Part 1: Introduction and anatomy

Understanding the shoulder’s anatomy can be crucial in diagnosing, treating and preventing injuries and building strength and power. Shoulder anatomy is a complex and intricate system of bones, joints, muscles, tendons, and ligaments that work together to allow a wide range of movement. It is considered a “freely movable” joint and has the greatest range of motion of any joint in the body, but stability is the trade-off for such mobility.

Joints

The shoulder comprises four joints; the acromioclavicular, glenohumeral, scapulothoracic, and sternoclavicular. The glenohumeral joint is a ball-and-socket joint made up of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone). The rounded end of the humerus fits into a shallow socket called the glenoid (located on the scapula) like a golf ball on a tee.
The shoulder has several critical structures to help stabilize the shoulder joint and control its movements. The rotator cuff is essential in controlling the arm’s movements and preventing the head of the humerus from dislocating from the glenoid. The rotator cuff comprises four muscles and their associated tendons surrounding the shoulder joint. These muscles originate from the scapula and attach to the humerus, and they help to keep the head of the humerus centred in the glenoid socket.

Rotator Cuff

The rotator cuff is a group of four muscles that help stabilize the shoulder joint and allow for smooth movement. The muscles of the rotator cuff are the supraspinatus, infraspinatus, teres minor, and subscapularis.

Supraspinatus

Origin: Supraspinous fossa of scapula

Insertions: Greater tubercle of humerus

Actions: Shoulder abduction (lifting arm away from body) of arm (0–15 degrees), and stabilization of the humeral head in the glenoid cavity

Infraspinatus

Origin: Infraspinous fossa of scapula

Insertion: Greater tubercle of humerus, Stabilization of humeral head in the glenoid cavity

Action: external rotation; Stabilizes humeral head in glenoid cavity

Subscapularis

Origin: Subscapular fossa of scapula

Insertions: Lesser tubercle of humerus

Action: Internal rotation, and Stabilizes humeral head in glenoid cavity

Teres Minor

Origin: Lateral border of scapula

Insertions: Greater tubercle of humerus

Action: Arm external rotation, arm adduction, and stabilizes humeral head in glenoid cavity

Ligaments and capsule

Underneath all the muscles discussed so far, the shoulder has several ligaments that help stabilize the joint and prevent dislocation. The most important of these is the glenohumeral ligament, which runs from the head of the humerus to the glenoid rim and helps control the joint’s movement. Several other ligaments help to stabilize the joint, including the coracohumeral ligament, the coracoacromial ligament, and the acromioclavicular ligament.

The bursae are tiny fluid-filled sacs near the shoulder joint that help reduce friction between bones and tendons. The tendons are strong, fibrous structures that attach muscle to bone and play a key role in transmitting the force generated by the muscles to the bones and joints.

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Deltoid

The deltoid muscle, also known as the delts, is named after the Greek letter delta, which has a similar triangular shape as the muscle. The deltoid muscle is a large triangular muscle that covers the shoulder joint. It has three distinct sections from different parts of the scapula and clavicle bones and converges to insert into the humerus bone.

The anterior deltoid originates from the anterior border of the lateral third of the clavicle. It is responsible for arm flexion and medial rotation and assists in shoulder abduction (ie. overhead press, bench press, punching, etc).

The middle deltoid originates from the acromion process of the scapula. It is responsible for the abduction (illustrated below) of the arm (beyond the initial 15°) and is the primary muscle used during overhead movements such as throwing, swimming, and serving a volleyball.

The posterior deltoid originates from the posterior border of the spine of the scapula. It is responsible for the extension and lateral rotation of the arm and assists in shoulder abduction (ie. rowing, pull-ups, and reverse fly exercises.)

Scapular Muscles

So far we have discussed the muscles and stabilizers of the glenohumeral joint. We will now focus on the muscles of the scapulothoracic junction. The scapulothoracic junction is where the scapula (shoulder blade) and thorax (rib cage) meet. It is a crucial area where the muscles surrounding it work in tandem to allow for optimal shoulder and upper limb movement. Among the muscles in this region are the trapezius, serratus anterior, rhomboids, and levator scapulae.

