Infraspinatus

The infraspinatus is a small, deep muscle located on the posterior surface of the scapula. As its name indicates, it is situated below the spine of the scapula and covers the entire infraspinous fossa. Triangular in shape, the fibers of the infraspinatus are broadest at the medial border of the scapula and converge laterally where they cross the glenohumeral joint, wrap around the humeral head, and attach anteriorly onto the greater tubercle of the humerus. The infraspinatus fascia covers the infraspinatus muscle and separates it from the neighboring teres major and teres minor muscles. The majority of the infraspinatus is framed by the posterior edge of the deltoid, the lateral edge of the lower trapezius, and the superior edge of the latissimus dorsi. The lateral portion is less accessible, as it is obscured by the bulk of the deltoid muscle.

The infraspinatus is one of four muscles that make up the rotator cuff. The supraspinatus, infraspinatus, teres minor, and subscapularis all function as a unit to stabilize the humeral head in the glenoid fossa. Each muscle has a specific role in steering the head of the humerus as the arm moves into different positions. Specifically, the infraspinatus works with the teres minor to seat the humeral head posteriorly in the glenoid fossa and prevent impingement on the coracoid process of the scapula.

The infraspinatus is one of the most powerful external rotators of the glenohumeral joint and is essential in pulling the upper extremity into backward extension and external rotation for shoulder movements such as pitching and hitting overhead. The horizontal and slight upward angle of the muscle fibers, as well as the strong attachment to the posterior scapula, provide excellent mechanical advantage for this "winding up" motion of the shoulder. The infraspinatus is also recruited eccentrically to slow the upper extremity during the "follow-through" or deceleration phase of these powerful movements.

Imbalances often develop between the powerful internal rotators of the shoulder (pectoralis major, latissimus dorsi, teres major, anterior deltoid, and subscapularis) and the smaller external rotators (posterior deltoid, infraspinatus, and teres minor), creating faulty mechanics in the glenohumeral joint. This presents as a variety of pathologies, including impingement syndrome, tendonitis, and bursitis. Trigger points are more common in the deeper subscapularis but may be responsible for referred pain in the upper extremity.

A Deeper Dive—Palpating the Infraspinatus

Positioning: client prone with their arm off the edge of the table

1. Palpate the lateral border of the scapula with your thumb.
2. Place the fingers of that same hand medially and superiorly to find the infraspinatus.
3. Locate the muscle belly in the infraspinous fossa of the scapula.
4. Follow the tendon of the infraspinatus superiorly and laterally around the head of the humerus to the greater tubercle of the humerus.
5. Have the client resist shoulder external rotation to ensure proper location.

Passive Stretching of the Infraspinatus

Positioning: client supine at the edge of the table with their arm relaxed at their side

1. Stand at the client's side and support the wrist and elbow, maintaining a slightly flexed elbow.
2. Move the arm into shoulder abduction until the elbow is even with the shoulder.
3. From this position, move the arm in and across the body as far as is comfortable.
4. Additional passive internal rotation may be applied to increase the stretch on the infraspinatus muscle.