Inverse kinematics associated with the degree of freedoms and constraints of shoulder models during the middle direct punch

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The shoulder complex consists of various joints which correspond to specific degrees of freedom (DOF). Musculoskeletal modeling is a method to reconstruct the real motions that involved a big challenge to simulation accuracy. Different shoulder models with various DOF and constraint definitions are available although the accuracy of the simulation is still debated. Thus, the objective of this study was to evaluate DOF and constraint definitions of shoulder models for inverse kinematics (IK) simulation during the middle direct punch (MDP). The experimental marker data in twenty elite martial arts players (65.4±5.8 kg, 172±7.8 cm, 29.5±8.5 years) were collected during the MDP. Four models were chosen as base models (M3, three-DOF between humerus and trunk Glenohumeral joint; M9, three-DOF for Scapulothoracic joint, three-DOF for Acromioclavicular joint, and three-DOF for Glenohumeral joint; Mst, coupled motions for scapula, clavicle, and humerus; Mel, an ellipsoid mobilizer for the Sternoclavicular joint). The subject-specific models were generated for each subject and model by marker data in static posture and scaling tools of OpenSim. The inverse kinematics tool of OpenSim was used to reconstruct MDP motion via models. RMS of marker error was used to compare models as indicators. Results represented significant differences in RMS of marker error for various models during the MDP tasks (P<0.05). Results illustrated the RMS of marker error for model Mel was minimum (12.87±0.09mm), whereas the RMS of marker error for all subject-specific models was lower than generic models. Based on the results the best model is model Mel which indicated this musculoskeletal model of the shoulder complex can reconstruct the MDP motion better than other represented models. In addition, our results indicated using the subject-specific model instead of generic models is vital to access reliable results. The results of this study would be utilized by sport science researchers, sports medicine doctors, coaches, trainers, and players to simulate shoulder motion by musculoskeletal modeling.