A Cross-Sectional and Longitudinal Study of Biceps Brachii Reinnervation Following Nerve Transfer Surgery

Brachial plexus injury (BPI) is a severe nerve trauma that often leads to profound motor deficits and disability, including systematic pain. Among affected nerves, the musculocutaneous nerve is crucial for elbow flexion through the biceps brachii, a fundamental function for performing routine tasks. Nerve transfer has become an effective surgical technique to restore motor function by rerouting axons from intact donor nerves directly to injured targets, reducing regeneration time and distance. Despite its clinical use, recovery variability remains high, and detailed understanding of neuromuscular adaptations after surgery is limited. This thesis investigates motor control recovery in patients who underwent motor nerve transfer, combining force measurements with high-density surface electromyography (HD-sEMG) of the biceps brachii. Two studies were performed on a total of twenty-one patients: a cross-sectional comparison between affected and healthy arms, and a longitudinal follow-up assessing changes over time. Participants performed isometric contractions at 20\% and 60\% of maximal voluntary contraction (MVC), following trapezoidal and sustained force tasks. However, only trapezoidal trials were included in subsequent investigation. Force analysis showed superior performance on the healthy side in terms of accuracy, correlation, and steadiness, as indicated by root mean square error (RMSE), Pearson correlation coefficient (PCC), and coefficient of variation (CoV), respectively, thereby confirming residual deficits in the affected arm. Motor unit (MU) behavior, assessed through HD-sEMG decomposition, revealed greater inter-spike interval (ISI) variability and increased coherence between cumulative spike trains and the force signal on the affected side, while firing rates remained comparable at 20\% MVC. The increased coherence reflects both a stronger shared synaptic input to motor neurons and a greater relative contribution of the biceps to the force output compared to other muscles involved in the task execution. Over time, improvements were observed in both task force accuracy and steadiness. Concurrently, HD-sEMG analyses at the MU level showed a significant reduction in CoV ISI and a trend toward decreased coherence, reflecting partial neuromuscular adjustments. Nevertheless, compensatory strategies or adaptations in muscles other than the biceps cannot be excluded. These findings highlight the value of combined biomechanical and neurophysiological assessments and support the clinical use of HD-sEMG to monitor nerve recovery. However, considerable variability exists among subjects in terms of the timing between injury and surgery, patient-related factors such as age, the type of intervention, and the donor nerve selected, in addition to limitations related to the small sample size and heterogeneity in follow-up intervals.