Motor units activation pattern during acute muscle pain and fatigue

Abstract

Musculoskeletal pain profoundly alters motor control, yet the underlying neural adaptations, particularly at the motor unit level, remain incompletely understood. Furthermore, the similarities and differences between motor adaptations to pain and fatigue require clarification. This thesis aimed to characterize the motor strategy to produce force during pain, using machine learning techniques and surface electromyography decomposition. Together with a novel motor unit clustering and segmentation analysis method, we investigated the motor unit's recruitment and firing strategies during experimentally induced muscle pain and muscle fatigue in humans to determine how the muscle can produce force during those conditions and to distinguish neural mechanisms underlying these adaptations. We investigated muscle activation on biceps brachii during repeated contractions, and the motor unit's behaviour in the vastus lateralis and rectus femoris muscles during isometric and isokinetic knee extension contractions. Key findings include: (1) A machine learning model successfully identified an EMG-based "pain signature," distinguishing painful from non-painful contractions with high sensitivity, highlighting central influence, despite placebo effects mimicked some aspects; (2) The developed clustering/segmentation method effectively differentiated motor unit groups and revealed significant differences in firing rates based on motor unit group, contraction type (concentric vs. eccentric), and specific joint angle (60°); (3) Experimentally induced pain triggered a consistent reorganization of motor units' activity, characterized by increased recruitment thresholds, decreased firing rates (in lower-threshold units), and altered motor units' action potential amplitudes, suggesting a shift towards recruiting higher-threshold motor units. This reorganization allowed for the maintenance of isokinetic torque; (4) Pain and fatigue elicited distinct patterns of motor units' adaptation and selectively impaired torque during different phases of dynamic contraction (pain: eccentric; fatigue: concentric), confirming that separate neural strategies are employed despite shared effects on maximal isometric force. Collectively, these findings demonstrate that the motor system utilizes complex, muscle-specific, and task-dependent strategies, primarily involving the reorganization of motor units' recruitment, to manage pain and fatigue. Pain induces adaptations distinct from those observed during fatigue. Understanding these adaptations at the motor unit level is crucial for elucidating the mechanisms of pain-related motor dysfunction and developing more targeted rehabilitation strategies. Palavras-chave: Eletromiografia, dor experimental, decomposição eletromiográfica, unidades motoras, fadiga, aprendizado de máquina.