Your shoulder is the most mobile joint in the human body. It allows you to raise your arm above your head, throw a ball, and reach behind your back. This exceptional mobility comes at a price: the shoulder sacrifices stability for 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 (head of the humerus) rests on a small, shallow surface (glenoid cavity). This structure sacrifices stability for exceptional range of motion.

Imagine a golf ball sitting 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 any direction: forward, sideways, backward, and in rotation. 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 well-fitting socket), the shoulder is inherently unstable. It relies heavily on muscles, tendons, and ligaments to hold 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 comprises four distinct joints: the glenohumeral (main joint), acromioclavicular, sternoclavicular, and scapulothoracic. These joints work together to create the full range of motion of the shoulder.

The glenohumeral joint

This is the main joint. The head of the humerus fits into the glenoid cavity of the shoulder blade. It is this joint that allows you to raise your arm and rotate it.

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

A joint capsule surrounds this joint. It is a pocket 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 transmitting 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 (bone in the center of the chest). It is the only bone connection between your arm and your torso. This joint moves when you raise your arm completely.

The scapulothoracic

The scapula slides over the rib cage. It is not a true joint with cartilage and a capsule, but this anatomical relationship is essential. Muscles control the position of the scapula on the thorax.

When the shoulder blade does not move properly (scapular dyskinesia), the entire range of motion of the shoulder is affected. This is why physical therapy for the shoulder often focuses on controlling the shoulder blade. Understanding these four joints reveals why shoulder pain can originate in several places. Now let's explore the bones that make up these joints.

What are the three main bones in the shoulder?

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

The humerus

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

The shoulder blade (scapula)

It 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 at 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

It is the horizontal bone that runs from the sternum (in the center of the chest) to the acromion. You can easily feel it under the skin. The clavicle acts as a spacer that keeps the arm away from the body.

These three bones create the basic structure. But it is the muscles and tendons that actually 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. These muscles keep the head of the humerus centered in the socket during movement. 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 the humerus. These four muscles work together to keep the ball (head of the humerus) properly positioned on the tee (glenoid cavity) while you move your arm.

The supraspinatus (or supraspinatus)

This muscle passes over the shoulder blade and attaches to the top of the humerus head. It initiates the sideways elevation of the arm. It is the most commonly injured tendon in the rotator cuff, as it passes through a narrow space under the acromion.

The infraspinatus (or subscapularis)

This muscle is located under the spine of the scapula (the bony ridge on the shoulder blade). It attaches to the back of the head of the humerus. It rotates the arm outward (external rotation).

The little circle

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

The subscapularis (or subscapular)

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

These four muscles are not very large. But their stabilizing role is crucial. Without them, large muscles such as the deltoid would pull the head of the humerus upward, causing it to catch under the acromion.

Rotator cuff tendinopathy accounts for two out of three cases of shoulder pain. Understanding these muscles helps you understand why physical therapy emphasizes gradual strengthening of the cuff. Now let's explore the other important muscles of the shoulder.

What other muscles control shoulder movements?

Other key muscles include the deltoid (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 stabilizes.

The deltoid muscle

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

The deltoid generates the power to lift the arm. But without the stabilizing rotator cuff, the deltoid would simply pull the humerus upward instead of creating a fluid motion.

The trapeze

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

The trapezius muscle 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 muscle

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 pull the shoulders forward.

The latissimus dorsi

This large back muscle pulls the arm toward the body and backward. It also contributes to internal rotation.

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

What is a bursitis and why does it become inflamed?

A bursa is a small sac filled with fluid 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 becomes inflamed (bursitis), it swells and causes pain when raising 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 raise your arm, the space narrows even further. The bursa allows the tendons to slide under the acromion without excessive friction.

When the bursa becomes irritated and inflamed, it swells. This swelling further reduces the already narrow space. Movement becomes painful. Nighttime pain is common, as swelling increases when you lie down.

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

The good news is that bursitis generally responds well to conservative treatment. With relative rest, ice, and appropriate exercises guided by a physical therapist, most cases of bursitis resolve 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 in favor of range of motion. This instability places a huge responsibility on the muscles and tendons to keep the joint centered. When these structures weaken or become fatigued, pain occurs.

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

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

This dependence on muscles creates vulnerability. When the rotator cuff muscles weaken, tire, or become injured, the head of the humerus can migrate upward. It moves closer to the acromion. This reduces the space available for the tendons and bursa. This proximity creates subacromial impingement.

In addition, the shoulder often works against gravity. When you raise your arm, you lift the weight of your entire upper limb. This constant load tires the structures over time.

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

Understanding this vulnerability reveals why physical therapy 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.

What should you remember about the anatomy of the shoulder?

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

The rotator cuff plays a central role in stabilization. The four muscles of the cuff keep the head of the humerus centered while large muscles such as the deltoid generate power.

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

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

This knowledge is liberating. 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, see our comprehensive guide to shoulder pain. If you are currently experiencing shoulder pain, find out how physical therapy can help you regain your mobility.