Examining Motor Unit Properties of Upper Limb Muscles Near the Zone of Injury in Chronic Cervical Spinal Cord Injury

Spinal cord injury (SCI) causes muscle weakness or paralysis, with functional deficits partly due to motor neuron damage near or below the injury level. Understanding how damage affects the morphology and function of motor neurons and the muscle fibers they innervate (i.e., motor units [MUs]) is important for characterizing motor pathophysiology. We assessed MU electrophysiological properties in muscles innervated near the injury zone (anconeus: C7; extensor indicis [EI]: C8-T1) in motor-incomplete cervical SCI and controls. The EI of 14 SCI participants (48.2 ± 14.6 years; 1 female; C2:1, C4:5, C5:6, C6:2) were compared with 14 controls (44.4 ± 15.9 years; 2 females), and the anconeus was sampled from 9 participants per group (SCI: 52.0 ± 15.8 years, 1 female; controls: 47.3 ± 17.4 years, 1 female). Participants completed multiple, 15-s low-intensity elbow (anconeus) and finger (EI) extensions to study MU potentials (MUPs) recorded using monopolar needle and surface electromyography (EMG). Automated signal-decomposition software provided quantitative data from needle-detected MUPs, surface-detected MUPs (S-MUPs), and near-fiber MUPs (NFMs). The anconeus had fewer MUP phases and turns (≥11%) in SCI than controls (p < 0.05), but MUP, S-MUP, and NFM parameters were not different (p ≥ 0.11). Conversely, multiple EI MUP, S-MUP, and NFM parameters were larger (≥23%) for SCI than controls (p < 0.05). Interestingly, the more caudally innervated EI demonstrated MU adaptations indicative of chronic denervation, while the more rostrally innervated anconeus did not. Thus, our findings indicate that MUs located in spinal segments farther from the injury may be more susceptible to transsynaptic degeneration than MUs located closer to the injury site.