Overall, the muscles of the scapulothoracic junction play a critical role in providing stability and mobility to the shoulder region. Dysfunction of these muscles can lead to pain, weakness, and difficulty performing everyday activities. Proper exercise and treatment can help improve these muscles’ function and alleviate any associated symptoms.

Trapezius

The trapezius muscle is a large, triangular muscle extending from the skull’s base to the thoracic vertebrae and down to the shoulder blade. It has three distinct regions: the upper, middle, and lower fibres. The upper fibres assist in the elevation and upward rotation of the shoulder blade, while the middle fibres aid in the retraction of the shoulder blade. The lower fibres help with depression and upward rotation of the shoulder blade.

The trapezius is a large muscle that runs from the base of the skull to the middle of the back. It plays a crucial role in shoulder movement and stability, particularly during overhead movements.

The trapezius muscles are divided into three parts: the upper fibers, middle fibers, and lower fibers. The upper fibers of the trapezius muscles are attached to the back of the skull and extend down to the shoulders. The middle fibers run horizontally across the upper back and attach to the spine, while the lower fibers are attached to the spine and extend down to the lower back.

Origin:

Descending part (superior fibers): medial third of the superior nuchal line, external occipital protuberance, nuchal ligament

Transverse part (middle fibers): spinous processes and supraspinous ligaments of vertebrae T1-T4 (or C7-T3)

Ascending part (inferior fibers): spinous processes and supraspinous ligaments of vertebrae T4-T12

Insertion:

Descending part (superior fibers): lateral third of clavicle

Transverse part (middle fibers): medial acromial margin, superior crest of spine of scapula

Ascending part (inferior fibers): lateral apex of the medial end of scapular spine

Function:

Descending part (superior fibers)

Scapulothoracic joint: draws scapula superomedially

Atlantooccipital joint: extension of head and neck, lateral flexion of head and neck (ipsilateral)

Altantoaxial joint: rotation of head (contralateral)

Transverse part (middle fibers)

Scapulothoracic joint: draws scapula medially

Ascending part (inferior fibers)

Scapulothoracic joint: Draws scapula inferomedially

Serratus Anterior

The serratus anterior muscle is located on the lateral aspect of the chest wall and runs from the upper eight or nine ribs to the scapula’s medial border. This muscle is responsible for the protraction of the shoulder blade, allowing the arm to raise above the head. It also aids in the upward rotation of the scapula, which is crucial in movements such as throwing.

Origin: Superior part: Ribs 1–2, Intercostal fascia

Middle part: Ribs-3–6

Inferior part: Ribs 7–8/9 (variably extends to rib 10 (+ external oblique muscle))

Insertion: Scapula

(Superior part: Anterior surface of superior angle

Middle part: Anterior surface of medial border

Inferior part: Anterior surface of inferior angle and medial border)

Action/function: Draws scapula anterolaterally, Suspends scapula on thoracic wall, Rotates scapula (draws inferiorly angle laterally)

Levator Scapulae

The levator scapulae muscle extends from the cervical vertebrae to the upper border of the shoulder blade. This muscle helps elevate the shoulder blade and assist in lateral flexion of the neck.

Origin: Transverse processes of vertebrae C1-C4

Insertion: Medial border of scapula (from superior angle to root of spine of scapula)

Action/functions:

Scapulothoracic joint: Draws scapula superomedially, rotates glenoid cavity inferiorly;

Cervical joints: Lateral flexion of neck (ipsilateral), extension of the neck

Rhomboids

The rhomboid muscles consist of two muscles, the rhomboid major and minor. The rhomboid major originates from the thoracic vertebrae and inserts onto the medial border of the shoulder blade. The rhomboid minor, located above the major, originates from the lower cervical and upper thoracic vertebrae and inserts onto the scapula’s medial border. Together, these muscles assist in retraction and downward rotation of the shoulder blade.

Origin:

Rhomboid minor: Nuchal ligament, Spinous processes of vertebrae C7-T1

Rhomboid major: Spinous process of vertebrae T2-T5

Insertion:

Rhomboid minor: medial end spine of scapula

Rhomboid major: medial border of scapula

Actions/functions:

Retraction of scaplae by drawing the scapula superomedially on the scapulothoracic junction, Rotates glenoid cavity inferiorly; Supports position of scapula

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