Functional Anatomy of the Shoulder

Original Editor - Ewa Jaraczewska

Top Contributors - Ewa Jaraczewska, Jess Bell, Wanda van Niekerk and Matt Huey  

Introduction

The shoulder is a complex anatomical structure consisting of four joints. Movements at the shoulder enable us to move and position our arm and hand in space. The muscles of the shoulder provide a stable base for these upper extremity movements. The dynamic relationship between the shoulder muscles, ligaments and bony articulations allows for significant movement, but this is at the expense of stability. The shoulder is, therefore, susceptible to dysfunction and instability.

Common shoulder injuries include sprains, strains, dislocation or subluxation and other degenerative conditions. It is essential to understand the anatomical features of the shoulder when assessing and treating these conditions. This article discusses the key anatomical structures of the shoulder complex, including the bony structures, articulations, ligaments, muscles, nerves and the vascular supply.

Key Terms

Axes: lines around which an object rotates. The rotation axis is a line that passes through the centre of mass. There are three axes of rotation: sagittal passing from posterior to anterior, frontal passing from left to right and vertical passing from inferior to superior. The rotation axes of the foot joints are perpendicular to the cardinal planes. Therefore, motion at these joints results in rotations within three planes. Example: supination involves inversion, internal rotation, and plantarflexion.

Bursae: reduces friction between the moving parts of the body joints. It is a fluid-filled sac. There are four types of bursae: adventitious, subcutaneous, synovial, and sub-muscular.

Capsule: one of the characteristics of the synovial joints. It is a fibrous connective tissue which forms a band that seals the joint space, provides passive and active stability and may even form articular surfaces for the joint. The capsular pattern is "the proportional motion restriction in range of motion during passive exercises due to tightness of the joint capsule."

Closed pack position: the position with the most congruency of the joint surfaces. In this position, joint stability increases. The closed pack position for interphalangeal joints is full extension.

Degrees of freedom: the direction of joint movement or rotation; there is a maximum of six degrees of freedom, including three translations and three rotations.

Ligament: fibrous connective tissue that holds the bones together.

Open (loose) pack position: position with the least joint congruency where joint stability is reduced.

Planes of movement: describe how the body moves. Up and down movements (flexion/extension) occur in the sagittal plane. Sideway movements (abduction/adduction) occur in the frontal plane. The transverse plane movements are rotational (internal and external rotation).

Shoulder Structure

The shoulder complex has four bones: the clavicle, sternum, scapula and proximal end of the humerus.

The clavicle is situated anteriorly. It connects to the acromion, part of the scapula, and the sternum (which is part of the thoracic spine complex). The clavicle is a sigmoid-shaped long bone which lies horizontally. It provides a connection point between the axial skeleton and the shoulder girdle and, therefore, is able to transfer weight from the upper limbs to the axial skeleton.

The sternum completes the anterior chest wall. It is a flat, cancellous bone with a thin layer of cortical bone. The sternum is slightly convex anteriorly, and it has multiple indentations along its lateral borders called costal notches. It consists of three parts:

1. Manubrium:
   • flat
   • has four-sides
   • widens superiorly
2. Body:
   • flat, rectangular bone
   • has facets on its lateral border for rib articulations
3. Xiphoid process:
   • a thin bone
   • projects inferiorly

The scapula is a thin, flat, triangular bone that sits on the postero-lateral aspect of the thoracic cage.

• It has two surfaces:
  • Anterior surface (or the costal surface): the shallow subscapular fossa is located on the anterior surface of the scapula
  • Posterior surface: divided into the supraspinous fossa and infraspinous fossa by the spine of the scapula
• It has three borders:
  • superior: extends from the superior angle of the scapula to the coracoid process
  • medial: runs parallel to the spine
  • lateral: runs superolaterally towards the apex of the axilla, with the glenoid fossa located superiorly
• It has three angles:
  • superior: covered by trapezius
  • inferior: covered by latissimus dorsi
  • lateral: location of the glenoid fossa
• It has three processes:
  • acromion
  • spine
  • coracoid process

The head of the humerus is at the proximal end of the humerus:

• The head of the humerus is an irregular hemisphere
• It articulates with the glenoid fossa of the scapula
• The anatomical neck of the humerus is the point between the head and the tuberosities
• The surgical neck is between the tuberosities and the shaft
• The greater tuberosity is lateral to the head at the proximal end
• The lesser tuberosity is located inferior to the head, on the anterior part of the humerus - the lesser tuberosity is very prominent and easily palpated
• The intertubercular groove (bicipital groove or sulcus) is located between the two tuberosities
  • Three muscles attach in the intertubercular grove. From medial to lateral, they are:
    • teres major
    • latissimus dorsi
    • pectoralis major
  • The long head of biceps tendon is also contained in the intertubercular groove

Bones, Articulations and Kinematics of the Shoulder

The shoulder complex has many soft tissues, including joint capsules, the labrum, ligaments, bursae, tendons, and muscles. Because of its mobility, shoulder stability depends on a coordinated effort between the static tissues (i.e. non-contractile tissues, such as ligaments) and dynamic tissues (i.e. contractile tissues, such as tendons and muscles).

