Muscle coordination strategies during Functional Reach across multiple directions in healthy individuals

Purpose. Understanding the neuromuscular strategies underlying postural stability is crucial for evaluating motor control and balance. This study examines muscle coordination during several variations of the Functional Reach (FR)—unilateral Functional Reach (uniFR), bilateral Functional Reach (biFR), and Lateral Reach (LR)—in healthy young adults. This study aims to establish a reference baseline for these tasks and assess whether fundamental motor control strategies are preserved across FR variations, despite distinct biomechanical demands and directional stability components. Methods. Seventeen healthy young adults performed ten repetitions for each FR task. Muscle synergies were derived using non-negative matrix factorization on surface electromyography (EMG) data recorded from eight muscles on each side of the body. Results. Results revealed that fewer synergies were required for anterior-posterior stability (uniFR and biFR), while medial-lateral control entailed in LR required an additional synergy. Despite the directional tuning of muscle contributions, both anterior-posterior and medial-lateral balance control relied on similar underlying motor functions, thus engaging the same muscle synergies in varying combinations. Furthermore, a greater similarity in muscle synergy structure and robustness was observed between uniFR and biFR compared to LR, suggesting a shared neural control strategy. Moreover, the LR task exhibited higher variability, reflecting a more complex and less standardized motor control process. Conclusion. Our findings indicate that similar motor functions are recruited across different balance tasks, with directionally tuned muscle synergies. This offers a deeper insight into the muscle coordination strategies during reaching tasks, providing a novel framework for evaluating the effects of aging or pathological conditions on neuromuscular control.