Clarification of glenohumeral joint alignment changes during the late cocking phase may reveal the mechanisms of throwing injuries. This study aimed to determine the effect of shoulder external rotation on humeral head center deviation relative to the scapular glenoid. Twenty-eight baseball players participated. The anteroposterior deviation of the humeral head center relative to the glenoid (humeral head translation) and the distance between the humeral head and posterior glenoid rim perpendicular to the glenoid articular surface (posterior glenohumeral distance) were measured. Magnetic resonance imaging of the throwing shoulder was performed at 90° abduction with 90°, 100°, and 110° external rotation; for the nonthrowing shoulder, measurements were conducted at 90° and 100°. In humeral head translation, the posterior translation of the humeral head relative to the glenoid was significantly greater at 110° compared to 90° external rotation position (P = .003). Humeral head translation was associated with posterior glenohumeral distance at the 90° (β-coefficient = 0.649) and 100° (β-coefficient = 0.556) external rotation positions. Increased shoulder external rotation resulted in posterior translation of the humeral head and proximity between the humeral head and the posterior glenoid rim. The factors identified as contributing to posterior deviation of the humeral head may trigger throwing shoulder injuries during the late cocking phase.
Keywords: humeral head translation; late cocking phase; physical assessment.