Pectoralis major activity prevents excessive horizontal abduction of the throwing arm in fastball pitching in baseball

Abstract

Abstract Background: In baseball pitching, repetitive excessive horizontal abduction of the upper (throwing) arm, or hyperangulation, can cause internal impingement to become pathological when glenohumeral contact intensity repeatedly reaches a threatening level. The pectoralis major, positioned anatomically and functionally in horizontal adduction, may counteract the throwing arm's moment of inertia, limiting excessive horizontal abduction. This study aimed to evaluate whether the pectoralis major actively and passively produces force before maximal horizontal abduction during fastball pitching, to counteract the throwing arm's moment of inertia, thereby limiting excessive horizontal abduction. Results: Moderate pectoralis major muscle activity was observed in the period preceding maximal horizontal abduction, indicating that the muscle actively generated force. The relative muscle length of the pectoralis major was approximately 125%, indicating that the muscle was stretched beyond its optimal muscle length, thereby also passively contributing to the force production. The relative muscle length of the pectoralis major increased in the period leading up to maximum horizontal abduction, which implies that the muscle provided an eccentric contraction. Conclusion: The pectoralis major actively and passively contributes to the force generation required to counteract the throwing arm’s moment of inertia through eccentric contraction in the period preceding maximal horizontal abduction during fastball pitching, potentially preventing the throwing arm of reaching excessive values in horizontal abduction or hyperangulation.