The scapula is clothed with four muscles on it's dorsal surface. They all insert into the humerus in the upper part of its shaft, its greater tuberosity, by tendon.

• The supraspinatus arises above the spine of the scapula. Its muscle bundles converge under the acromion and its tendon inserts into the highest part of the tuberosity. It assists the deltoid in raising the arm and is covered by the trapezius.
• The ifraspinatus arises from a large part of the dorsal surface of the scapula, just below the spine. It is a thick muscle and its rounded form is seen bounded by the trapezius and deltoid. It acts with the posterior part of the deltoid to pull the arm back and rotate the head of the humerus backward.
• The teres minor arises from the lateral border of the scapula and inserts into the tuberosity just below the infraspinatus. It helps rotate the humerus backward.

• The teres major arises from the dorsal surface of the inferior angle of the scapula. It passes across the arm pit to attach on the anterior side of the humerus. The muscle therefore when it contracts rotates the head of the humerus forward in it's socket, and pulls the humerus back. In a person with well developed muscles the forms of these last three can be seen clearly. Otherwise they appear as one or two forms, slightly rounded on the surface, above the upper border of the latissimus dorsi.
• The subscapularis, not seen on the surface because it is on the ventral side of the scapula, covers the whole surface. It's tendon inserts into the anterior aspect of the tuberosity. These five muscles are known as the "rotator cuff" as they control the action of the head of the humerus rotating in the socket.
• The triceps, as the name indicates, has three heads. Two of them are attached to the shaft of the humerus, the medial head lying under the other two, and the long head spanning the back of the arm pit to attach to the scapula just under the glenoid fossa. They have a common flat tendon which appears as a flattened area on the back of the upper arm above the elbow. This tendon spans the elbow joint and inserts into the olecranon process of the ulna, which can be seen on the surface. The triceps is the only muscle on the back of the upper arm and is the great extensor of it, straightening the elbow joint.

In the figure to the left:

The arms are being strongly extended backward. The triceps' fleshy and contracting heads are emerging from under the posterior part of the deltoid. The forms of two parts of the deltoid, the middle and posterior, can be seen. The middle part is puckering because of it's septa. The posterior parts are the strong rounded bands leading from the spine area of the scapula over to the humerus. On the left side, the deltoid has a richer form and is more strongly puckered because it is rising that arm slightly higher, with more muscle bundles in it's middle part contracting.

Both the scapulae are being pulled back to the midline of the back. Their vertebral borders are deep in the two furrows which lead down to their inferior angles. His left inferior angle is higher because that scapula is being pulled up toward the head. This action is caused by the trapezius contracting and it can be seen as a prominent mound on his left shoulder. It's contracting form leads right up to the skull.

Between the buried vertebral borders of the scapula and the midline, where the vertebrae are also buried, the two rhomboids are contracting and also the lower fleshy part of the trapezius, causing the mounds.

In the two drawings to the left:

The lower muscle bundles of the trapezius are contracting and appear as definite small columns, either side of the midline of the back. The flat area between them is the aponeurotic tendinous attachment of the trapezius to the spines of the thoracic vertebrae. The middle and upper fleshy parts of the trapezius which are attached to the medial edge of the acromions and the upper borders of the spines of the scapulae are also very obvious mounds as the muscle works to rotate the scapulae. In full rotation of the scapulae, with the arms raised forward and high above the head, the whole lower part of the trapezius is stretched, and would appear quite flattened against the rib cage.

The deltoids are both contracting, lifting the upper arms. The biceps and brachialis muscles are working with them, both to lift the ulna and radius in the lower arms but mainly here to flex the arms against resistance.

The latissimus dorsi muscles are holding the inferior angles of the scapulae against the rib cage. The infraspinatus and the teres major are bulging forms over the upper edges of this muscle.

Note the form of the brachialis. It lies partially under the biceps and is a very powerful muscle. It has a V-shaped attachment on either side of the insertion of the deltoid into the deltoid tuberosity of the humerus, and it is crossing under the biceps to insert into the ulna. Also, on the left arm, the brachioradialis is working with the biceps and brachialis to strongly flex the arm against resistance.

The forms of both inferior angles of the scapulae can be seen. The left one is more apparent on the
edge.

The quadartus lumborum and the spinalis on his right side are stronger contracting forms as he is
bending slightly backward as well as to his right.