A baseball pitch is an explosive and dynamic motion during which one of the highest angular rates achieved by the human body in sports activity was noted. The nature of the baseball pitch puts shoulder structures at risk, and one of the most common injuries among pitchers is a rotator cuff tear, frequently connected with internal impingement. It has been found that the amount of internal impingement increases for the angle of horizontal abduction beyond the coronal plane. To better understand the mechanics in the shoulder that leads to excessive horizontal abduction, it was decided to study muscle behavior (activation and produced torque) during the acceleration and deceleration phases of a baseball pitch. The study focused on muscles that are believed to have the most significant contribution during the aforementioned phases: pectoralis major and rotator cuff muscles (infraspinatus, supraspinatus and teres major). The analysis was performed with the use of the Thoracoscapular Shoulder Model implemented in OpenSim software. About 20 pitches of 5 different baseball pitchers were simulated based on the collected marker motion data. Visible differences in the horizontal abduction/adduction angle were found between participants, with the angle of horizontal abduction between -10 degrees to 18 degrees at the moment of maximal external rotation (MER). Next, the muscle length was analyzed. The normalized muscle length of the rotator cuff muscles at MER was found to be similar to one found in the literature, as well as the changes in muscle length over time. However, discrepancies were found in the pectoralis major behavior. The conclusion was drawn that the thoracic part of the pectoralis major is too long. Moreover, it was found that the pectoralis major does not wrap around the humeral head and shaft for the horizontal abduction angle exceeding daily-live tasks. Next, static optimization was done to find the activation of the muscles in the glenohumeral joint and, consequently, to calculate the contribution of the individual muscles to the total torque in the joint. The contribution of the rotator cuff muscles was in accordance with the information found in the literature.
However, the pectoralis major was not as active as expected. Instead of it, high activity of the deltoideus muscle and coordinate actuators was found. The results indicate that the pectoralis major was not working as expected, and other muscles needed to compensate for its normal function. Several improvements to the model and data collection were proposed. For the model development, it is suggested to ensure wrapping around the humeral head and shaft for the pectoralis major. Moreover, further evaluation of the model scaling is recommended, as problems with scaling the thoracic part of the pectoralis major were found. It is believed that after their implementation, it will be possible to find a potential correlation between the range of horizontal abduction and muscle activity while using the methodology and tools presented in this study.