Shoulder elevation involves:
- humeral movement (at glenohumeral joint)
- scapular movement (at scapulothoracic joint)
Scapulohumeral rhythm:
(describes the timing of movement at these joints during shoulder elevation.)
- first 30 degrees of shoulder elevation involves a "setting phase":
- The movement is largely glenohumeral.
- Scapulothoracic movement is small and inconsistent.
- after the first 30 degrees of shoulder elevation:
- The glenohumeral and scapulothoracic joints move simultaneously.
- Overall 2:1 ratio of glenohumeral to scapulothoracic movement.
You can observe scapulohumeral rhythm by palpating the scapula's position as a person elevates the shoulder. Helpful scapular landmarks for palpation are the base of the spine and the inferior angle.
Scapulohumeral rhythm serves at least two purposes.
- It preserves the length-tension relationships of the glenohumeral muscles; the muscles do not shorten as much as they would without the scapula's upward rotation, and so can sustain their force production through a larger portion of the range of motion.
- It prevents impingement between the humerus and the acromion. Because of the difference in size between the glenoid fossa and the humeral head, subacromial impingement can occur unless relative movement between the humerus and scapula is limited. Simultaneous movement of the humerus and scapula during shoulder elevation limits relative (arthrokinematic) movement between the two bones.