Are Rotator Muscle Performance and Posterior Shoulder Capsule Tightness Related to Glenohumeral Internal Rotation...

Posterior inferior capsule (PIC) tightness in the shoulder joint, a known risk factor for shoulder injury, can be assessed using shear-wave ultrasound elastography (SWE). However, to date, the correlation between PIC elasticity and shoulder rotator muscle performance in college baseball players with glenohumeral internal rotation deficit (GIRD) has not been reported. This study aimed to analyze the differences in PIC elasticity measured using SWE and shoulder rotator muscle performance between male college baseball players with and without GIRD.

Twenty-four male college baseball players participated in this study: 10 with GIRD (G group) vs. 14 without GIRD (NG group). PIC elasticity was measured using SWE in the lateral decubitus position. Shoulder rotator muscle performance tests were performed for the internal rotators (IRs) and external rotators (ERs) at an angular velocity of 180°/sec with 90° shoulder abduction using an isokinetic test device.

Mean PIC elasticity was significantly greater in the throwing shoulders of the G group than in those of the NG group (4.8 ± 1.2 kPa vs. 3.7 ± 1.2 kPa, p = 0.036). In the throwing shoulders of the G group, compared with those of the NG-group, the ER/IR ratio was significantly lower (35.7% ± 5.0% vs. 55.5% ± 6.2%, p < 0.001) and IR muscle strength was significantly greater (75.0 ± 7.6 Nm kg^-1 × 100 vs. 55.7 ± 6.4 Nm kg^-1 × 100, p = 0.002). The mean elasticity of the PIC showed a significant negative correlation with the ER/IR ratio in the throwing shoulders of the G group (r = -0.640, p = 0.046).

Among the male college baseball players with GIRD, SWE could quantitatively assess PIC tightness, and an imbalance in shoulder rotator muscle strength was found in these baseball players. Therefore, clinicians and therapists need to focus on the restoration of shoulder rotator muscle imbalance in addition to improving internal rotation in these players.