We aimed to determine whether subjective shoulder instability in baseball players is more influenced by forward or backward translations based on US test.
Twenty-three healthy college baseball players (baseball group) and 11 healthy males (control group) without overhead sports experiences were included. Humeral head translation was measured using a US (SONIMAEGEMX1, KONICAMINOLTA, Japan). The translations were calculated separately for forward and backward directions. Measurement items included ER ROM and Oxford shoulder instability score (OSIS). Statistical analyses compared humeral head translations and the ROM between the groups, and multiple regression analysis was used to examine the relationship between the ROM and forward translation in the baseball group. The baseball group was divided into stability and instability groups by OSIS scores. Multiple regression analysis determined whether forward or backward translation affected OSIS scores in the baseball group.
The baseball group showed significantly higher ER ROM (control; 97.6 ± 9.9°, baseball; 111.5 ± 7.7°) and forward humeral head translation (control; 0.94 ± 0.45 mm, baseball; 1.84 ± 0.92 mm) than the control group (p0.001). Multiple regression analysis showed that higher forward translation in the baseball group was associated with higher ER ROM (p0.001). Comparison of the stability and instability groups showed higher backward translation (stability: 0.64 ± 0.34 mm, instability: 1.15 ± 0.34 mm) in the instability group, with multiple regression analysis suggesting an association between the higher backward translation and the increased OSIS (p0.001).
In baseball, ER motion of 140° to 180° occurs during pitching with the shoulder joint in approximately 110° of abduction. In these positions, the anterior bundle of the inferior glenohumeral ligament, located anteroinferior part of the joint, is subjected to intensive lengthening stress and relaxation. In the baseball group, the daily repetition of the throwing motion is thought to have caused an increase the forward translation with the increase ER ROM.
The backward translation reflects the posterior support of the glenohumeral joint, and poor stiffness cause the slip of the humeral head downward during upper limb elevation and instability of shoulder girdle motion. In baseball pitching, the posterior part of the glenohumeral complex has an important role in resisting compression and extension stress. Therefore, high backward translation based on US was suspected to be extracted as a factor in subjective shoulder instability.
The US performed may be a useful testing technique to address shoulder problems for college baseball players with subjective shoulder instability.
Humeral head translation
Ultrasound imaging