Bones and Articulations

Bones	Articulations	Characteristics	Key palpation points
Scapula, Ribs	Scapulothoracic joint	The scapulothoracic joint is not a true anatomic joint. Instead, it is the articulation between the scapula and the thorax. One of its roles is to maintain the centralised position of the humeral head in the glenoid fossa during arm elevation. The resting position of the scapula is as follows: approximately in the middle of upward-downward rotation, 35° internal rotation, and 10° anterior tilt.	To palpate the scapula, position the patient in prone. If the patient can tolerate it, position the arm on the tested side at the small of the patient's back (i.e. in internal rotation). This will help you to visualise the borders of the scapula. With one hand, cup the shoulder anteriorly to lift the humerus. With this passive retraction manoeuvre, you can clearly identify the borders of the scapula. Please note that the lateral border is difficult to palpate, as it is covered with muscles. To palpate the ribs posteriorly, position the patient in sitting or standing with their back towards you, trunk flexed, and scapula protracted. In order to palpate the first rib, locate the long spinous process of the C7 vertebra. The first rib is about two centimetres lateral and slightly inferior to the C7 spinous process. It can be difficult to palpate the first rib as it is covered by trapezius and levator scapulae. Use the same method to palpate the second rib located between T1 and T2.T2 can be found at the level of superior angle of scapula. Ribs three and four are located under the medial end of the spine of the scapula. Rib seven is located under the inferior angle of the scapula. The costal angle of rib nine is at the level of the spinous process of L1.
Sternum, Clavicle	Sternoclavicular joint (SC)	The SC joint is the articulation between the sternal end of the clavicle, the manubrium of the sternum, and the first costal cartilage. It contains an articular disc, which separates the joint into two compartments.	The sternum is located on the anterior wall of the thorax. It is in the middle of the rib cage. The broadest part of the manubrium articulates with the costal cartilage of the first rib. The xiphoid process is located at the inferior end of the sternum, which is at the level of the seventh costal cartilage. You can palpate the xiphoid process by finding the small, pointed projection at the end of the sternum. The xiphoid process articulates directly with the costal cartilage of ribs seven and indirectly with the cartilage of ribs eight, nine, and ten. When palpating the clavicle, start at the centre of the clavicle, as it is easy to find. Gently hold the posterior and anterior aspects of the clavicle, and move your fingers towards the medial end of the clavicle where the clavicle meets the manubrium of the sternum. This is where the sternoclavicular joint is located. You can confirm you are on the joint by passively moving the clavicle up and down, which creates a small motion at the joint.
Acromion of the scapula, Clavicle	Acromioclavicular joint (AC)	The AC joint is the articulation between the lateral end of the clavicle and the acromion of the scapula. It moves with the glenohumeral joint, enabling functional movement at the shoulder girdle.	To palpate the acromion of the scapula, position the patient in sitting. First, locate the spine of the scapula. Place your finger on the upper and the lower portion of the spine of the scapula and move it laterally. Pass the thickening portion of the spine called the tubercle and continue laterally to find a sharp edge. This is the acromial angle and the beginning of the acromion process, which turns upwards towards the front of the shoulder. From there, you can palpate the posterior, lateral, and anterior parts of the acromion process. The acromioclavicular joint (AC) is palpated at the lateral end of the clavicle. The joint line may be difficult to palpate as it is covered by trapezius. Place your finger on the AC joint to feel the joint line and passively move the clavicle up and down.
Humerus, Glenoid fossa of the scapula	Glenohumeral joint	The glenohumeral joint is made up of the glenoid fossa of the scapula and the head of the humerus. It has the largest range of motion of all the joints in the body, relying on its surrounding muscles and connective tissue structures for stability.	To palpate the glenohumeral joint, position the patient in supine. First, locate the coracoid process. To do this, slightly abduct the patient's arm. Ask the patient to flex their arm to activate the anterior deltoid. You will then be able to palpate the medial border of the anterior deltoid. Move your finger laterally and feel for a prominence, shaped like a knuckle, under the skin. It is the coracoid process of the scapula. The glenohumeral joint is palpated approximately 1 cm inferior to the coracoid process.

Shoulder Kinematics

Shoulder range of motion and strength can be affected by various factors.

• A study by Pike et al. found that shoulder range of motion (ROM) and strength significantly decrease as age increases, especially in the over-65 age group.
• Increased body mass index (BMI) can also have a negative effect on shoulder flexibility.

