Withdrawal reflexes in the upper limb adapt to arm posture and stimulus location.

ABSTRACT

INTRODUCTION: Withdrawal reflexes in the leg adapt in a context-appropriate manner to remove the limb from noxious stimuli, but the extent to which withdrawal reflexes adapt in the arm remains unknown. METHODS: We examined the adaptability of withdrawal reflexes in response to nociceptive stimuli applied in different arm postures and to different digits. Reflexes were elicited at rest, and kinetic and electromyographic responses were recorded under isometric conditions, thereby allowing motorneuron pool excitability to be controlled. RESULTS: Endpoint force changed from a posterior-lateral direction in a flexed posture to predominantly a posterior direction in a more extended posture [change in force angle (mean ± standard deviation) 35.6 ± 5.0°], and the force direction changed similarly with digit I stimulation compared with digit V (change = 22.9 ± 2.9°). CONCLUSIONS: The withdrawal reflex in the human upper limb adapts in a functionally relevant manner when elicited at rest.