Your shoulder is the most mobile joint in the human body. It allows you to lift your arm above your head, throw a ball, and reach behind your back. This exceptional mobility comes at a cost: the shoulder sacrifices stability for a greater range of motion. Understanding how your shoulder works helps you better understand why certain pains occur and how to treat them. This guide explores the key anatomical structures of your shoulder in accessible language.

Why is the shoulder so mobile?

The shoulder allows movement in all directions thanks to a unique anatomical configuration where a large ball (the head of the humerus) rests on a small, shallow surface (the glenoid cavity). This structure sacrifices stability for exceptional range of motion.

Imagine a golf ball resting on a tee. That's what your shoulder looks like. The head of the humerus (the top of your arm bone) is a sphere that fits into the glenoid cavity of the shoulder blade. This cavity is shallow and small.

This configuration creates unparalleled mobility. You can move your arm in almost all directions: forward, sideways, backward, and rotating. No other joint in the body offers this range of motion.

But this mobility comes at a cost. Unlike the hip (where the head of the femur fits deeply into a snug socket), the shoulder is inherently unstable. It relies heavily on muscles, tendons, and ligaments to keep the joint in place. This is why muscle problems often cause shoulder pain. Now that you understand this fundamental trade-off, let's explore the structures that make up this complex joint.

What are the four joints of the shoulder?

The shoulder consists of four distinct joints: the glenohumeral (main joint), acromioclavicular, sternoclavicular, and scapulothoracic. These joints work together to create the shoulder's full range of motion.

The Glenohumeral Joint

This is the main joint. The head of the humerus fits into the glenoid cavity of the shoulder blade. This joint allows you to lift and rotate your arm.

A rim of cartilage called the labrum surrounds the glenoid cavity. It slightly deepens the socket and helps stabilize the joint. When the labrum tears (often during dislocations), the shoulder becomes even more unstable.

An articular capsule surrounds this joint. It's a sac of connective tissue that holds the joint together. In adhesive capsulitis (frozen shoulder), this capsule thickens and contracts, severely limiting movement.

The Acromioclavicular (AC) Joint

The acromion (part of the shoulder blade) connects to the collarbone. This small joint is located on top of your shoulder. You can feel it by following your collarbone to its end.

This joint moves very little but plays a key role in transferring forces from the arm to the torso. Falls onto the shoulder often injure this joint (shoulder separation).

The Sternoclavicular Joint

The collarbone connects to the sternum (the bone in the center of your chest). It is the only bony connection between your arm and your torso. This joint moves when you fully raise your arm.

The Scapulothoracic

The shoulder blade glides over the rib cage. It's not a true joint with cartilage and a capsule, but this anatomical relationship is essential. Muscles control the position of the shoulder blade on the chest.

When the shoulder blade doesn't move correctly (scapular dyskinesis), all shoulder movement is affected. This is why shoulder physiotherapy often focuses on shoulder blade control. Understanding these four joints reveals why shoulder pain can originate from multiple areas. Now let's explore the bones that make up these joints.

What are the three main bones of the shoulder?

The shoulder is made up of three bones: the humerus (arm bone), the shoulder blade or scapula (a triangular bone in the back), and the clavicle (a horizontal bone in the front). These bones articulate with each other to form the shoulder complex.

The Humerus

This is the long bone of your upper arm. The head of the humerus (the rounded upper end) forms the ball-shaped part of the main joint. Just below the head are two bony bumps: the greater tubercle (on the side) and the lesser tubercle (in the front). These bumps serve as attachment points for the rotator cuff tendons.

The Shoulder Blade (Scapula)

This is the flat, triangular bone in your back. It contains several important parts:

• The glenoid cavity: the shallow surface where the head of the humerus fits.
• The acromion: the bony projection that forms the roof of the shoulder.
• The coracoid process: a beak-shaped projection in the front.

The shoulder blade essentially floats on your rib cage. It is not directly attached to your spine by bones. Muscles hold it in place and control its movements.

The Collarbone

This is the horizontal bone that runs from the sternum (breastbone, in the center of your chest) to the acromion. You can easily feel it under your skin. The collarbone acts like a strut, keeping your arm away from your body.

These three bones form the basic structure. However, it's the muscles and tendons that truly enable movement and provide stability. Let's now explore the most important muscles of the shoulder.

What is the Rotator Cuff and Why is it So Important?

The rotator cuff is a group of four muscles that surround and stabilize the glenohumeral joint (shoulder joint). These muscles keep the head of the humerus (upper arm bone) centered in its socket during movements. The four muscles are: the supraspinatus, infraspinatus, teres minor, and subscapularis.

Think of the rotator cuff as a muscular cuff that wraps around the head of your humerus (upper arm bone). These four muscles work together to keep the ball (head of the humerus) well-positioned on the tee (glenoid cavity or shoulder socket) as you move your arm.

The Supraspinatus

This muscle runs above the shoulder blade and attaches to the top of the humerus (upper arm bone). It initiates the upward movement of your arm to the side. Its tendon is the most commonly injured in the rotator cuff because it passes through a narrow space under the acromion.

The Infraspinatus

This muscle is located below the spine of the shoulder blade (the bony ridge on the shoulder blade). It attaches to the back of the humerus (upper arm bone). It rotates your arm outwards (external rotation).

The Teres Minor

This muscle works with the infraspinatus for external rotation. It is located just below the infraspinatus.

The Subscapularis

This muscle is located at the front, between the shoulder blade and the ribs. It attaches to the front of the humerus (upper arm bone). It rotates your arm inwards (internal rotation).

