Extensor reflexes in human spinal cord injury: activation by hip proprioceptors

Extensor spasms, which are a significant component of spasticity in spinal cord injury (SCI), were investigated in an attempt to identify the role that hip proprioceptors play in triggering an extensor reflex response. In ten SCI subjects, a controlled hip extension movement was imposed on one leg while the knee and ankle were held in an isometric position using an instrumented leg brace. Isometric joint torques of the hip, knee, and ankle were measured following a constant velocity (30 degrees /s), 45 degrees -75 degrees extension movement of the hip that was applied using the motor of a Biodex rehabilitation/testing system. Electromyograms (EMGs) from four to eight muscles were also recorded during the ten movement trials. The stereotypical torque response to an imposed hip extension consisted of hip flexion, knee extension, and ankle plantarflexion, although all components were not observed in every subject. EMGs indicated coactivation at the knee and ankle joints, with extensor activity generally outlasting flexor activity. These observations are consistent with clinical descriptions of extensor spasms. In contrast, the response to imposed hip flexion, which was observed in six of the ten subjects, comprised hip extension, knee flexion and ankle extension. This difference between the response to hip flexion and the response to extension indicates a specificity of the reflex, suggesting that organized pathways for coordinating leg movements are involved.