Effects of multijoint spastic reflexes of the legs during assisted bilateral hip oscillations in human spinal cord injury.

OBJECTIVE

To investigate the timing and magnitude of muscle activation during an active-assist bilateral hip motor task in human spinal cord injury (SCI).

DESIGN

A single test session using a novel robotic system to alternately flex and extend the hips from 40 degrees of hip flexion to 10 degrees of hip extension at 1 of 3 frequencies (.25, .50, .75Hz). Subjects were asked either to actively assist the movements or to remain relaxed during the imposed oscillations.

SETTING

All data were collected in a research laboratory.

PARTICIPANTS

Ten subjects with motor incomplete (American Spinal Injury Association grade C or D) SCI and 10 individuals without neurologic injury participated in this study.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Electromyograms and joint torques were recorded from the lower extremities of SCI subjects and compared with electromyograms and joint torque patterns recorded from 10 neurologically healthy individuals completing the same tasks.

RESULTS

In trials involving active assistance of the imposed hip oscillations, SCI subjects produced muscle activation patterns that were phased differently from muscle activity of neurologically intact subjects. SCI subjects generated peak torque at the end ranges of movement (ie, 40 degrees hip flexion, 10 degrees extension), whereas control subjects generated the greatest torque midway through the movements. Moreover, the phasing of active-assist hip torque in SCI subjects was similar to the phasing of reflexive hip torques produced during the unassisted condition (ie, SCI subjects instructed to relax), while control subjects produced no reflexive torques during unassisted trials.

CONCLUSIONS

The differences in the timing of muscle activity during the active-assist task in controls and SCI subjects highlights problems in generating appropriately timed muscle activity during ongoing movements. The similarity in muscle activity patterns for the active-assist and unassisted trials in SCI subjects further suggests that reflex feedback from hip afferents contributes substantially to muscle activation during active-assist movements. These findings demonstrate the disruptions in reflex regulation of movement in people with incomplete SCI and suggest that spastic reflexes might disrupt motor control.