• The position of the thoracic spine must be considered when assessing shoulder range of motion as it significantly affects scapular kinematics during scapular plane abduction. A slouched posture is associated with decreased shoulder range of motion and muscle force.

• Other factors influencing shoulder range of motion include biological sex, comorbidities, and hand dominance:
  • active shoulder flexion and abduction are greater in males
  • active external rotation is greater in females
  • mean range of motion in all planes is lower among people with diabetes
  • mean range for all assessed movements except external rotation of the right shoulder is lower for those who are right-hand dominant compared to those who are left-hand dominant

The table below indicates values of the active range of motion in the shoulder joints:

Joint	Type of joint	Plane of movement	Motion	Kinematics	Closed pack position	Open pack position
Scapulothoracic joint	Not a true joint	Frontal Transverse Sagittal	Joint kinematics at the scapulothoracic joint include translation and rotation of the scapula on the ribcage. Scapular movements include elevation, depression, adduction, and abduction. The scapula also rotates through upward and downward rotation, internal and external rotation, and anterior and posterior tilt.	Scapular elevation: 40 degrees. Scapular depression: 10 degrees. Scapular protraction: 20 degrees. Scapular retraction: 15 degrees. Scapula rotation: Upwards rotation : 60 degrees. Downwards rotation : 30 degrees. Internal rotation: 30-45 degrees. External rotation: 10-20 degrees. Anterior tilt: approximately 10 degrees. Posterior tilt: approximately 30 degrees.	Because it is not a true joint, the scapulothoracic joint does not have a close-packed position.	Arms resting by the side of the body.
Sternoclavicular joint	Synovial- saddle-type	Sagittal Frontal Transverse	Anterior/posterior axial rotation, Elevation/depression, Protraction /retraction	Anterior rotation: 40-50 degrees. Posterior rotation: 40-50 degrees. Elevation: 45 degrees. Depression: 10 degrees. Combined protraction and retraction: 35 degrees.	Clavicle in maximal rotation, which is produced by maximal arm elevation and full scapular rotation.	Arms resting by the side of the body.
Acromioclavicular joint	Synovial-plane type		Acromioclavicular joint range of motion is difficult to measure as it combines the clavicle and glenohumeral joint. Maximum rotation range of motion is achieved when the upper limb is resting in a horizontal position.	Axial rotation: 25 degrees	90 degrees of shoulder abduction.	Arms resting by the side of the body.
Glenohumeral joint	Ball and socket	Sagittal Frontal Transverse	Shoulder flexion/extension, Shoulder abduction/adduction, Shoulder internal rotation/external rotation, Shoulder circumduction: a combination of flexion, abduction, extension, and adduction.	Flexion/extension: 150-180 degrees/45 degrees. Abduction/adduction: 160/30 degrees. External/internal rotation: 85/85 degrees.	Abduction and external rotation.	40-50 degrees of abduction and 30 degress horizontal adduction.

Shoulder Passive Range of Motion Assessment

There are no set guidelines on selecting a testing position (e.g. sitting, supine, or standing) when assessing passive range of motion at the shoulder. The therapist must, therefore, consider various factors when choosing a testing position for the passive range of motion assessment. For example:

• if the therapist is shorter than the patient, it may be difficult to assess the patient in standing
• if a patient has multiple disabilities, they may not be able to assume certain positions

It is recommended that the clinician document the testing position and use the same position when re-assessing passive range of motion.

Shoulder flexion

• The patient will ideally be in a sitting position to avoid compression on the scapula when supine
• The patient's arm is positioned with the palm facing inwards, and the forearm in the mid-position
• The therapist grasps the distal humerus, close to the elbow
• The therapist moves the patient's humerus in an anterior and upward direction to the limit of shoulder flexion
• If possible, the elbow should remain in extension to minimise the risk of passive insufficiency of the 2-joint triceps muscle affecting results
• In the absence of pathology, the end feel will be firm
• Normal range: pure glenohumeral flexion: 80-90 degrees, but with the clavicle and scapula moving: 180 degrees

Shoulder extension

• The patient can be in sitting, standing or side-lying
• The patient's arm is positioned with the palm facing inwards, and the forearm in the mid-position
• The therapist grasps the patient’s humerus as distally as possible
• The therapist moves the patient's arm into extension
• In the absence of pathology, the end feel will be firm
• Normal range: 60 degrees

Shoulder abduction

• The patient can be in sitting, standing or supine
• The patient's arm is positioned with the palm facing inwards, and the forearm in the mid-position
• The therapist grasps the patient’s humerus as distally as possible
• The therapist moves the patient's arm into abduction
• In the absence of pathology, the end feel will be firm
• Normal range: 90 degrees of pure glenohumeral abduction. Further motion is expected if the clavicle and scapula are not fixated

