The shoulder or glenohumeral joint is a ball and socket joint formed between the humerus and scapula. Due to the shallowness of the socket and relatively loose connections, the shoulder joint allows for a wide range of motion; however, this wide range makes the joint unstable and thus more prone to dislocation and injury than other joints.

Two other joints make up the shoulder; the acromioclavicular joint of the clavicle and scapula, which allows the arm to be raised above the head, and the sternoclavicular joint of the clavicle and sternum, which plays an important role in facilitating movement of the upper arm and connecting it to the rest of the skeleton.

Muscles that act on the shoulder can be classified as extrinsic, intrinsic, pectoral, or upper arm. Upper arm muscles will be discussed in a later section since they primarily promote forearm movement.

Extrinsic Shoulder Muscles

Extrinsic muscles of the shoulder originate from the trunk and attach to the bones of the shoulder. They can be further subdivided into superficial and deep layers.

Superficial

As suggested by the name, superficial muscles lie on the surface. There are two superficial extrinsic muscles.

Trapezius – The trapezius is the most superficial muscle of the back and forms a broad flat triangle.

• Attachments – The trapezius originates from the skull and spine of the upper back and neck. It attaches to the clavicle and scapula.
• Actions – The superior region supports the arm and elevates and rotates the scapula, the intermediate region retracts the scapula, and the inferior region rotates and depresses the scapula.

Latissimus Dorsi – The latissimus dorsi originates from the lower back and covers a wide area.

• Attachments – The latissimus dorsi originates from the lower spine and ribs and the upper pelvis and fascia of the deep trunk muscles. The muscle converges into a tendon attaching to the humerus.
• Actions – Extends, adducts, and medially rotates the upper arm.

Deep

Three deep muscles lie below the superficial muscles of the shoulder.

Levator Scapulae – A small, strap-like muscle that joins the neck to the scapula.

• Attachments – Originates from the side of the spine in the neck and attaches to the scapula.
• Actions – Elevates the scapula.

Rhomboid Major – Sits inferiorly to the levator scapulae.

• Attachments – Originates from the spine in the upper back and attaches to the scapula in an inferior position to the levator scapulae attachment.
• Actions – Retracts and rotates the scapula.

Rhomboid Minor – Sits between the levator scapulae and  rhomboid major, with which it is paired in action and function. It retracts and rotates the scapula.

Location of the trapezius muscle

Highlighted in orange, the trapezius is a large, broad muscle of the back that acts on the shoulder.

This diagram illustrates the lumbar triangle in relation to the deltoideus, infraspinatus, teres major, lattissimus dorsi, and lumbodorsal fascia.

Intrinsic

Intrinsic muscles originate from the scapula or clavicle and attach to the humerus. There are six intrinsic muscles, four of which form the rotator cuff.

Deltoid – The deltoid muscle is a triangular muscle which covers the shoulder. The action of the muscle is complex, with the components acting in opposing and separate ways during the course of a contraction.

• Attachments – The deltoid muscle originates from the scapula and clavicle and attaches to the lateral surface of the humerus.
• Actions – The anterior region assists the pectoralis major during transverse flexion of the shoulder and acts weakly in strict transverse flexion. The lateral region assists in shoulder flexion when the shoulder is rotating, although it also assists the transverse abduction of the shoulder. The posterior region is the hyperextensor of the shoulder, contributing to transverse extension.

Teres Major – The teres major is a thick flattened muscle connecting the lower scapula with the humerus.

• Attachments – Originates from the posterior of the scapula and attaches to the humerus.
• Actions – Adducts the shoulder and assists in rotation of the arm.

Location of the deltoid muscles

Highlighted in orange, the deltoids cover the rounding of the shoulder joint.

This diagram illustrates the deltoideus in relation to the lumbar triangle, infraspinatus, teres major, lattissimus dorsi, and lumbodorsal fascia.

Rotator Cuff

The rotator cuff is a group of four muscles that pull the ball of the humerus into the shallow socket of the scapula, adding required stability. The rotator cuff complex is composed of the supraspinatus, infraspinatus, subscapularis, and teres minor all of which originate from the scapula and connect to the humerus. The supraspinatus is involved in abduction of the arm in association with the deltoid, while the other muscles facilitate rotation of the arm.

Muscles of the rotator cuff

Muscles of the rotator cuff and presented with the triceps brachii.

This image displays the rotator cuff muscles, including the supraspinatus, infraspinatus, subscapularis, and teres minor.

Pectoral

Pectoral muscles lie in the chest and exert force through the shoulder to move the upper arm. Three pectoral muscles interact with the shoulder.

Pectoralis Major – The pectoralis major is a large, fan-shaped muscle covering the chest. It is comprised of clavicular and sternocostal regions.

• Attachments – The clavicular region originates from the clavicle and the sternocostal region originates from the sternum and the fascia of the oblique muscles of the abdomen. Both attach to the humerus.
• Actions – Adducts and rotates the upper arm.

Pectoralis Minor – The pectoralis minor muscle is smaller and lies beneath the pectoralis major. 

• Attachments – The pectoralis minor originates from the third to fifth ribs and attaches to the scapula.
• Actions – Supports and depresses the scapula.

Serratus Anterior – The serratus anterior is located in the lateral wall of the chest.

• Attachments – The muscle is formed of several strips originating from the second to eight ribs, each of which attaches to the scapula.
• Actions – Supports the scapula allowing for elevation of the upper arm.

Key Movements

Extension (upper limb backwards behind back)

• Produced by the posterior deltoid, latissimus dorsi, and teres major.

Flexion (upper limb forwards past chest)

• Produced by the biceps brachii (both heads), pectoralis major, anterior deltoid, and corocobrachialis.

Abduction (upper limb away from trunk, spreading arms wide)

• Produced by the supraspinatus and deltoid. Past 90 degrees, the scapula needs to be rotated by the trapezius and serratus anterior to achieve abduction.

Adduction (upper limb towards trunk, bringing arms down to side)

• Produced by contraction of pectoralis major, latissimus dorsi, and teres major.

Medial Rotation (rotation of arm inwards to cover abdomen)

• Produced by contraction of subscapularis, pectoralis major, latissimus dorsi, teres major, and anterior deltoid.

Lateral Rotation (rotation of arm outwards away from the abdomen)

• Produced by contraction of the infraspinatus and teres minor.