Resultant Force Analysis of the Shoulder during Throwing Activities

Image-based graphical modeling has been used to quantify the joint reaction forces within the shoulder during dynamic pitching activities. For this model, kinematic pitching data from living subjects was applied to a generic model of the musculoskeletal system to animate the pitching motion. The model also was used to apply inverse dynamics analysis to the pitching motion to quantify the joint reaction forces at the shoulder and elbow. The joint reaction forces and moments were also incorporated into the pitching animation to improve interpretation of the results.

For this analysis, the kinematics of collegiate baseball pitchers pitching from an artificial mound within a motion analysis laboratory were captured. Force plates were built into the wood mound to measure the ground reaction forces during the pitching motion [30]. Reflective markers were taped to the skin of each pitcher at the wrist, elbow, shoulder, hip, knee, and ankle. A marker also was fixed to the baseball. The pitching motions were recorded using a 5-camera, 200-Hz video capture system. Following testing, the marker positions were digitized throughout the pitching motion. The marker positions were recreated on a generic skeleton model. The markers were moved to match the digitized positions throughout the pitching motion, creating an animation of the pitching motion on the model. Local coordinate systems were defined within the skeleton model to quantify the joint rotations during the pitching motion. The upper arm and forearm also were modeled as rigid links connected to ball and socket joints at the elbow and shoulder. Inertial properties were assumed for each link based on anthromorphometric data, and inverse dynamics techniques were used to quantify the joint reaction force at the elbow and the shoulder (Fig. 3).

The force in the shoulder tended to reach a maximum near the ball-release phase of the pitching motion. The largest component of the force was a compressive force along the humerus shaft, which generally exceeded 500 N. The moment acting on the shoulder generally peaked between the position of maximum external rotation and ball release. In this region, the moments acting on the shoulder were internally rotating the humerus and bringing the humerus down and across the chest. This study showed that the graphic model can be used to display kinematic results and quantify joint kinetics. The model can be used to quantify the variation in joint forces from one individual to another or between pitches for a single individual. This data will be valuable both for performance enhancement and for injury prevention in athletes.

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