Configuration of the Shoulder Complex during the Arm-Cocking Phase in Baseball Pitching

Background: The role of the scapula during high-velocity baseball pitching has been described without 3-dimensional kinematic data. It has been speculated that the scapula functions to align the humerus with the spine of the scapula on both the transverse and scapular planes at the end of the arm-cocking phase. Hypothesis: Two hypotheses were formulated: (1) the scapulothoracic protraction angle correlates with the humerothoracic horizontal adduction angle among participants, and (2) the scapulohumeral rhythm of the humerothoracic elevation is not the same as the normal ratio (2:1) observed widely in controlled abductions. Study Design: Descriptive laboratory study. Methods: A total of 20 Japanese professional baseball pitchers were asked to pitch 3 fastballs as they would normally during pitching practice. The 3-dimensional kinematic data of the thorax, scapulae, humeri, and pelvis were recorded using an electromagnetic tracking device operating at 240 Hz. Humerothoracic, scapulothoracic, and glenohumeral joint configurations were determined at the instant of stride-foot contact (SFC) and the end of the arm-cocking phase (MER). Results: The mean (±SD) glenohumeral horizontal adduction (-6° ± 7°) and elevation (85° ± 10°) angles at the MER indicated that the humerus was positioned almost parallel to the spine of the scapula. The mean scapulothoracic protraction angle (15° ± 10°) was significantly correlated with the humerothoracic horizontal adduction angle (10° ± 11°) at the MER (r = 0.76, P <.001) but not at the SFC (r = 0.13, P =.58). The scapulohumeral rhythm (4.2 [±1.9]:1) expressed as the ratio of the glenohumeral elevation angle to the scapulothoracic upward rotation angle at the MER was significantly greater than the normal ratio (2:1) (P <.01). Conclusion: The results supported the hypotheses, providing evidence to corroborate the widely accepted concept that the scapula functions to align the humerus with the spine of the scapula so as to limit the glenohumeral joint configuration within the "safe zone" at the MER. Clinical Relevance: Disruption of coordination, such as abnormal patterns including "SICK" scapula (scapular malposition, inferior medial border prominence, coracoid pain, and dyskinesis) and scapular dyskinesis, may result in an abnormal configuration of the glenohumeral joint at the MER.