Preparatory balance adjustments precede withdrawal response to noxious stimulation in standing humans

Self-initiated leg movement in standing humans is preceded by a medio-lateral preparatory balance adjustment (PBA); however, such preparatory balance control is often absent in reflex-like stepping responses evoked by whole-body instability. The presence or absence of the PBA may reflect a task-dependent modulation of the response serving to preserve lateral stability (PBA present) or avoid delay in the lifting of the foot (PBA absent). To examine whether such task-dependent modulation can occur during more stereotypical limb movements, we examined spinally-mediated withdrawal responses evoked by noxious stimulation of the foot. Results showed that rapid limb withdrawal was preceded by a large PBA when subjects were standing but not when they were supine. The PBA caused limb withdrawal to the noxious stimulation to be delayed. However, the onset of the PBA in the standing trials was equivalent in timing to the onset latency of the classic withdrawal responses recorded during the supine trials. Evidence of a preparatory balance adjustment evoked, in advance of a delayed withdrawal response, at very rapid latencies (underlying muscle activation at 70-120 ms) may raise new questions about the neural mechanisms underlying the co-ordination of balance and movement.