Early activation of arm muscles follows external perturbation of upright stance.

Grasping, counterbalancing and protective arm movements are an important defence against external postural perturbation, but are commonly constrained in studies of postural control. We measured muscle activity at the shoulder, and the lower leg, during unconstrained responses to platform translation. Results revealed very early activation in shoulder muscles, similar in timing to the 'automatic' ankle responses. The arm activation occurred even when the reaction provided no immediate defence against destabilization but would appear to be more than a 'startle' response, since the activation was scaled to the perturbation magnitude and persisted even when perturbations were expected. The arm activation would appear to be driven from a remote sensory source, since there was negligible loading or stretch of the arm muscles.