Shoulder adduction

• The patient can be in sitting, standing or supine
• The patient's arm is positioned with the palm facing inwards, and the forearm in the mid-position
• The therapist grasps the patient’s humerus as distally as possible
• The therapist moves the patient's arm into adduction
• In the absence of pathology, the end feel will be firm
• Normal range: 30 degrees

Shoulder external rotation

• The patient can be sitting, standing, supine or side-lying
• The therapist stabilises the patient's scapula with one hand and brings the patient's arm into 45 degrees of glenohumeral abduction. The therapist's other hand holds the patient's arm at the elbow, keeping it at 0 degrees of rotation
• The therapist externally rotates the patient's arm while observing any scapular retraction
• The movement is stopped when a firm end feel is reached or when the scapula starts to displace
• Normal range: approximately 60 degrees

Shoulder internal rotation

• The patient can be sitting, standing, side-lying
• The therapist stabilises the patient's scapula with one hand and brings the patient's arm into 45 degrees of glenohumeral abduction. The therapist's other hand holds the patient's arm at the elbow, keeping it at 0 degrees of rotation
• Next, the therapist brings the patient's hand behind the patient's back and lifts it off the thorax
• In the absence of pathology, the end feel will be firm
• Normal range: around 100 degrees from the anatomic position
• An alternative testing position for internal and external glenohumeral joint rotation is prone with the shoulder at 90 degrees of abduction and the elbow at 90 degrees of flexion. The forearm hangs over the edge of the table. The therapist stabilises the distal part of the shoulder with one hand and moves the patient's forearm towards their head for external rotation or towards their hip for internal rotation.

Shoulder Bursae

The bursae of the shoulder allow for functional movement in the shoulder joint. However, they are also a leading cause of shoulder pathology and dysfunction.

The shoulder has the following bursae:

• Subscapular bursa or the scapulothoracic bursa: between the tendon of the subscapularis muscle and the shoulder joint capsule.
• Subdeltoid bursa: between the deltoid muscle and the shoulder joint cavity.
• Subacromial bursa: below the acromion process and above the greater tubercle of the humerus. This bursa is considered a primary source of shoulder pain, and it is a well-vascularised structure.
• Subcoracoid bursa: between the coracoid process of the scapula and the shoulder joint capsule.
• Infraspinatus bursa: between the infraspinatus tendon and the joint capsule.
• Subcutaneous acromial bursa: above the acromion, beneath the skin.

The subacromial and the subdeltoid bursae are often considered a single bursa, referred to as the subacromial deltoid bursa.

Glenoid Labrum

The glenoid labrum is a fibrocartilaginous, ridge-like connective tissue that increases the articular surface area for the humeral head by deepening the glenoid fossa. It is also the primary attachment for the glenohumeral ligaments.

Shoulder Joint Capsule

The glenohumeral joint capsule is an important passive stabiliser of the shoulder joint. The glenohumeral joint capsule is thickened anteriorly, and it is twice the size of the humeral head. It provides most of the shoulder's extensibility anteriorly and inferiorly, and it “winds up” during abduction and external rotation. The joint capsule and glenohumeral ligaments function as stabilisers at the extremes of motion.

Ligaments of the Shoulder

Scapulothoracic Joint Ligaments

The scapulothoracic joint has no ligaments as it is not a true joint. The attaching muscles and structures of the sternoclavicular and acromioclavicular joints support it.

Sternoclavicular Joint Ligaments

Key ligaments	Origin	Insertion	Action/role	Key palpation points
Anterior sternoclavicular ligament	Anterosuperior surface of the sternal end of the clavicle	Anterosuperior surface of the manubrium and the adjacent part of the first costal cartilage	Reinforces the anterior aspect of the joint	Position the patient in supine. Find the centre of the clavicle. Palpate the posterior and anterior aspects of the clavicle and move your fingers towards the medial end of the clavicle, where the clavicle meets the manubrium of the sternum.
Posterior sternoclavicular ligament	Posterior aspect of the sternal end of the clavicle	Posterosuperior manubrium	Posterior reinforcement of the joint capsule
Interclavicular ligament	Sternal end of the left clavicle	Sternal end of the right clavicle	Superior reinforcement of the joint capsule. Resists excessive depression or downward glide of the clavicle
Costoclavicular ligament	First rib and costal cartilage	Inferior surface of the clavicle	Limits elevation of the pectoral girdle. Acts as a fulcrum for elevation-depression and protraction-retraction. Limits the clavicular elevation and superior glide of the clavicle.	Position the patient in supine with their scapula depressed. To palpate the first rib, find the clavicle. Place your finger on the lateral end of the clavicle and roll behind it. Next, with the finger, push the belly of the trapezius muscle back and apply downward pressure to feel a bony prominence. This is the first rib. To confirm you are on the first rib, ask the patient to take a deep breath. The first rib should elevate during inhalation.

