Flexor reflexes in chronic spinal cord injury triggered by imposed ankle rotation

Hypersensitivity of the flexor reflexes to input from force-sensitive muscl e afferents may contribute to the prevalence and severity of muscle spasms in patients with spinal cord injuries. In the present study, we triggered f lexor reflexes with constant-velocity ankle movements into end-range dorsif lexion and plantarflexion positions in 8 individuals with spinal cord injur ies. We found that all 8 subjects had coordinated increases in flexion torq ue at the hip and ankle following externally imposed plantarflexion movemen ts at the ankle. In addition, end-range dorsiflexion movements also trigger ed flexor reflexes in 3 subjects, although greater loads were required to t rigger such reflexes using dorsiflexion movements (compared to plantarflexi on movements). These three-joint reflex torque patterns triggered by ankle movement were broadly comparable to flexion withdrawal responses elicited b y electrocutaneous stimuli applied to a toe, although the amplitude of the torque response was generally lower. We conclude that excitation of muscle and joint-related afferents induced by end-range movements may be responsib le for exaggerated flexion reflex responses in spinal cord injury. (C) 2000 John Wiley & Sons, Inc.