both the anticipatory and reactionary increases in muscle activation was invariant across perturbation amplitudes. The characteristics of the reactionary responses have a similar organization to the long latency muscle responses to support surface perturbations.