Proper shoulder function depends on intricate relationships between muscle groups, tendons, bone structures and more. Comprehensive shoulder assessment and treatment should focus on the Thoracic Spine, Scapula, Rotator Cuff and the Gleno-Humeral joint while also addressing posture and energy transfer throughout the entire kinetic chain.
If an athlete has limited mobility in the thoracic spine, the scapula will not have a sound foundation from which to operate. Thoracic extension and rotation are frequently compromised due to kyphotic posture with typical forward head and rounded shoulder position. This can result in an anteriorly tilted and abducted scapula and subsequently a poor foundation upon which the humerus has to move. Furthermore this creates compression in the sub-acromial space, resulting in typical shoulder pathologies such as impingement, bursitis and Rotator Cuff tendonitis.
Altered position or movement mechanics of the scapula affect shoulder stability and mobility. Common pathologies include labrum injuries, MDI (multi directional instability), tendonitis and elbow injuries. Trying to throw or hit with an unstable scapula is like trying to shot put while balancing on a gymball. Transferring forces up the kinetic chain from the legs through the hips, pelvis and spine into an unstable or poorly positioned scapula is a recipe for injury. Force couples, mobility and stability all help maintain instantaneous center of rotation (ICR) of the humerus in/on the glenoid surface. Having an ICR helps to ensure proper bio-mechanical function of the joint, which helps avoid labral tearing. When the joints are centered, they perform optimally and injury risk is reduced.
The gleno-humeral joint is subject to tremendous acceleration, deceleration and distraction forces, creating stress on the joint surface, the joint capsule and the rotator cuff. Gleno-humeral Internal Rotation Deficit (GIRD) is one of the most common adaptations that can affect throwers or hitters. GIRD develops due to the great deceleration forces placed on the posterior capsule and RTC, causing shortening of the muscles and tightening of the posterior-inferior capsule and a shift of the gleno-humeral rotation point. This migration and changing of the capsule contributes to decentration, or loss of the ICR.
GIRD leads to an increase in total range of motion of the dominant vs. non-dominant side though with an increase in external rotation and a loss of internal rotation. Some of this can be explained by “retroversion” or bony change to the humerus but it’s mostly caused by shortening of the posterior capsule and the RTC.