Shoulder Anatomy: Bones, Muscles, and Biomechanics

The shoulder is the most mobile joint in the human body, capable of orienting the upper limb in all directions of space. This extraordinary mobility, however, comes with a compromise: the shoulder sacrifices part of its stability to ensure a range of motion that no other joint can match. Understanding shoulder anatomy and its biomechanics is fundamental to correctly interpreting the most common pathologies — from subacromial impingement to rotator cuff tears — and to setting up effective prevention and rehabilitation programs.

This guide analyzes in detail the bones, joints, muscles, and biomechanics of the shoulder joint complex, using accessible yet scientifically rigorous language.

Table of Contents

• The Bones of the Shoulder
• The Four Joints of the Shoulder
• The Rotator Cuff: The Four Stabilizing Muscles
• Other Shoulder Muscles
• The Subacromial Space
• Shoulder Biomechanics
• The Vascularization of the Supraspinatus Tendon
• Functional Aspects and Kinetic Chains
• Frequently Asked Questions (FAQ)
• Conclusion
• Frequently Asked Questions
• Resources
• Sources and Scientific References

The Bones of the Shoulder

The shoulder bones—comprising the clavicle, scapula, and humerus—form the skeletal framework of the upper shoulder girdle and upper arm. The shoulder complex consists of three main bones which, articulating with each other and with the thorax, form a unique functional structure.

Humerus

The humerus is the long bone of the arm. Its proximal (upper) extremity features:

• Humeral head: a hemispherical surface covered with cartilage, which articulates with the glenoid of the scapula. Its diameter is about 3-4 times larger than the glenoid cavity, which explains the high mobility but also the relative instability of the joint.
• Anatomical neck: the groove that separates the humeral head from the tuberosity region.
• Greater tuberosity: a lateral bony prominence where three of the four rotator cuff muscles (supraspinatus, infraspinatus, teres minor) insert.
• Lesser tuberosity: an anterior bony prominence where the subscapularis muscle inserts.
• Bicipital groove: the furrow between the greater and lesser tuberosities through which the long head of the biceps tendon passes.

Scapula

The scapula is a flat, triangular bone, located on the posterior wall of the thorax between the 2nd and 7th ribs. Its main anatomical landmarks are:

• Glenoid cavity: the shallow concave surface that accommodates the humeral head. It covers only about 25-30% of the humeral head, which necessitates the glenoid labrum (a fibrocartilaginous ring that increases the depth of the cavity) and capsuloligamentous structures to maintain stability.
• Acromion: the bony process that projects laterally from the spine of the scapula, forming the “roof” of the shoulder. Its morphology (Type I flat, Type II curved, Type III hooked) influences the subacromial space and can predispose to subacromial impingement.
• Coracoid process: an anterior beak-shaped bony protuberance that serves as an insertion point for muscles (pectoralis minor, coracobrachialis, short head of the biceps) and ligaments.
• Spine of the scapula: a posterior bony crest that divides the scapula into the supraspinous fossa (superior) and infraspinous fossa (inferior), housing the eponymous rotator cuff muscles.

Clavicle

The clavicle is an “S”-shaped long bone that connects the sternum to the scapula, representing the only direct bony connection between the upper limb and the trunk. Its main functions are:

• To keep the shoulder away from the thorax, preserving space for arm movements.
• To transmit forces from the upper limb to the axial skeleton.
• To protect the brachial plexus and subclavian vessels that pass inferiorly.

The Four Joints of the Shoulder

Speaking of the “shoulder joint” is actually reductive. Shoulder movement is the result of the collaboration of four joints that work in synergy, forming the so-called shoulder joint complex.

1. Glenohumeral Joint

This is the “true” shoulder joint, a ball-and-socket joint between the head of the humerus and the glenoid cavity of the scapula. It is the joint with the greatest freedom of movement in the entire body, allowing movements in all three planes of space.

Stability is ensured by:

• Glenoid labrum: a fibrocartilaginous ring that deepens the glenoid cavity by 50% and increases the contact surface.
• Joint capsule: wide and loose, to allow movement.
• Glenohumeral ligaments (superior, middle, inferior): capsular reinforcements that limit excessive translations.
• Coracohumeral ligament: limits external rotation and inferior translation.
• Rotator cuff: the main dynamic stabilizer (described later).

