Modulation of flexor reflexes by static and dynamic hip proprioceptors in chronic human spinal cord injury

The aim of this study was to investigate the influence of hip proprioceptors on the organisation of the flexor reflex elicited by nociceptive stimulation in individuals with spinal cord injury. The influence of hip position and passive movement were tested in 10 subjects with chronic spinal cord injury. Stimuli were tested isometrically with the hip in three positions. Additionally, the response was also measured to stimuli applied with the hip at midposition during imposed hip flexion and extension movement. The torque and EMG responses were compared in order to identify the postural and movement-dependent modulation of the withdrawal reflex. Ankle and hip torques were significantly modulated by hip position (ANOVA, p < 0.05), with the largest torque response obtained in the hip extended position, compared with the flexed position. We also observed a significant difference between the flexor reflex during movement and with the leg isometric. Ankle and hip torque and tibialis anterior electromyograms were significantly higher in the movement conditions than the isometric condition (Tukey test, p < 0.05). We postulate that inputs from hip proprioceptors enhance the withdrawal reflex response. Movement appears to increase the response, regardless of movement direction, suggesting a novel role for the dynamic components of hip afferents.