Extensor spasms triggered by imposed knee extension in chronic human spinal cord injury

Extensor spasms, which are a significant component of spasticity in spinal cord injury (SCI), are still incompletely understood. In this study, contributions of knee proprioceptors to the origination of extensor spasms were examined in fifteen subjects with SCI. Ramp and hold knee extension perturbations were imposed to one leg while the hip and ankle were held in an isometric position using an instrumented leg brace. Isometric joint torques of knee, ankle and hip, and electromyograms (EMGs) from six muscles of the leg were recorded following controlled knee extension at four different velocities. Tests were conducted with the hip in both flexed and extended positions. A stereotypical torque response consisting of hip flexion, knee extension, and ankle plantar flexion was observed following knee perturbations, although not all components were demonstrated in every subject. During the hold periods with the knee extended, EMG activity recorded from the vastus medialis, medial gastrocnemius and rectus femoris demonstrated patterns consistent with clinical observations of extensor spasms. Furthermore, larger responses were observed with the hip in the extended vs. flexed position (p<0.05). Such behaviors emphasize the role of knee and hip proprioceptors in the initiation of extensor spasms in human SCI. This knowledge may be especially helpful in identifying rehabilitation strategies for producing functional movements in human SCI.