In the shoulder girdle, no one bone or joint works alone. The term "scapulohumeral rhythm" refers to the cooperative movements of the scapula and the humerus – or, more accurately, the glenohumeral joint and the scapulothoracic joint.
Bone Rhythm in Shoulder Abduction
Scapulohumeral rhythm is most clearly seen in shoulder abduction. The humerus, or upper arm bone, reaches out to the side and all the way up to the ear. This movement requires the head of the humerus to roll in the glenoid fossa, its shallow socket on the side of the scapula; depending on the individual's bony structure, the humerus also needs to rotate at some point to avoid butting against the acromion process.
However, glenohumeral movement alone is usually not enough to achieve full range of motion in the shoulder complex. The scapula should upwardly rotate to assist the movement of the humerus. In upward rotation, the acromion process moves upward, while the inferior angle of the scapula moves toward the side of the body. The left scapula spins clockwise, and the right scapula counter-clockwise.
Measuring Scapulohumeral Rhythm
Individual movement patterns, including bone rhythms, vary from person to person, but there are guidelines for typical or ideal scapulohumeral rhythm. Ideally, pure glenohumeral abduction can bring the humerus out 30 degrees. For every two degrees thereafter, the scapula should upwardly rotate one degree. When the humerus has reached 180 degrees of abduction (reaching straight up beside the ear), the inferior angle of the scapula should roughly bisect the ribs when viewed from the side.
In some people, this scapular motion can be easily seen under the skin. In others, it's easier to assess by palpating the inferior angle of the scapula, allowing the scapula to slide beneath the palpating hand.
Muscles Involved with Scapulohumeral Rhythm
The primary abductors of the shoulder are the lateral deltoid and supraspinatus, both of which act on the glenohumeral joint. In addition, the trapezius pulls the scapula into upward rotation, and the serratus anterior supports the scapula as it pulls toward the side of the body.
Though it works in concert with the deltoid, supraspinatus plays an important role in the rhythm of the shoulder: it rolls the humerus down out of the way of the acromion process, preventing it from impinging for as long as possible. This action, along with sufficient upward rotation of the scapula, enables easy movement of the arm all the way up to the ear.
Improving Scapulohumeral Rhythm
If these rhythms have been inhibited, often by shoulder injuries, movement teachers and physical therapy practitioners can assist clients to regain functional rhythm by strengthening the muscles that are weak, stretching those that are restricting movement, and guiding the client to allow the scapula to move in its ideal pattern.
For further information, see
Bony Anatomy of the Shoulder Girdle
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