Relation of Muscles Coordination to the Major Degrees of Freedom in Three Upper Limb Motions

The development of musculoskeletal models aims to facilitate the evaluation of muscle function and coordination during daily activities. These models are validated using surface electromyography (EMG) data to ensure accurate representation of neuromuscular activation patterns. After validation, dominant muscles in each phase of motion are identified through non-negative matrix factorization (NNMF) and inverse kinematics (IK) approaches. The study considers three specific motions: flexion/extension, abduction/adduction, and shrugging. Using the NNMF approach, four synergies are identified for the flexion and abduction motions, while two synergies are identified for the shrugging motion. Given the high degrees of freedom (DOF) in the shoulder complex, the IK approach is employed to detect the major DOF involved in each movement. Therefore, the dominant muscles associated with the major DOF are determined based on the literature. This study investigates how the central nervous system generates muscle activation patterns to coordinate complex joint movements, a challenge in motor control. The results show high conformity between muscle coordination and the dominant muscles identified by the IK and NNMF approaches, confirming the validity of using NNMF to describe muscle coordination strategies. Based on the findings, a simplified musculoskeletal model (SMM) is proposed for predicting joint angles in the upper limbs of prostheses and robotic applications. This model incorporates identified synergy modules and key joint DOF for the three basic motions, reflecting the underlying principles of motor control for efficient movement execution.