Corticospinal inhibition investigated in relation to upper extremity motor function in cervical spinal cord injury

Objective Corticospinal inhibitory mechanisms are relevant to functional recovery but remain poorly understood after spinal cord injury (SCI). Post-injury characteristics of contralateral silent period (CSP), a measure of corticospinal inhibition evaluated using transcranial magnetic stimulation (TMS), is inconsistent in literature. We envisioned that investigating CSP across muscles with varying degrees of weakness may be a reasonable approach to resolve inconsistencies and elucidate the relevance of corticospinal inhibition for upper extremity function following SCI. Methods We studied 27 adults with chronic C1-C8 SCI (age 48.8±16.1 years, 3 females) and 16 able-bodied participants (age 33.2±11.8 years, 9 females). CSP characteristics were assessed across biceps (muscle power = 3-5) and triceps (muscle power = 1-3) representing stronger and weaker muscles, respectively. We assessed functional abilities using the Capabilities of the Upper Extremity Test (CUE-T). Results Participants with chronic SCI had prolonged CSPs for biceps but delayed and diminished CSPs for triceps compared to able-bodied participants. Early-onset CSPs for biceps and longer, deeper CSPs for triceps correlated with better CUE-T scores. Conclusions Corticospinal inhibition is pronounced for stronger biceps but diminished for weaker triceps muscle in SCI indicating innervation relative to the level of injury matters in the study of CSP. Significance Nevertheless, corticospinal inhibition or CSP holds relevance for upper extremity function following SCI.