Prior intention to mimic a balance disorder: does central set influence normal balance-correcting responses?

Abstract

This study investigated the alterations caused by active, potentially destabilizing, postural movements to normal balance-correcting responses when subjects pretended to be unstable. The destabilising muscle synergies were pre-selected by the test subjects and, presumably, could be influenced by central set. After practice in responding to two series of 4 deg dorsiflexion rotations of a support surface presented with random inter-stimulus intervals, subjects were requested to mimic a balance disorder by generating a consistent pattern of unstable movement at the onset of each of the third series of identical rotations. Subjects either excessively rotated the trunk forwards or backwards, or allowed themselves to fall backwards like an inverted pendulum. These changes in the postural response strategy were accompanied by a suppression of early (120–220 ms) stabilising ankle muscle activity which preceded destabilising voluntary activation of the back and neck muscles at 200–250 ms. Increased activation post-400 ms was observed in all recorded muscles except in the ankle muscles when the trunk was rotated forwards. Latency alterations to the normal patterns of balance correcting responses were not observed. These results indicate that prior intention to mimic a balance disorder modifies response synergies by imposing first a premovement silent period followed by voluntary activity on normal balance-correcting responses. An interaction between automatic balance corrections and rapid voluntary movements best explains the observations rather than any central-set effects.