Abstract
Weakness of shoulder external rotator has been proposed as a possible risk factor for developing an injury. However, shoulder strength characteristics in youth baseball players is unknown. The purpose of this study was to determine if the shoulder used for throwing was weaker, compared to the nondominant hand in youth baseball players. There were 19 players from an elementary school,23 from a junior high school, and 21 from a high school, all of whom did not have any throwing shoulder pain. In addition,24 nonthrowing athletes in high schools were also tested as a control. Muscle strength in the following muscles was measured bilaterally with a hand-held dynamometer (MicroFet): supraspinatus at 45 degrees of scaption, external and internal glenohumeral rotators at hanging arm. Three measurements were taken and averaged. The statistical analysis was done using Mann-Whitney U-test and one-way ANOVA. There were no significant differences of shoulder strength in the elementary, the junior high school players, and in the nonthrowing athletes. However, the throwing arm's external rotators were significantly weaker than those of the nonthrowing arm (p=0.0152), and the throwing arm's internal rotators were significantly stronger than those of the nonthrowing arm (p=0.0063). The external-to-internal rotator muscles strength ratio in the dominant side was 0.68 in the elementary,0.60 in the junior high, and 0.55 in the high school players. Those on the non-dominant side were 0.68,0.63, and 0.82respectively. Repetitive throwing action might cause weakness of the external glenohumeral rotators. It was suggested that this weakness might have a relation to the quantity and quality of the baseball practice.
