Evaluation of Biceps Brachii Reinnervation Following Nerve Transfer Surgery

Brachial Plexus Injuries (BPIs) often lead to significant motor function impairment, severely limiting a patient's ability to control the affected muscles. Nerve transfer surgery is a commonly used intervention designed to restore muscle function by reinnervating paralyzed muscles. This study aims to evaluate the effectiveness of reinnervation in the biceps brachii following nerve transfer surgery, using High-Density Surface Electromyography (HD-sEMG) and measurements of functional force production. The study involved patients who had undergone nerve transfer surgery after BPI, with data collected from both healthy and affected biceps. HD-sEMG signals and force measurements were recorded during specific motor tasks. Patients performed both trapezoidal contractions and fatiguing tasks at two distinct force levels: 20\% and 60\% of Maximum Voluntary Contraction (MVC). The data collected allowed for an in-depth analysis of Motor Unit (MU) activation patterns and an assessment of the quality of force control in both conditions. The results demonstrated that, in general, healthy biceps exhibited superior motor control, characterized by a lower error relative to the target force profile, which translated into more stable and precise force production. The ability to control force in a muscle is influenced by the coordinated activity of MUs, which is modulated by the central nervous system. For this reason, coherence analysis between the force signal and the cumulative spike train was used to evaluate the effectiveness of MU reinnervation in motor control. The findings indicated that, in general, healthy biceps showed higher coherence values compared to affected biceps, which highlights challenges in achieving fine motor control in reinnervated muscles. This suggests that fine motor adjustments, which are crucial for precise force modulation, remain particularly challenging after nerve transfer. However, there were cases where the affected biceps showed relatively high coherence values, even higher than the healthy biceps, suggesting that reinnervation can achieve functional recovery in some patients, allowing effective recruitment and coordination of MU. These results underscore the variability in reinnervation outcomes, which are influenced by several factors, including patient-specific characteristics, the timing of surgery, and the effectiveness of rehabilitation strategies. However, the relatively positive outcomes observed in certain patients indicate a promising potential for nerve transfer surgery in restoring motor function after BPI. The use of HD-sEMG has proven valuable in assessing reinnervation success and MU behavior, supporting it as a possible effective technique for evaluating neuromuscular recovery and rehabilitation progress. This could be further explored and validated by future studies, such as through long-term monitoring of neuromuscular recovery at multiple time points.