Medio-lateral balance adjustments preceding reflexive limb withdrawal are modified by postural demands

We have recently observed medio-lateral balance adjustments (BA) preceding reflexive stepping elicited by noxious stimulation. While task specific mod ulation is evident for BA prior to voluntary leg movement, it is unclear wh ether rapid BA reactions (prior to 'reflexive' stepping) represent a generi c response to evoked limb withdrawal or can be modified to suit task-condit ions. This study was designed to establish whether the CNS is able to modif y rapid onset latency BAs to match task conditions. Reflexive stepping was evoked by applying a noxious stimulus (50 ms stimulus train, I ms pulses, 3 00 Hz, 4Xperceptual threshold) to the plantar surface of the either the lef t or right foot. Task conditions were varied prior to stimulation by having subjects maintain one of three different static positions: (I) lean left ( 70% body weight (BW) on left), (2) neutral (50% BW both sides), (3) lean ri ght (70% BW on right). BAs were denoted by centre-of-pressure (Col?) excurs ions towards the swing foot after the onset of noxious stimulation (average onset latency of 128 ms). There was a significant increase in frequency of occurrence and a significant increase in magnitude of CoP shift when the s timulation was applied to a loaded limb (leaning with 70% BW on the stimula ted foot) as compared to an unloaded limb (30%BW). In addition, 78% of load ed trials featured steps taken with the unstimulated foot, which delayed re moval of the stimulated foot. Collectively, the results indicate modifiabil ity of the very rapid onset balance adjustments that precede the onset of l imb withdrawal revealing complex control of balance exists even over very b rief latencies. (C) 2001 Elsevier Science BY All rights reserved.