These four muscles are not very large. However, their role in stabilization is crucial. Without them, larger muscles like the deltoid would pull the head of the humerus (upper arm bone) upwards, causing impingement under the acromion.

Rotator cuff tendinopathy accounts for two out of three shoulder pain cases. Understanding these muscles helps you see why physiotherapy focuses on progressively strengthening the rotator cuff. Let's now explore the other important muscles of the shoulder.

What Other Muscles Control Shoulder Movements?

Other key muscles include the deltoid (a powerful muscle on top of the shoulder), the trapezius (controls the shoulder blade), and the pectoralis major (brings the arm forward). These muscles generate force, while the rotator cuff provides stability.

The Deltoid

This is the large muscle that forms the rounded shape of your shoulder. It has three parts: anterior (front), middle (side), and posterior (back).

The deltoid generates the power to lift your arm. However, without the stabilizing rotator cuff, the deltoid would simply pull the humerus (upper arm bone) upwards instead of creating a smooth movement.

The Trapezius

This large, diamond-shaped muscle extends from your neck down to the middle of your back. It controls the movements of your shoulder blade. The upper part raises your shoulders. The middle part retracts the shoulder blade (pulls it backward). The lower part lowers the shoulder blade.

The trapezius is often tense in people who spend a lot of time at the computer. This tension can cause neck and shoulder pain.

The Pectoralis Major

This large muscle at the front of the chest brings the arm forward and rotates it inward. Men who do a lot of bench presses often have tight pectoral muscles, which pulls their shoulders forward.

The Latissimus Dorsi

This large back muscle brings the arm towards the body and backward. It also helps with internal rotation.

These muscles work in complex coordination. If a muscle is weak or tight, the entire movement pattern changes. This is why physiotherapy always assesses the muscle balance around the shoulder. Now, let's explore the structures that reduce friction in the joint.

What is a bursa and why does it get inflamed?

A bursa is a small, fluid-filled sac that reduces friction between structures that slide over each other. The subacromial bursa is located between the rotator cuff tendons and the acromion. When it gets inflamed (bursitis), it swells and causes pain when lifting the arm.

Your shoulder contains several bursae. The most important one is the subacromial bursa. It is located in the space between the rotator cuff tendons (below) and the acromion (above).

This space is already narrow. When you lift your arm, the space narrows even further. The bursa allows the tendons to slide under the acromion without excessive friction.

When the bursa gets irritated and inflamed, it swells. This swelling further reduces the already narrow space. Movements become painful. Night pain is common because swelling increases when you lie down.

Bursitis often occurs with rotator cuff tendinopathy. Inflammation of one structure irritates neighboring structures. Treatment must address both issues.

The good news is that bursitis generally responds well to conservative treatment. With relative rest, ice, and appropriate exercises guided by a physiotherapist, most bursitis resolves within a few weeks. Now that you understand the individual structures, let's explore why this mobility creates vulnerability.

Why does shoulder mobility create vulnerability?

The shoulder's exceptional mobility sacrifices bone stability for range of motion. This instability places a huge responsibility on the muscles and tendons to keep the joint centered. When these structures weaken or get tired, pain occurs.

Compare your shoulder to your hip. Both are ball-and-socket joints. But the hip is deeply embedded. The hip socket almost completely surrounds the head of the femur (thigh bone). This configuration provides intrinsic bone stability.

Your shoulder works differently. The glenoid cavity (shoulder socket) covers only about one-third of the head of the humerus (upper arm bone). The bone alone is not enough to keep the joint stable. The rotator cuff muscles must constantly work to keep the head of the humerus centered.

This reliance on muscles creates a vulnerability. When the rotator cuff muscles weaken, get tired, or are injured, the head of the humerus can move upward. It gets closer to the acromion. This reduces the space available for the tendons and the bursa. This narrowing creates subacromial impingement.

Furthermore, the shoulder often works against gravity. When you lift your arm, you are lifting the weight of your entire upper limb. This constant load tires out the structures over time.

Aging also affects the shoulder. The rotator cuff tendons gradually lose their elasticity and ability to repair themselves. Small tears accumulate. Tendon degeneration develops. But remember: these structural changes don't always cause pain.

Understanding this vulnerability reveals why physiotherapy emphasizes strengthening the rotator cuff and scapular control. By improving muscle function, you compensate for natural bone instability. You reduce the risk of compression of vulnerable structures.

Key takeaways about shoulder anatomy?

Your shoulder is a marvel of biomechanical engineering. It sacrifices stability to offer you exceptional mobility. This mobility depends on the precise interaction between four joints, three bones, and numerous muscles and tendons.

The rotator cuff plays the central role in stabilization. The four rotator cuff muscles keep the head of the humerus centered while larger muscles like the deltoid generate power.

The subacromial space is a critical area. This is where the rotator cuff tendons and the bursa must slide under the acromion. When this space narrows (due to rotator cuff weakness, inflammation, or improper shoulder blade position), pain occurs.

Understanding this anatomy helps you understand your pain. When your physiotherapist explains that your supraspinatus tendon is irritated, you now know what they are talking about. When they prescribe rotator cuff strengthening exercises, you understand why.

This knowledge is empowering. Your shoulder is not mysterious or defective. It is a complex structure that works remarkably well most of the time. And when it doesn't work well, you can now understand why and what can help.

To learn more about specific shoulder problems and their treatments, consult our complete guide to shoulder pain. If you are currently experiencing shoulder pain, discover how physiotherapy can help you regain your mobility.