Ankle motor skill is intact in spinal cord injury, unlike stroke: implications for rehabilitation

After an incomplete spinal cord injury (iSCI) or stroke, the impairment of both muscle strength and accurate muscle coordination (timing and amplitude) is an expected clinical finding, although these aspects are not well-distinguished by clinical tests. The objective was to determine whether iSCI patients with impaired corticospinal tract function (reduced strength and prolonged transcranial magnetic stimulation latencies) experience a similar deterioration in muscle coordination as stroke patients.

We assessed ankle dorsal and plantar flexion strength, as well as the ability to accurately control the activation of these muscle groups, using a visuomotor torque tracking task. The task was adjusted to the level of muscle weakness, which enabled a distinction between impairment in strength and coordination to be made.

Reference strength and visuomotor task performance values were obtained in 47 healthy subjects. In 27 iSCI patients with significant muscle weakness (ankle dorsal flexion 65% of healthy values, plantar flexion 76%), task performance improved at a similar rate and the final performance level equaled that of healthy subjects. However, in 10 stroke subjects the tracking task performance was significantly impaired in both legs, while strength was mainly reduced in the affected leg.

These findings indicate that strength is predominantly affected in incomplete spinal cord injury (iSCI) patients, while accurate muscle activation remains largely unaffected. In stroke patients, muscle coordination deteriorates in both legs, independent of muscle weakness. Therefore, iSCI patients might benefit more from rehabilitation interventions that improve muscle strength than stroke patients, where supraspinal areas involved in motor control are additionally affected.