Estimation of the Optimal Shoulder Orientation from the Viewpoint of Minimal Shoulder Joint Load in 183 Adolescent Baseball Pitchers
Authors: Tanaka Hiroshi , Hayashi Toyohiko , Inui Hiroaki , Muto Tomoyuki , Ninomiya Hiroki 
Published: July 2017

Objectives: Adolescent baseball pitchers are prone to shoulder injury as a result of the extreme force and torque placed on the shoulder, especially tremendous joint force applies to the shoulder near ball release. In our clinical experience, shoulder pain at ball release is common complaint. Little analysis is available about the relationship between shoulder orientation and shoulder joint force at the ball release. Methods: A total of 183 adolescent baseball pitchers (14 to 18 years old) participated after providing written informed consents approved by the hospital’s institutional review board. The procedures to be performed were also explained to their parent(s). Each participant was not currently injured or recovering from an injury at time of testing.For data collection, a set of 14-mm spherical reflective markers was placed on the skin overlying 34 anatomical landmarks. Subsequently, the motion capture automatic digitizing system was used to collect 500-Hz from 7 synchronized cameras was set up around the regulation pitching mound in an indoor laboratory. After stretching and warm-up pitching, each player pitch to 5 fastball pitches. Then, the best pitch thrown for a strike was chosen for kinematic and kinetic analysis.The local coordinate systems were used to calculate 3-dimesional rotation at the shoulder using the typical Eulerian sequence. The standard inverse dynamic equation was used to estimate resultant joint force at throwing shoulder. In order to normalize data between subjects, force was expressed as percent using body weight.To estimate the optimal shoulder orientation at ball release, Rz was defined a shoulder abduction angle, which indicated zero value of shoulder superior/inferior force at ball release by linear regression analysis. Similarly-defined Ry was a shoulder horizontal adduction/abduction angle, which indicated zero value of shoulder anterior/posterior force. Afterward, 2 groups with consideration for variations of shoulder orientations at ball release were created: {A- < Rz - 5, Rz - 5 ≤ A ≤ Rz + 5, Rz + 5 < A+}, {B- < Ry - 5, Ry - 5 ≤ B ≤ Ry + 5, Ry + 5 < B+}. Because shoulder range of motion is generally measured 5 degrees intervals in the orthopaedics. Then, we employed MANOVA to investigate significant differences of averages of inferior/superior and anterior/posterior forces applied to the shoulder at ball release among 2 groups respectively. Results: Superior/inferior force was significantly correlated with shoulder abduction. Also anterior/posterior force was strongly correlated with shoulder horizontal adduction/abduction (Table1). The average superior force at A- and the average inferior force at A+ were significantly greater than that at A. Also, the average anterior force at B- and the average posterior force at B+ were significantly higher than at that B (Table 2). Conclusion: The optimal shoulder orientation for minimizing shoulder force at the instant of ball release was 80.6 degrees of shoulder abduction and 10.7 degrees of shoulder adduction. Excessive shoulder horizontal abduction can lead to SLAP lesion, internal impingement and any more pitching-related shoulder injuries has been published. The results of this study demonstrated that increasing amounts of shoulder horizontal abduction were shown to increase anterior force on the shoulder. Therefore, excessive shoulder horizontal abduction can be responsible for several pitching-related shoulder injuries from the viewpoint of shoulder joint load. [Table: see text][Table: see text]