Acromioclavicular Joint Ligaments

Key ligaments	Origin	Insertion	Action/role	Key palpation points
Coracoclavicular ligament: Conoid ligament (CL) Trapezoid ligament (TL)	CL: coracoid process of the scapula TL: coracoid process of the scapula	CL: conoid tubercle of the clavicle TL: trapezoid line of the clavicle	Prevents superior dislocation of the scapula. Resists scapular rotation. Supports the weight of the upper limb suspended from the clavicle.	To palpate the coracoid process of the scapula, position the patient in a supine and slightly abduct the patient's arm. Ask the patient to flex their arm to activate the anterior deltoid. Palpate the medial border of the anterior deltoid. Move your finger laterally and feel for a bony prominence, shaped like a knuckle, under the skin. This is the coracoid process of the scapula. The conoid tubercle and the trapezoid line cannot be palpated as they are located on the underside of the clavicle.
Acromioclavicular ligament	Superior surface of the acromion	Acromial end of the clavicle	Resists posterior translation at the joint. Resists posterior axial rotation of the joint.	Find the coracoid process of the scapula. The acromioclavicular joint is more lateral and superior to the coracoid process.

Glenohumeral Joint Ligaments

Key ligaments	Origin	Insertion	Action/role	Key palpation points
Coracoacromial ligament	Summit of the acromion of the scapula	Lateral border of the coracoid process of the scapula	Stabilises the head of the humerus. Limits inferior translation and excessive external rotation of the humerus.
Transverse humeral ligament	Lesser tubercle of the humerus	Greater tubercle of the humerus	Holds the tendon of the long head of the biceps muscle in place.	To palpate the lesser tubercle of the humerus, start by finding the anterolateral portion of the acromion. Just below, you will find the greater tubercle of the head of the humerus. Next, place your finger on the anterior surface of the head of the humerus and passively move the patient’s arm into lateral rotation. You will feel the bicipital groove under your palpating finger as the patient's arm moves into lateral rotation. As you move the patient's arm further into lateral rotation, you will be able to palpate the lesser tubercle.
Superior glenohumeral ligament	Glenoid labrum	Lesser tubercle	Prevents posterior and inferior translation of the humeral head.
Middle glenohumeral ligament	Inferior to the superior ligament along the anterior margin of the glenoid	Anterior surface of the lesser tubercle	Provides anterior stability when the shoulder is abducted to between 45 and 60 degrees.
Inferior glenohumeral ligament	Glenoid labrum	Beyond the lesser tubercle	Prevents extremes of motion. Stabilises against anteroinferior dislocation.

Muscles of the Shoulder

The dynamic stabilisers of the shoulder complex include the rotator cuff muscles, the deltoid, and the scapular muscles, which control scapulohumeral rhythm. For optimal shoulder stabilisation, the dynamic stabilisers must function efficiently and synergistically. Dynamic stabilisation enables significant mobility at the shoulder and provides adequate stability when the shoulder complex is functioning normally. The rotator cuff muscles provide stability at the glenohumeral joint, primarily through the compression of the head of the humerus against the glenoid during movement. The rotator cuff muscles are supraspinatus, infraspinatus, teres minor, and subscapularis.

The following tables include muscles that act on the scapula and the humerus. Other functions of these muscles not related to the scapula and shoulder motion are not included in these tables (e.g. when the scapula is fixed, levator scapulae assists with the lateral flexion of the cervical spine).

Scapular Elevation

Muscle	Origin	Insertion	Innervation	Action
Levator scapulae	Transverse processes of C1 to C4	Medial border of the scapula	Dorsal scapular nerve C5 Cervical nerves C3-C4	Elevates the scapula. Tilts the glenoid cavity inferiorly by rotating the scapula downward.
Upper trapezius	Occipital bone and the nuchal ligament	Lateral third of the clavicle	Accessory nerve (CN XI) Spinal nerves C3-C4	Produces scapular elevation.

Scapular Depression

Muscle	Origin	Insertion	Innervation	Action
Pectoralis minor	Ribs three to five	Medial border and coracoid process of the scapula	Medial pectoral nerve C8-T1	The primary actions of pectoralis minor include stabilisation, depression, abduction or protraction, internal rotation and downward rotation of the scapula.
Lower trapezius	The spinous processes of T4 to T12	Lateral third of the clavicle, acromion, and the spine of the scapula	Accessory nerve (CN XI) Spinal nerves C3-C4	Shoulder depression and scapula upward rotation.

Scapular Protraction

Muscle	Origin	Insertion	Innervation	Action
Serratus anterior	Ribs one to nine	Costal surface of the medial scapula	Long thoracic nerve C5-C6-C7	The main actions of serratus anterior are protraction and upward rotation of the scapulothoracic joint. It is also a key scapular stabiliser, keeping the scapula against the ribcage at rest and during movement. Known as the "Boxer's muscle", as it protracts the scapula during a punch.
Pectoralis minor	Ribs three to five	Medial border and coracoid process of the scapula	Medial pectoral nerve C8-T1	The primary actions of pectoralis minor include stabilisation, depression, abduction or protraction, internal rotation and downward rotation of the scapula.

