Compensatory movement is a phenomenon that occurs wherein alternative muscle groups abnormally contribute to an action when the primary muscle group of the action experiences fatigue or injury. As compensatory movement increases the risk of physical injury, it should be avoided during exercises or rehabilitation. However, quantifying how fatigue induces compensatory movement and changes exercise patterns remains unclear and unstandardized. This study utilizes muscle synergy and joint kinematic analysis to observe and discuss the impact of exercise-induced fatigue on humans during exercise. Using bicep curl as the case study, we collected upper limb joint kinematics and surface electromyography signal (sEMG) from eight muscles in 12 healthy subjects under standard and fatigue stages. Two muscle synergies were generated from the sEMG signal and thoroughly analyzed along with the joint kinematics. Our findings highlight changes in muscle synergy and joint kinematics between the standard and fatigued states. Compared to the standard stage, the result indicates a shift in relative contribution from the forearm muscles to the shoulder muscles, and the activation amplitude for both synergies is significantly increased during the fatigue stage. In addition, more vigorous movement is captured on the shoulder joint during fatigue. Our findings also indicate that joint kinematics demonstrated greater reliability as an indicator of fatigue than muscle synergy. This work offers valuable insights into identifying fatigue in sports science and rehabilitation and provides numerical standards for sensor development that capture the fatigue stage.
Muscle Synergies and Kinematic Analysis of Bicep Curls and Compensatory Movements
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