Scapulohumeral Rhythm

Motion of the scapula, synchronous with motions of the humerus, allows for 150° to 180° of shoulder ROM into flexion or abduction with elevation. The ratio has considerable variation among individuals but is commonly accepted to be 2:1 (2° of glenohumeral motion to 1° of scapular rotation) overall motion. During the setting phase (0° to 30° abduction, 0° to 60° flexion), motion is primarily at the glenohumeral joint, whereas the scapula seeks a stable position. During the mid-range of humeral motion, the scapula has greater motion, approaching a 1:1 ratio with the humerus; later in the range, the glenohumeral joint again dominates the motion.

• During humeral elevation, the synchronous motion of the scapula allows the muscles moving the humerus to maintain an effective length-tension relationship throughout the activity and helps maintain congruency between the humeral head and fossa while decreasing shear forces.

• Muscles causing the upward rotation of the scapula are the upper and lower trapezius and the serratus anterior. Weakness or complete paralysis of these muscles results in the scapula being rotated downward by the contracting deltoid and supraspinatus as abduction or flexion is attempted. These two muscles then reach active insufficiency, and functional elevation of the arm cannot be reached, even though there may be normal passive ROM and normal strength in the shoulder abductor and flexor muscles.

• During elevation of the humerus, the pectoralis minor is lengthened as the scapula upwardly rotates, retracts, and tips posteriorly. Restricted scapular movement during humeral elevation from a shortened pectoralis minor results in patterns similar to those seen in patients with impingement symptoms and could be a risk factor for development of the syndrome.

Clavicular Elevation and Rotation With Humeral Motion

It is commonly accepted that initially the first 30° of upward rotation of the scapula occurs with elevation of the clavicle at the SC joint. Then, as the coracoclavicular ligament becomes taut, the clavicle rotates 38° to 55° about its longitudinal axis, which elevates its acromial end (because it is crank-shaped). This motion allows the scapula to rotate an additional 30° at the AC joint. Loss of any of these functional components decreases the amount of scapular rotation and thus the ROM of the upper extremity.

Focus on Evidence

A recent three-dimensional study of clavicular motion during humeral flexion, scaption, and abduction to 115° using surface electromagnetic sensors on 30 asymptomatic subjects and 9 individuals with shoulder pathology documented 11° to 15° of clavicular elevation, 15° to 29° of retraction, and 15° to 31° of posterior long axis rotation, showing similar patterns but different ranges from previously reported studies. Ranges of clavicular motion above 115° were not reliable owing to movement of the clavicle under the skin.

External Rotation of the Humerus With Full Elevation

During abduction of the arm in the frontal plane, for the greater tubercle of the humerus to clear the coracoacromial arch, the humerus must externally rotate as it is elevated above the horizontal. Weak or inadequate external rotation results in impingement of the soft tissues in the suprahumeral space, causing pain, inflammation, and eventually loss of function.

Internal Rotation of the Humerus With Full Elevation

Medial rotation begins at around 50° of passive shoulder flexion when all structures are intact. With full range of shoulder flexion and elevation, the humerus medially rotates 90°, and the medial epicondyle faces anteriorly.

• As the arm elevates above the horizontal position in the sagittal plane, the anterior capsule and ligaments become taut, causing the humerus to rotate medially. The bony configuration of the posterior aspect of the glenoid fossa contributes to the inward rotation motion of the humerus as the shoulder flexes. Most of the shoulder flexor muscles are also medial rotators of the humerus.

• The infraspinatus and teres minor stabilize the humeral head against the inward rotating forces, helping to maintain alignment and stability of the head in the fossa. Weakness in these muscles may contribute to excessive anterior translation and instability.

Elevation of the Humerus Through the Plane of the Scapula—Scaption

The plane of the scapula is described as 30° anterior to the frontal plane. Motion of the humerus in this plane is popularly called scaption or scapular plane abduction. In this range there is less tension on the capsule, and greater elevation is possible than with pure frontal or sagittal plane elevation. Neither internal nor external rotation of the humerus is necessary to prevent greater tubercle impingement during elevation in scaption. Many functional activities occur with the shoulder oriented in this plane.

Deltoid-Short Rotator Cuff and Supraspinatus Mechanisms

• Most of the force of the deltoid muscle causes upward translation of the humerus; if unopposed, it leads to impingement of the soft tissues in the suprahumeral space between the humeral head and the coracoacromial arch.

• The combined effect of the short rotator muscles (infraspinatus, teres minor, subscapularis) causes stabilizing compression and downward translation of the humerus in the glenoid.

• The combined actions of the deltoid and short rotators result in a balance of forces that abduct the humerus and control the humeral head.

• The supraspinatus muscle has a significant stabilizing, compressive, and slight upward translation effect on the humerus; these effects, combined with the effect of gravity, lead to abduction of the arm.

• Interruption of function leading to fatigue or poor coordination of any of these muscles can cause microtrauma and eventual dysfunction in the shoulder region.

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