Scapular Retraction

Muscle	Origin	Insertion	Innervation	Action
Rhomboids: Rhomboid Major Rhomboid Minor	Rhomboid Major: the spinous processes of T2 to T5 Rhomboid Minor: the spinous processes of C7 to T1	Rhomboid Major and Minor: Medial border of the scapula	Dorsal scapular nerve C4-C5	Retract, elevate and rotate the scapula. Protract the medial border of the scapula, keeping it in position at the posterior thoracic wall.
Middle Trapezius	The spinous processes of T1-T4 (other sources indicate C7-T3)	Lateral third of the clavicle, as well as the acromion and the spine of the scapula.	Accessory nerve (CN XI), Spinal nerves C3-C4	Scapula retraction.

Scapular Upward Rotation (Lateral Rotation)

Muscle	Origin	Insertion	Innervation	Action
Serratus anterior	Ribs one to nine	Costal surface of the medial scapula	Long thoracic nerve C5-C6-C7	The main actions of serratus anterior are protraction and upward rotation of the scapulothoracic joint. It is also a key scapular stabiliser, keeping the scapula against the ribcage at rest and during movement. Known as the "Boxer's muscle", as it protracts the scapula during a punch.
Upper trapezius	Occipital bone and the nuchal ligament	Lateral third of the clavicle	Accessory nerve (CN XI) Spinal nerves C3-C4	The upper and lower fibres of trapezius work concurrently to produce rotation of the scapula.
Lower trapezius	The spinous processes of the T4 to T12.	Lateral third of the clavicle, acromion, and the spine of the scapula	Accessory nerve (CN XI) Spinal nerves C3-C4	Shoulder depression and scapula upward rotation

Scapular Downward Rotation (Middle Rotation)

Muscle	Origin	Insertion	Innervation	Action
Rhomboids: Rhomboid Major Rhomboid Minor	Rhomboid Major: the spinous processes of T2 to T5 Rhomboid Minor: the spinous processes of C7 to T1	Rhomboid Major and Minor: Medial border of the scapula.	Dorsal scapular nerve C4-C5	Retract, elevate and rotate the scapula. Protract the medial border of the scapula, keeping it in position at the posterior thoracic wall.
Levator scapulae	Transverse processes of C1 to C4	Medial border of the scapula	Dorsal scapular nerve C5, Cervical nerves C3-C4	Elevates the scapula Tilts the glenoid cavity inferiorly by rotating the scapula downward.
Pectoralis minor	Ribs three to five	Medial border and coracoid process of the scapula	Medial pectoral nerve C8-T1	The primary actions of pectoralis minor include stabilisation, depression, abduction or protraction, internal rotation and downward rotation of the scapula.

Shoulder Flexion

Muscle	Origin	Insertion	Innervation	Action
Coracobrachialis	Coracoid process of the scapula	Medial surface of the humeral shaft	Musculocutaneous nerve	Assists with shoulder flexion and adduction.
Biceps brachii: Short head (SH) Long head (LH)	SH: coracoid process of the scapula LH: supraglenoid tubercle of the scapula	SH/LH: radial tuberosity		Flexes the shoulder. Helps to stabilise the head of the humerus in the glenoid cavity.
Deltoid: Anterior deltoid (AD) Lateral deltoid (LD) Posterior deltoid (PD)	AD: lateral third of the clavicleLD: acromion of the scapula PD: spine of the scapula	AD/LD/PD: deltoid tuberosity on the humerus	Axillary nerve	Only the anterior deltoid contributes to shoulder flexion.
Pectoralis major: Clavicular head (CH) Sternocostal head (SCH)	CH: anterior surface of the medial half of the clavicle SCH: anterior surface of the sternum, superior six costal cartilages, and aponeurosis of external oblique muscle	CH/SCH: Lateral lip of intertubercular groove of the humerus	Lateral and medial pectoral nerves	When acting alone, the clavicular head flexes the shoulder.

