Evaluating motor unit properties after nerve transfer surgery

Nerve transfer surgery (NTS) shows promise in restoring movement to muscles paralyzed by spinal cord (SCI) and peripheral nerve injury (PNI). Yet, motor outcomes vary, and the neurophysiological factors influencing responders and non-responders remain unclear. As the fundamental goal of NTS is to reinnervate paralyzed muscles by creating new motor units (MUs), we examined MU properties after NTS for individuals with SCI and PNI. Nine participants (3 SCI, 6 PNI, 50.3 ± 13.9 years) >18 months post-NTS were evaluated and compared to either age-matched controls (SCI) or the contralateral limb (PNI). We used a sophisticated, signal processing software known as Decomposition-Based Quantitative Electromyography (DQEMG) and near-fiber EMG to examine MU characteristics sampled from needle electromyography signals recorded during low-intensity contractions. The NTS muscle MU potentials (MUP) were larger, and near-fiber MUPs (NFM) were more temporal dispersed than controls. Measures of neuromuscular junction instability were greater in NTS muscles compared to controls (p < 0.05). Firing rates of MU, and MUP phases and turns were not different between groups (p > 0.05). Overall, these data suggest the quality of reinnervation post-surgery requires further investigation as a potential mediator of motor outcome and the required time for successful reinnervation may be longer than currently predicted.