Testing a Novel Wearable Device For Motor Recovery of the Elbow Extensor Triceps Brachii in Chronic Spinal Cord Injury.

After corticospinal tract damage, reticulospinal connections to motoneurons strengthen preferentially to flexor muscles. This could contribute to the disproportionately poor recovery of extensors often seen after spinal cord injury (SCI) and stroke.In this study, we paired electrical stimulation over the triceps muscle with auditory clicks, using a wearable device to deliver stimuli over a prolonged period of time. Healthy human volunteers wore the stimulation device for ∼6 hours and a variety of electrophysiological assessments were used to measure changes in triceps motor output. In contrast to previous results in the biceps muscle, paired stimulation: 1) did not increase the StartReact effect; 2) did not decrease the suppression of responses to transcranial magnetic brain stimulation (TMS) following a loud sound; 3) did not enhance muscle responses elicited by a TMS coil oriented to induce anterior-posterior current.In a second study, chronic cervical SCI survivors wore the stimulation device for ∼4 hours every day for 4 weeks; this was compared to a 4 week period without wearing the device. Functional and electrophysiological assessments were repeated at week 0, week 4 and week 8. No significant changes were observed in electrophysiological assessments after paired stimulation. Functional measurements such as maximal force and variability and speed of trajectories made during a planar reaching task also remained unchanged.Our results suggest that the triceps muscle shows less potential for plasticity than biceps; pairing clicks with muscle stimulation does not seem beneficial in enhancing triceps recovery after SCI. Significance Statement A wearable device which pairs auditory clicks with electrical stimulation over a muscle was previously shown to strengthen putative reticulospinal connections to the biceps brachii muscle. Here, we tested whether this device could also strengthen connections to the triceps muscle, both in healthy volunteers and in people with spinal cord injury. Surprisingly, there was no evidence of an effect. This suggests that connections to triceps differ from those to biceps in their ability to undergo plastic changes. This may underlie the poor recovery of triceps compared to biceps typically seen after spinal cord injury.