BACKGROUND: Tightness of the posteroinferior capsule is assumed to be the cause of internal rotation loss in baseball pitchers. Although the relationship between posterior capsule and subacromial impingement has been recognized, this relationship during the baseball-pitching motion is unclear.HYPOTHESIS: Contact pressure during baseball-pitching motion increases with posterior capsule tightness.STUDY DESIGN: Controlled laboratory study.METHODS: Eight fresh-frozen shoulders were used. The peak contact pressure and area on the coracoacromial arch were measured on a custom-designed shoulder experimental device capable of 6 degrees of freedom motion. Simultaneously, the sites of peak pressure on the coracoacromial arch and humerus were observed from various angles. The posteroinferior capsule tightness was simulated by plicating the capsule in the region from 6 to 8 o'clock. The static testing positions correlated to the early cocking, late cocking, acceleration, deceleration, and follow-through phases of the pitching motion.RESULTS: The peak contact pressure during the follow-through phase (0.63 + or - 0.50 MPa) significantly increased with posteroinferior capsule tightness (1.00 + or - 0.65 MPa) (P = .014). Additionally, the contact area on the coracoacromial ligament during the follow-through phase (0.98 + or - 0.67 cm(2)) significantly increased with posteroinferior capsule tightness (1.47 + or - 0.91 cm(2)) (P < .001). The site of the peak contact pressure did not change between the 2 conditions.CONCLUSION: Our findings demonstrate that posteroinferior capsule tightness leads to higher contact pressure under the subacromial arch and increased contact area, particularly on the coracoacromial ligament during the follow-through phase.CLINICAL RELEVANCE: This tightness may affect risk of injury of the rotator cuff and its surrounding tissues by increasing subacromial contact during pitching.