2. Acromioclavicular Joint

Connects the lateral end of the clavicle to the acromion of the scapula. It is a plane joint that allows small gliding movements, essential for the rotation and tilting of the scapula during arm elevation.

The acromioclavicular ligament and the coracoclavicular ligaments (conoid and trapezoid) ensure its stability. Acromioclavicular joint separation is a frequent injury to this joint.

3. Sternoclavicular Joint

Unites the medial end of the clavicle to the manubrium of the sternum. It is a saddle joint with an interposed articular disc. It allows movements of elevation-depression, protraction-retraction, and rotation of the clavicle, amplifying the shoulder’s range of motion.

4. Scapulothoracic Joint

It is not a joint in the classic anatomical sense (lacking a joint capsule and cartilaginous surfaces), but it is a functional pseudo-joint between the anterior surface of the scapula and the posterior thoracic wall. Movement is ensured by the gliding of the scapula over the serratus anterior and subscapularis muscles.

The scapulothoracic contribution to overall shoulder movement is enormous: for every 2° of glenohumeral abduction, the scapula rotates approximately 1° (the so-called scapulohumeral rhythm 2:1). Out of 180° of total abduction, approximately 120° come from the glenohumeral joint and 60° from scapular rotation.

The Rotator Cuff: The Four Stabilizing Muscles

The rotator cuff is a group of four muscles whose tendons envelop the humeral head like a “cuff,” providing dynamic stability to the glenohumeral joint. It is the most important structure for shoulder function and, at the same time, one of the most vulnerable to injury and degeneration.

1. Supraspinatus

• Origin: supraspinous fossa of the scapula.
• Insertion: superior facet of the greater tuberosity.
• Function: it is the primary initiator of abduction (the first 15-20°). It stabilizes the humeral head in the glenoid during arm elevation, preventing its superior migration.
• Clinical relevance: it is the most frequently injured rotator cuff tendon, as it passes through the subacromial space where it is subject to compression and friction.

2. Infraspinatus

• Origin: infraspinous fossa of the scapula.
• Insertion: middle facet of the greater tuberosity.
• Function: primary external rotator of the shoulder (contributes 60% to external rotation strength). Stabilizes the humeral head posteriorly.
• Clinical relevance: its weakness alters joint kinematics and contributes to impingement.

3. Teres Minor

• Origin: lateral border of the scapula.
• Insertion: inferior facet of the greater tuberosity.
• Function: external rotator and inferior stabilizer of the humeral head. Works in synergy with the infraspinatus.
• Clinical relevance: it is rarely injured in isolation.

4. Subscapularis

• Origin: subscapular fossa (anterior surface of the scapula).
• Insertion: lesser tuberosity.
• Function: primary internal rotator of the shoulder and powerful anterior stabilizer of the humeral head.
• Clinical relevance: its injury is often underestimated and leads to anterior instability.

The Force Couple Mechanism

The rotator cuff does not act as a single muscle, but as a system of force couples. The external rotators (infraspinatus and teres minor) and the internal rotator (subscapularis) balance each other in the transverse plane, centering the humeral head in the glenoid. The supraspinatus and the inferior rotator cuff muscles counteract the superior translational force of the deltoid in the coronal plane. This balance of forces is essential for smooth, conflict-free movement.

Other Shoulder Muscles

In addition to the rotator cuff, numerous muscles contribute to shoulder movement and stability.

Deltoid

The deltoid is the largest muscle of the shoulder, divided into three portions:

• Anterior: flexion and internal rotation.
• Middle (acromial): abduction (from 15° to 90° it is the primary mover).
• Posterior: extension and external rotation.

The deltoid is a powerful mover, but without the rotator cuff centering the humeral head, its contraction would push the humerus upwards against the acromion, causing impingement.

Trapezius

The trapezius is a large superficial back muscle, fundamental for scapular movement:

• Upper trapezius: elevates the scapula and contributes to upward rotation.
• Middle trapezius: retracts the scapula (brings it closer to the spine).
• Lower trapezius: depresses the scapula and contributes to upward rotation.

An imbalance between the trapezius portions (typically hyperactivity of the upper and weakness of the middle and lower) is one of the most frequent causes of scapular dyskinesis and subacromial impingement.

