Identification of Static and Dynamic Properties Induced by Hip Changes to Flexion Withdrawal Reflex in Chronic Human Spinal Cord Injury

Abstract

The aim of this study was to investigate the static and dynamic properties of the flexion withdrawal reflex modulated by hip changes in human spinal cord injury. The influence of the hip position and passive movement were tested in six subjects with chronic spinal cord injury. Each subject was placed in a supine position and lower limb was fixed with the knee at 23.7±16.0 degrees flexion and the ankle at 33.3±5.2 degrees plantar flexion. A train of 10 stimulus pulses were applied at 200Hz to the skin of the medial arch to elicit the nociceptive flexion reflexes. From results of the regression analysis, activation of flexor muscles (tibialis anterior and illiopsoas) during static hip angle changes has a linear relationship with respect to hip angle (P<0.05). As the hip flexion angle increases, the EMG activity decreases. A significant linear relationship was also found between the reflex activity of flexor muscle (illiopsoas) and hip angular velocity during both hip flexion and hip extension movement (P<0.05). In order to validate the reflex response, we compared the static and dynamic gains of estimated activation with measured electromyogram of tibialis anterior. From this study, we postulate that flexor muscle activities of nociceptive flexion reflex have linear relationship with hip angle and angular velocity.