Scapulohumeral rhythm describes the coordinated movement between the glenohumeral joint and the scapulothoracic articulation. What is the approximate ratio of arm abduction to scapulothoracic upward rotation?

• A. Glenohumeral joint and scapulothoracic articulation with about a 1:2 ratio
• B. Glenohumeral joint and acromioclavicular articulation with about a 2:1 ratio
• C. Glenohumeral joint and scapulothoracic articulation with about a 2:1 ratio
• D. Scapulothoracic articulation alone, without glenohumeral involvement

Answer: (C)

The movement pattern being tested is that arm elevation relies on a coordinated interaction between the glenohumeral joint and the scapulothoracic articulation in about a 2:1 ratio. In practical terms, when the arm is raised to about 180 degrees, roughly 120 degrees comes from abduction at the shoulder joint (glenohumeral) and about 60 degrees comes from upward rotation of the scapula on the thorax (scapulothoracic). This 2:1 relationship keeps the shoulder moving smoothly and prevents impingement, distributing the work between the two joints.

The correct choice reflects that the ratio involves the shoulder joint and the scapulothoracic articulation, not the acromioclavicular joint, and not a scenario where the scapula moves without any glenohumeral involvement.