Shoulder Extension

Muscle	Origin	Insertion	Innervation	Action
Latissimus dorsi	Spinous processes of T7 to T12 Iliac crest Ribs nine to twelve	Intertubercular groove in the front of the humerus	Thoracodorsal nerve	Shoulder extension, adduction, and internal rotation
Teres major	Dorsal surface of the inferior angle of the scapula	Medial lip of intertubercular groove of the humerus	Lower subscapular nerve	Assists with shoulder extension. Internally rotates the shoulder. Contributes to the stabilisation of the shoulder joint.
Triceps brachii: Long head (LH) Medial head (MH) Lateral head (LLH)	LH: infraglenoid tubercle of the scapula MH: posterior surface of the humerus, inferior to the radial groove LLH: posterior surface of the humerus	LH/MH/LLH: olecranon of the ulna	Radial nerve	Only the long head is involved in the shoulder extension. Stabilises the humeral head during shoulder abduction. Extends the elbow.
Deltoid: Anterior deltoid (AD) Lateral deltoid (LD) Posterior deltoid (PD)	AD: lateral third of the clavicle LD: acromion of the scapula PD: spine of the scapula	AD/LD/PD: deltoid tuberosity on the humerus	Axillary nerve	The posterior deltoid helps with shoulder extension.

Shoulder Abduction

Muscle	Origin	Insertion	Innervation	Action
Supraspinatus	Supraspinous fossa in the scapula	Greater tubercle of the humerus	Suprascapular nerve	Initiate abduction up to 15 degrees, after which the lateral deltoid muscle takes over.
Deltoid: Anterior deltoid (AD) Lateral deltoid (LD) Posterior deltoid (PD)	AD: lateral third of the clavicle LD: acromion of the scapula PD: spine of the scapula	AD/LD/PD: deltoid tuberosity on the humerus	Axillary nerve	The lateral deltoid abducts the shoulder.

Shoulder Adduction

Muscle	Origin	Insertion	Innervation	Action
Subscapularis	Subscapular fossa of the scapula	Lesser tuberosity of humerus	Upper and lower subscapular nerves	Internal rotation and adduction. Assists in holding the humeral head in the glenoid cavity of the scapula.
Teres minor	Superior part of the lateral border of the scapula	Inferior facet on the greater tuberosity of humerus	Axillary nerve	Shoulder internal rotation. Assists in holding the humeral head in the glenoid cavity of the scapula.
Teres major	Dorsal surface of the inferior angle of the scapula	Medial lip of intertubercular groove of the humerus	Lower subscapular nerve	Assists with shoulder extension. Acts as a functional unit with latissimus dorsi to internally rotate, adduct and extend the shoulder. Contributes to the stabilisation of the shoulder joint.
Latissimus dorsi	Spinous processes of T7 to T12 Iliac crest Ribs nine to twelve	Intertubercular groove in the front of the humerus	Thoracodorsal nerve	Shoulder extension, adduction, and internal rotation.
Pectoralis major: Clavicular head (CH) Sternocostal head(SCH)	CH: anterior surface of the medial half of clavicle SCH: anterior surface of sternum, superior six costal cartilages, and aponeurosis of the external oblique muscle	CH/SCH: Lateral lip of intertubercular groove of the humerus	Lateral and medial pectoral nerves	Shoulder adduction and internal rotation. Draws the scapula anteriorly and inferiorly.

External Rotation

Muscle	Origin	Insertion	Innervation	Action
Infraspinatus	Infraspinous fossa on the scapula	Middle facet on the greater tuberosity of humerus	Suprascapular nerve	Shoulder external rotation. Assists in holding the humeral head in the glenoid cavity of the scapula.

Internal Rotation

Muscle	Origin	Insertion	Innervation	Action
Subscapularis	Subscapular fossa of scapula	Lesser tuberosity of humerus	Upper and lower subscapular nerves	Internal rotation and adduction. Assists in holding the humeral head in the glenoid cavity of the scapula.
Teres major	Dorsal surface of the inferior angle of the scapula	Medial lip of intertubercular groove of the humerus	Lower subscapular nerve	Assists with shoulder extension. Acts as a functional unit with the latissimus dorsi to internally rotate, adduct and extend the shoulder. Contributes to the stabilisation of the shoulder joint.
Latissimus dorsi	Spinous processes of T7 to T12 Iliac crest Ribs nine to twelve	Intertubercular groove in the front of the humerus	Thoracodorsal nerve	Shoulder extension, adduction, and internal rotation.
Pectoralis major: Clavicular head (CH) Sternocostal head(SCH)	CH: anterior surface of the medial half of clavicle SCH: anterior surface of sternum, superior six costal cartilages, and aponeurosis of external oblique muscle	CH/SCH: Lateral lip of intertubercular groove of the humerus	Lateral and medial pectoral nerves	Shoulder adduction and internal rotation. Draws the scapula anteriorly and inferiorly.
Deltoid: Anterior deltoid (AD) Lateral deltoid (LD) Posterior deltoid (PD)	AD: the lateral third of the clavicle LD: acromion of the scapula PD: spine of the scapula	AD/LD/PD: deltoid tuberosity on the humerus	Axillary nerve	The anterior deltoid contributes to shoulder internal rotation.

Innervation of the Shoulder

The majority of the muscles in the shoulder are innervated by nerves originating from the brachial plexus. The brachial plexus is a network of nerves formed from the ventral rami of nerve roots C5 to T1. From proximal to distal, the brachial plexus is organised by roots, trunks, divisions, and cords.

