Injuries to the shoulder and elbow are common amongst baseball players and the topic of many studies in peer reviewed journals. Previous studies have determined that joint forces at maximum external rotation (MER) and anterior joint instability are 
prime factors that lead to injury.(1, 2) It has also been established that anterior sheer force increases with horizontal abduction at MER which places extreme stress to the rotator cuff interval as anterior humeral head translation occurs (2). This same combination has been reported to be the cause of internal impingement, which if left untreated, leads to injury of the rotator cuff attachments and labrum.(2, 3, 4)
Below are a couple of recent studies that reveal findings of extreme shoulder joint forces and faulty mechanics that can lead to the injuries stated above by prior research.
Researchers from UNC Chapel Hill analyzed the throwing mechanics of 
72 high school pitchers with at least 2 years pitching experience. The pitches were thrown from a full wind-up on a standard sized mound set up in a biomechanical motion analysis lab. After warming up and becoming familiar with the mound each pitcher was instructed to throw as hard as possible while attempting to hit the strike zone. The three strikes with the highest velocity were used for analysis.
Pelvic and upper torso rotation velocities were measured and analyzed. Shoulder joint angles were calculated as were shoulder and elbow moments and forces which were then compared to the data collected on pelvic and upper torso rotation sequencing. Pitchers were defined to have improper trunk rotatin sequence when the peak pelvic rotation velocity was reached later than peak upper torso rotation velocity. Of the 72 pitchers, 33 demonstrated improper trunk rotation sequences. It was also found that pitchers with improper rotation sequences also demonstrated greater maximum external rotation (MER) angles and greater shoulder proximal force.
The authors performed a biomechanical analysis of 213 pitchers from various ages and participation levels in order to examine the upper arm angle relative to trunk position and shoulder rotation and estimate the resultant joint forces in the shoulder. Horizontal abduction
angle and joint forces at the moment of maximum external rotation (MER) were the focus of the analysis.
Pitchers were instructed to complete a standard warm-up and then throw from a regulation mound using their most comfortable pitching style. Each pitcher was allowed 3 trials with the highest pitch velocity being used for analysis. After calculation of joint angles and forces the authors concluded that there was a significant correlation between horizontal abduction angle and anterior/posterior shear force at the moment of MER. The larger the horizontal abduction angle is at MER, the more severe the anterior/posterior shear force is at the glenohumeral joint.
Clinical Application: These studies used high tech biomechanical equipment to analyze pitching mechanics that may be difficult to see with the naked eye, however its important to know that these faults exist and can create severe damage to the shoulder. While both studies were analytical in nature and did not compare groups in a randomized control trial, they each reached a common conclusion that will aid in the rehab and injury risk reduction of pitchers. That is, faulty pitching mechanics lead to excessive joint forces at MER which seems to be the pathogenesis for throwing injuries. Finally, when working with pitchers it’s important to take these study conclusions and apply them to your exercise prescription. Avoid exercises that stress the anterior shoulder joint. Facilitate glenohumeral stability throughout the throwing motion, specifically at abduction + external rotation. Train in the transverse plane with a variety of stances for efficient pelvic/torso sequencing and optimal core to extremity transfer of force.
References
1. Fleisig, G.S., Andrews, J.R., Dillman, C.J., and Escamilla, R.F. Kinetics of baseball pitching with implications about injury mechanisms. Am J Sports Med. 1995; 23: 233–239
2. Nobuhara, K. and Ikeda, H. Rotator interval lesion. Clin Orthop Relat Res. 1987; 223: 44–50
3. Walch, G., Boileau, P., Noel, E., and Donell, S. Impingement of the deep surface of the supraspinatus tendon on the posterosuperior glenoid rim: an arthroscopic study. J Shoulder Elbow Surg. 1992; 1: 238–245
Chris Butler Sports PT 