In phase II (90-180 degrees) of scapulohumeral rhythm, how much motion is contributed by the scapulothoracic joint?

In phase II of scapulohumeral rhythm, which occurs when the arm is abducted from 90 to 180 degrees, approximately 30 degrees of motion is contributed by the scapulothoracic joint.

This phase of movement is characterized by the coordinated motion between the glenohumeral joint (the ball-and-socket joint of the shoulder) and the scapulothoracic joint (the articulation of the scapula against the thoracic wall). As the arm continues to elevate beyond 90 degrees, the scapula must rotate upward to maintain the proper alignment of the glenoid fossa with the humerus. This upward rotation allows for continued range of motion while preventing impingement between the humeral head and the acromion process.

Therefore, recognizing the specific contribution of the scapulothoracic joint in this phase helps to understand the overall mechanics of shoulder elevation and the importance of scapular motion in optimizing shoulder function.