Serratus Anterior

The serratus anterior is the muscle that keeps the scapula against the thoracic wall and rotates it upwards during arm elevation. Its weakness causes scapular winging and severely compromises shoulder function.

Pectoralis Major and Latissimus Dorsi

• The pectoralis major is a powerful flexor, adductor, and internal rotator of the humerus.
• The latissimus dorsi is the primary extensor, adductor, and internal rotator, fundamental in pulling movements.

Biceps Brachii

The long head of the biceps originates from the supraglenoid tubercle and crosses the glenohumeral joint before traversing the bicipital groove. It contributes to shoulder flexion and anterior stabilization of the humeral head. Long head of the biceps tendinitis is a common pathology.

The Subacromial Space

The subacromial space is the corridor between the acromion (superiorly) and the humeral head (inferiorly). It normally measures 10-15 mm and through it pass:

• The supraspinatus tendon
• The subacromial bursa (a synovial cushion that reduces friction)
• The long head of the biceps tendon

The reduction of this space — due to structural causes (hooked acromion, osteophytes), functional causes (scapular dyskinesis, rotator cuff weakness), or postural causes (thoracic kyphosis) — leads to subacromial impingement syndrome, one of the most widespread shoulder pathologies.

The coracoacromial arch, formed by the acromion, the coracoacromial ligament, and the coracoid process, constitutes the rigid roof under which impingement occurs.

Shoulder Biomechanics

Shoulder biomechanics are complex because they involve coordinated movements of all four joints of the complex. The main movements are described below.

Abduction (Lateral Arm Elevation)

Abduction moves the arm away from the body in the frontal plane, from 0° to 180°:

• 0°-15°: the supraspinatus is the primary mover, with assistance from the deltoid.
• 15°-90°: the deltoid (middle portion) becomes the primary mover, assisted by the supraspinatus.
• 60°-120°: painful arc in case of impingement, as the greater tuberosity and supraspinatus tendon pass through the restricted subacromial space.
• 90°-120°: external rotation of the humerus is necessary to rotate the greater tuberosity posteriorly and allow it to “pass” under the acromion.
• 90°-180°: the trapezius and serratus anterior rotate the scapula superiorly to complete the range. The glenohumeral:scapulothoracic ratio is 2:1 (Inman’s scapulohumeral rhythm).

Flexion (Anterior Arm Elevation)

Flexion moves the arm forward from 0° to 180°:

• 0°-60°: primary movers are the anterior deltoid and the clavicular head of the pectoralis major.
• 60°-120°: the trapezius and serratus anterior contribute with scapular rotation.
• 120°-180°: extension of the spine is necessary to complete the range. The last third of the movement involves the thoracic spine.

External and Internal Rotation

• External rotation (0°-90°): infraspinatus and teres minor. Fundamental for positioning the arm in overhead movements and for “freeing up” the subacromial space.
• Internal rotation (0°-70°): subscapularis, pectoralis major, latissimus dorsi. The gesture of bringing the hand behind the back combines extension, adduction, and internal rotation.

Stability vs. Mobility: The Shoulder’s Compromise

The shoulder is designed for mobility at the expense of bony stability. Unlike the hip, where the femoral head is deeply embedded in the acetabulum, the humeral head rests on a shallow glenoid — comparable to a golf ball on a tee.

Stability is ensured by a sophisticated multi-level system:

When one of these levels is compromised, the shoulder becomes vulnerable to instability, dislocations, and tendon injuries.

The Vascularization of the Supraspinatus Tendon

A fundamental aspect for understanding the vulnerability of the rotator cuff is the vascularization of the supraspinatus tendon. Approximately 1 cm from its insertion on the greater tuberosity, there is a hypovascular critical zone (described by Codman as the “critical zone”) where blood supply is reduced. This poor vascularization slows down healing processes and predisposes to tendon degeneration, explaining why most rotator cuff tears originate precisely in this area.

Functional Aspects and Kinetic Chains

The shoulder never works in isolation. Every daily gesture and every sports activity involves a kinetic chain that starts from the feet and is transmitted through the pelvis, spine, scapula, and finally the shoulder.

• An accentuated thoracic kyphosis limits scapular rotation and reduces the subacromial space.
• Thoracic spine stiffness compromises full arm elevation.
• Alterations of the pelvis and lower limbs have a chain reaction on shoulder mechanics, especially in overhead athletes.

