Muscles laterally rotating the arm at the shoulder joint

Teres minor

Infraspinatus

Deltoid(posterior fibres)

Teres minor

When seen from the back, teres minor forms the upper boundary of both the upper triangular and quadrilateral spaces. It is a thin muscle which arises by two heads, separated by a groove for the circumflex scapular artery, from the upper two-thirds of the lateral border of the scapula, and the fascia between it and teres major(below) and infraspinatus(above). The fibres run upwards and laterally forming a narrow tendon which attaches to the lowest of the facets on the greater tubercle of the humerus and to the bone immediately below. The tendon reinforces and blends with the lower posterior part of the capsule of the shoulder joint.

Nerve supply

Teres minor is supplied by the axillary nerve, root value C5 and 6. The skin over the muscle is supplied by roots T1, 2 and 3.

Action

In the anatomical position teres minor is a lateral rotator, but when the arm is abducted it laterally rotates and adducts.

Palpation

Teres minor can be felt contracting if the examiner’s fingers are placed half way up the lateral border of the scapula and the arm is then actively laterally rotated. The tendon is found just below that of infraspinatus as determined above. 

Infraspinatus

Infraspinatus is a thick, triangular muscle which arises from the medial two-thirds of the infraspinous fossa of the scapula, tendinous intersections attached to ridges in this fossa, and the thick fascia covering the muscle. The fibres converge to a narrow tendon which inserts onto the middle facet on the greater tubercle of the humerus, and into the posterior part of the capsule of the shoulder joint. A bursa, which occasionally communicates with the shoulder joint, separates the muscle from the neck of the scapula. The upper part of the muscle lies deep to trapezius, deltoid and the acromion process; however, the lower part is superficial.

Nerve supply

Infraspinatus is supplied by the suprascapular nerve, root value C5 and 6. The skin over the muscle is supplied by the dorsal rami of T1 to T6.

Action

Infraspinatus is a lateral rotator of the arm at the shoulder joint.

When the arm is laterally rotated, contraction of infraspinatus can be felt in the medial part of the infraspinous fossa. Its tendon can be palpated if the greater tubercle is moved from below the acromion process. To accomplish this the subject lies prone supporting him or herself on the elbows and forearms. The arm is then laterally rotated some 25° and slightly adducted. The tendon can now be palpated just below the acromial angle. It is at this point that soft tissue techniques, such as transverse frictions or electrical treatments are applied if the tendon becomes inflamed.

Functional activity

Infraspinatus and teres minor are of importance during the sequence of movements which occur when the arm is fully abducted. During the latter part of this movement the humerus is laterally rotated so that the greater tubercle moves clear of the coracoacromial arch thereby enabling the remaining part of the humeral head to come into contact with the glenoid fossa, and full abduction to occur.

Teres minor, infraspinatus, supraspinatus and subscapularis, the musculotendinous “rotator cuff” of extensible ligaments around the shoulder joint, are all concerned with its stability; the proximity of their tendons to the joint enhances their effect. During movements of the head of the humerus on the glenoid fossa, interplay between these muscles reduces the sliding and shearing movements which tend to occur. When carrying a weight in the hand these same four muscles brace the head of the humerus against the glenoid fossa.