It is known that pairing noninvasive stimulation of the primary motor cortex and a peripheral nerve, known as paired corticospinal transmission-motoneuronal stimulation (PCMS), enhances corticospinal transmission in patients with chronic spinal cord injury (SCI). This study examined whether voluntary activity can also increase the excitability of spinal motoneurons, subsequently potentiating PCMS effects on corticospinal transmission in chronic SCI patients.

Subjects were 17 patients with SCI, and 14 age-matched controls. Corticospinal volleys evoked by TMS over the hand representation of the primary motor cortex were timed with peripheral electrical stimulation of the ulnar nerve. The PCMS was applied at rest (PCMSactive), and during a small level of isometric index finger abduction (PCMSactive). Motor evoked potentials (MEPs) elicited by TMS and electrical stimulation were measured in the first dorsal interosseous muscle in both case and control groups.

In patients with SCI, MEPs elicited by the combined TMS and peripheral electrical stimulation were increased when compared to controls. Those SCI patients who did not respond to PCMSrest did respond with PCMSactive. Those who responded to both, demonstrated larger increments with active PCMS.

Muscle contraction during paired corticospinal transmission-motor neuronal stimulation potentiates corticospinal transmission, suggesting a technique that may be used to increase spinal plasticity after spinal cord injury.