The infraspinatus muscle activity during pitching motion in baseball players with shoulder instability

Abstract Background Shoulder microinstability is often overlooked, which can be problematic, especially in overhead athletes. The slipping phenomenon is defined as posterior or lateral sliding of the humeral head in an elevated arm. When the shoulder is close to the end range of stability, the infraspinatus is highly activated and keeps the shoulder in the glenoid cavity. This study aimed to examine the characteristic physical function and infraspinatus activity during the pitching motion in baseball pitchers with shoulder instability. Methods Twenty-one male baseball pitchers participated and were divided into two groups based on radiograph findings at zero position: slipped (group S) and non-slipped (group N) groups. Physical function using Hara test and infraspinatus muscle activity during pitching were evaluated. Results The infraspinatus muscle activity during the acceleration phase was significantly greater in group S (59.5±33.0%MVC) than in group N (33.0±16.9%MVC) (P Conclusions This study shows that baseball pitchers with slipping phenomenon have capsular laxity and scapular instability that indicate high infraspinatus muscle activity during the acceleration phase. Therefore, repetitive pitching with hyperactivity of the infraspinatus on the slipping shoulder may cause fatigue and dysfunction.