Scapulohumeral Rhythm

1. 0-30 degrees (setting phase): Movement is variable and inconsistent. The scapula may move slightly medially ("setting" against the thorax) or not move at all; most motion is purely glenohumeral. This phase is often excluded from the ratio calculation because it's inconsistent between individuals.
2. 30-180 degrees: From here on, the 2:1 rhythm applies consistently. Of the remaining 150 degrees of abduction:
   • 100 degrees occurs at the glenohumeral joint
   • 50 degrees occurs via scapular upward rotation (rotation of the scapula so the glenoid fossa faces more superiorly)

• Glenohumeral motion: deltoid and rotator cuff (especially supraspinatus)
• Scapular upward rotation: trapezius (all parts) and serratus anterior working as a force couple; the serratus anterior rotates the scapula laterally around the chest wall while the trapezius stabilizes/rotates it upward

• Loss of normal scapulohumeral rhythm is seen in conditions like rotator cuff tears, subacromial impingement, scapular winging (long thoracic nerve/serratus anterior palsy), trapezius palsy (spinal accessory nerve injury), adhesive capsulitis ("frozen shoulder"), and glenohumeral instability.
• A common compensatory pattern: with rotator cuff dysfunction or shoulder pain, patients often show early and excessive scapular elevation/rotation ("hiking" the shoulder) to compensate for reduced glenohumeral motion - this alters the normal 2:1 rhythm and is often assessed clinically (e.g., during shoulder abduction observation, "scapular dyskinesis").
• Assessing scapulohumeral rhythm is part of the physical exam for shoulder pain, especially in evaluating rotator cuff pathology and scapular dyskinesis.

Scapulohumeral Rhythm

• 0-30 degrees of abduction (setting phase): Motion is variable; the scapula may not move much, with most movement occurring at the glenohumeral joint.
• 30-180 degrees: A consistent 2:1 ratio applies - for every 15 degrees of further arm elevation, about 10 degrees comes from the glenohumeral joint and 5 degrees from scapular rotation.
• Per the textbook: "As elevation increases above 90 degrees, so does the proportion of scapulothoracic motion relative to glenohumeral motion."

• Glenohumeral motion: deltoid (prime mover) with the rotator cuff (supraspinatus, infraspinatus, teres minor, subscapularis) acting as a force couple with the biceps to depress and stabilize the humeral head, converting the deltoid's vertical pull into rotation rather than superior translation of the humeral head.
• Scapular rotation: trapezius and serratus anterior acting together to upwardly rotate the scapula; the clavicle also rotates 30-40 degrees during this motion, largely at the sternoclavicular joint, with the acromioclavicular joint's coracoclavicular ligaments maintaining the scapula-clavicle relationship.

• "Disturbance of the normal scapulohumeral rhythm affects the biomechanics of the shoulder joint and may result in secondary impingement" (Rheumatology, p. 700).
• Rotator cuff weakness or tears reduce dynamic stabilization of the humeral head, causing superior migration of the humeral head and increased risk of subacromial impingement - this alters the normal rhythm.
• Seen clinically in: rotator cuff tears/tendinopathy, subacromial impingement syndrome, scapular dyskinesis, long thoracic nerve palsy (winged scapula), trapezius (spinal accessory nerve) palsy, and adhesive capsulitis.
• Assessment of scapulohumeral rhythm (observing for early scapular "hiking" or asymmetric scapular movement during arm elevation) is a standard part of the shoulder physical exam.