Bilateral Oscillatory Hip Movements Induce Windup of Multijoint Lower Extremity Spastic Reflexes in Chronic Spinal Cord Injury

After spinal cord injury (SCI), alterations in intrinsic motoneuron properties have been shown to be partly responsible for spastic reflex behaviors in human SCI. In particular, a dysregulation of voltage-dependent depolarizing persistent inward currents (PICs) may permit sustained muscle contraction after the removal of a brief excitatory stimulus. Windup, in which the motor response increases with repeated activation, is an indicator of PICs. Although windup of homonymous stretch reflexes has been shown, multijoint muscle activity is often observed following imposed limb movements and may exhibit a similar windup phenomenon. The purpose of this study was to identify and quantify windup of multijoint reflex responses to repeated imposed hip oscillations. Ten chronic SCI subjects participated in this study. A custom-built servomotor apparatus was used to oscillate the legs about the hip joint bilaterally and unilaterally from 10° of extension to 40° flexion for 10 consecutive cycles. Surface electromyograms (EMGs) and joint torques were recorded from both legs. Consistent with a windup response, hip and knee flexion/extension and ankle plantarflexion torque and EMG responses varied according to movement cycle number. The temporal patterns of windup depended on the muscle groups that were activated, which may suggest a difference in the response of neurons in different spinal pathways. Furthermore, because windup was seen in muscles that were not being stretched, these results imply that changes in interneuronal properties are also likely to be associated with windup of spastic reflexes in human SCI