A three-dimensional, six segment chain analysis of forceful overarm throwing

In an effort to understand the mechanical cause-effect relationship and muscular coordination of overarm throwing which effectively produce the desired high speed of the baseball, a three-dimensional six segment model was developed to investigate the pelvis, upper trunk and both upper extremities. Kinematic parameters were obtained for all six segments. Kinetic parameters were obtained for upper trunk anterior/posterior lean, medial/lateral lean, and counterclockwise/clockwise rotation, for upper arm horizontal adduction/abduction, adduction/abduction, external/internal rotation, and for elbow flexion/extension. High speed (250 fps) 3D film data were collected and all muscular and interaction moments were analyzed, using the angular momentum principle with respect to the proximal endpoint of each segment, for three professional baseball pitchers. It was found that trunk rotators acting counterclockwise accelerate trunk then prevent too much deceleration. Horizontal adductors largely prevent horizontal abduction but provide some horizontal adduction bringing humerus in line with shoulders. Internal rotators slow external rotation and accelerate internal rotation. Elbow extensors initiate and accelerate rapid elbow extension. All subjects showed a similar muscular coordination pattern for the throwing upper extremity. For the trunk rotation two different muscular activation pattern were observed. Data in this study generally confirmed the proximal-to-distal principle kinematicly but not kineticly. The muscular moments for elbow extension and humerus internal rotation displayed stretch-shortening contraction patterns indicating that this pattern is an important muscular coordination strategy used in throwing. Also, the interaction moment at the elbow joint due to the linear acceleration of this joint center, as well as the humerus internal rotation velocity played important roles in initiating and stopping the rapid elbow extension before release. The former assisted the elbow extensor to accelerate the ball. Unchecked it might cause elbow damage. The latter assisted the flexor in stopping the rapid extension thereby preventing injury to the joint.