Reflex response to imposed bilateral hip oscillations in human spinal cord injury

In human spinal cord injury (SCI), imposed unilateral hip movements trigger multijoint, coordinated reflexes that might incorporate interneuronal circuitry involved in normal motor control, such as neural pathways associated with the reflex control of locomotion. To further investigate the complexity of these hip-triggered reflexes, we measured the effects of kinematics of the contralateral hip on this type of spastic reflex activity in 11 chronic SCI subjects. A novel servomotor drive system was constructed to impose bilateral hip oscillations while the knees and ankles were held stationary in instrumented leg braces. Surface electromyograms (EMGs) and joint torques were recorded during the imposed hip oscillations. Tests were conducted at two different frequencies to test for velocity dependence of the reflexes and the following four paradigms were used to examine the effects of contralateral hip afferents on hip-triggered spastic reflexes: 1) bilateral alternating, 2) bilateral synchronous, 3) unilateral leg oscillation with the contralateral leg held stationary in hip extension, and 4) unilateral leg oscillation with the contralateral leg held stationary in hip flexion. The response to bilateral alternating movements resulted in a significantly larger reflex magnitude compared with the bilateral synchronous movements (P < 0.001). Unilateral leg perturbations yielded reflex patterns that were consistent with the reflex patterns observed during alternating and synchronous hip oscillations. These observations suggest that spastic reflex excitability is modulated through afferent input from the contralateral hip in a manner that is generally consistent with locomotion.