For this reason, an effective rehabilitation program cannot be limited to the shoulder but must consider the entire kinetic chain.

Frequently Asked Questions (FAQ)

How many joints does the shoulder have?
The shoulder joint complex comprises four joints: glenohumeral, acromioclavicular, sternoclavicular, and scapulothoracic. All contribute to overall movement.

What are the rotator cuff muscles?
The rotator cuff is composed of four muscles: supraspinatus, infraspinatus, teres minor, and subscapularis. They act as dynamic stabilizers, centering the humeral head in the glenoid during all movements.

Why is the shoulder so prone to injuries?
The shoulder sacrifices bony stability to achieve maximum mobility. The humeral head is much larger than the glenoid cavity, and stability depends almost entirely on capsuloligamentous and muscular structures, which can weaken or tear.

What is the subacromial space and why is it important?
It is the space between the acromion and the humeral head, normally 10-15 mm. The supraspinatus tendon and the subacromial bursa pass through it. A reduction in this space causes subacromial impingement syndrome.

What is the scapulohumeral rhythm?
It is the ratio between the movement of the glenohumeral joint and the rotation of the scapula during arm elevation. The physiological ratio is 2:1: for every 2° of glenohumeral abduction, the scapula rotates 1°.

Why does the supraspinatus tendon tear so frequently?
The supraspinatus is vulnerable for three reasons: it passes through the restricted subacromial space where it is subject to compression, it has a hypovascular zone (“critical zone”) about 1 cm from its insertion, and it is continuously stressed as the initiator of abduction.

Conclusion

The anatomy of the shoulder represents an extraordinary example of biological engineering, where mobility and stability are balanced through a complex system of bones, joints, muscles, and ligaments. Understanding the functioning of this system — from the four joints to the rotator cuff’s force couple mechanism, from the scapulohumeral rhythm to the subacromial space — is the first step to effectively preventing and treating shoulder pathologies.

In case of pain, limited movement, or concerns about your shoulder health, it is advisable to consult your doctor or physical therapist.

Frequently Asked Questions

Why is the shoulder joint particularly susceptible to certain issues?

The shoulder joint is the most mobile in the human body, allowing for a vast range of motion. This exceptional mobility inherently compromises its stability, making it more susceptible to various issues compared to less mobile joints.

What are the key anatomical structures responsible for shoulder stability?

Essential stability of the shoulder joint is primarily provided by the shallow glenoid cavity, which is augmented by the labrum and surrounding ligaments. Additionally, the four muscles of the rotator cuff play a crucial role in dynamic stabilization during movement.

How does the shape of the acromion influence shoulder health?

The acromion, which forms the bony roof of the shoulder, has a shape that can significantly influence the risk of subacromial impingement. Variations in its morphology may reduce the space available for tendons, potentially leading to irritation or compression.

What is the importance of understanding shoulder anatomy for injury prevention and rehabilitation?

A comprehensive understanding of the shoulder’s complex anatomy and biomechanics is fundamental for effective injury prevention and rehabilitation strategies. This knowledge allows for accurate interpretation of pathologies and the development of targeted intervention programs.

For a broader overview of related conditions, see our shoulder pain guide.

Sources and Scientific References

1. Gallardo Vidal MI et al. (2022). [Physiotherapy and health education protocol in chronic musculoskeletal shoulder pain. Experience in Primary Care]. Aten Primaria. 54:102284. DOI | PubMed
2. Stokes DJ et al. (2023). Physical Therapy for the Treatment of Shoulder Instability. Phys Med Rehabil Clin N Am. 34:393-408. DOI | PubMed
3. Satpute K et al. (2022). Efficacy of mobilization with movement (MWM) for shoulder conditions: a systematic review and meta-analysis. J Man Manip Ther. 30:13-32. DOI | PubMed
4. Mostafaee N et al. (2022). Shoulder and scapula muscle training plus conventional physiotherapy versus conventional physiotherapy only: a randomized controlled trial of patients with lateral elbow tendinopathy. Physiother Theory Pract. 38:1153-1164. DOI | PubMed
5. Kara D et al. (2024). Blood Flow Restriction Training in Patients With Rotator Cuff Tendinopathy: A Randomized, Assessor-Blinded, Controlled Trial. Clin J Sport Med. 34:10-16. DOI | PubMed