Muscle-specific motor unit firing characteristics in elbow flexors and extensors after cervical spinal cord injury

Abstract

Individuals with cervical spinal cord injury (SCI) often exhibit asymmetric recovery of upper-limb function, with greater weakness in elbow extensors than flexors. To determine whether muscle-specific changes in motor unit (MU) behavior contribute to this disparity, we identified MU firing instants from high-density surface electromyography to characterize MU firing characteristics in the biceps brachii (BIC) and triceps brachii (TRI) of individuals with cervical SCI (n = 20) and non-injured controls (n = 18). We quantified rate-coding behavior and metrics related to persistent inward currents (PICs), including onset-offset hysteresis ({Delta}F), ascending firing rate nonlinearity, and self-sustained firing. At the group level, BIC MUs in SCI participants showed reduced rate coding and altered ascending firing rate nonlinearity relative to controls. In contrast, TRI MUs showed no clear group-level differences. However, subgroup analysis revealed that SCI participants with low-strength during extension (n = 9) exhibited lower {Delta}F and longer self-sustained firing durations in TRI MUs than those with high-strength (n = 6). In BIC, SCI participants with low-strength during flexion (n = 8) showed reduced rate-coding behavior relative to high-strength SCI participants (n = 9), with no differences in PIC-related metrics. Together, these results demonstrate muscle-specific alterations in MU firing after cervical SCI that may relate to strength recovery or preservation and underscore the need for nuanced analyses in heterogeneous SCI populations. Key pointsO_LIRate coding and nonlinear firing behaviors are significantly altered in the biceps brachii, but not triceps brachii, of participants with cervical spinal cord injury. C_LIO_LIStrength based subgroup analyses revealed muscle-specific differences in motor unit behaviors that may be associated with strength preservation or recovery following spinal cord injury. C_LIO_LIFunctional heterogeneity following spinal cord injury may mask group differences in motor unit behaviors and warrants careful interpretation of results of future studies. C_LI