Nerve	Origin	Branches	Motor fibres	Sensory fibres
Accessory nerve (cranial nerve XI)	Upper spinal cord (C1-C5/C6). Lateral aspect of the medulla oblongata.	Internal branch External branch	Purely somatic motor function, innervating trapezius	None
Dorsal scapular nerve	Anterior ramus of spinal nerve C5	None	Rhomboid major, rhomboid minor and levator scapulae	None
Long thoracic nerve	Spinal nerves C5, C6, and C7		Serratus anterior	None
Subclavian nerve	Anterior ramus of spinal nerves C5-C6		Subclavius	None
Suprascapular nerve	Superior trunk of brachial plexus	Motor muscular branches Sensory articular branches	Supraspinatus and infraspinatus	Acromioclavicular joint Glenohumeral joint
Medial pectoral nerve	Medial cord of brachial plexus (C8, T1)	Muscular branches	Pectoralis major and minor	None
Lateral pectoral nerve	Lateral cord of brachial plexus (C5, C6, C7)		Pectoralis major and minor	None
Musculocutaneous nerve	Lateral cord of brachial plexus (C5-C7)		Coracobrachialis, biceps brachii	Anterolateral skin of the forearm
Upper subscapular nerve	Posterior cord of the brachial plexus (C5-C6)		Superior portion of the subscapularis	None
Lower subscapular nerve	Posterior cord of the brachial plexus (C5-C6)		Inferior portion of the subscapularis Teres major
Thoracodorsal nerve	Posterior cord of the brachial plexus (C6-C8)	Small terminal muscular branches	Latissimus dorsi	None
Axillary nerve	Posterior cord of brachial plexus (C5-C6)	Anterior, posterior, and articular branches	Deltoid, teres minor, lateral head of triceps brachii	Glenohumeral joint, and the skin of the deltoid region/upper arm
Radial nerve	Posterior cord of brachial plexus (C5-T1)	Posterior brachial cutaneous nerve, inferior lateral brachial cutaneous nerve, posterior antebrachial cutaneous nerve, muscular branches, deep branch of radial nerve, superficial branch of radial nerve	Triceps brachii	Lower outer aspect and posterior surface of the arm

Vascular Supply of the Shoulder

The blood supply for the shoulder is provided primarily by the subclavian artery. As it enters the shoulder region and axilla, the subclavian artery becomes the axillary artery.

Artery	Origin	Branches	Supply
Dorsal scapular artery	Subclavian artery		Levator scapulae, rhomboids, and trapezius
Suprascapular artery		Suprasternal branch Acromial branch	Supraspinatus and infraspinatus
Superior thoracic artery	Axillary artery	Collateral and terminal branches	Pectoralis major, pectoralis minor, serratus anterior, serratus anterior, subclavius
Thoracoacromial artery		Pectoral, acromial, clavicular and deltoid branches	Pectoralis major, pectoralis minor, deltoid muscles. The skin covering the clavipectoral fascia
Lateral thoracic artery		Lateral mammary branches, lateral cutaneous branches	Serratus anterior, pectoralis major, pectoralis minor and subscapularis. The axillary lymph nodes, breasts, skin of the anterior thoracic wall
Subscapular artery		Circumflex scapular branch, thoracodorsal arteries	Latissimus dorsi, deltoid, long head of triceps brachii, subscapularis, supraspinatus, infraspιnatus, and serratus anterior and adjacent skin
Anterior circumflex humeral artery (AC) Posterior circumflex humeral artery (PC)		AC: ascending branch PC: descending branch	AC: Glenohumeral joint, teres major and minor, deltoid, and the head of the humerus. PC: Glenohumeral joint, deltoid, teres major, teres minor, and long and lateral heads of triceps brachii

Clinical Relevance

1. Forward head posture increases tension on the levator scapulae muscles. Increased tightness in the levator scapulae due to increased activity from a forward head posture can lead to cervicogenic headaches.
2. Upper trapezius is frequently affected in neck injuries, including high-velocity accidents, such as motor vehicle accidents.
3. Scapular winging occurs when the muscles of the scapula are weak or paralysed, resulting in reduced ability to stabilise the scapula. Damage to the long thoracic nerve, which provides sole innervation to the serratus anterior muscle, can lead to scapula winging.
4. When the position and motion of the patient's scapula are altered, they can experience a condition called scapular dyskinesia or scapular dysrhythmia. It is characterised by inferior angle prominence, medial border prominence, excessive superior border elevation, or symmetric scapular motion during arm movement. Learn more about scapular dyskinesia from here.
5. Shoulder instability is defined as loss of shoulder comfort and function due to undesirable translation of the humeral head on the glenoid fossa.
6. Rotator cuff tears are the leading cause of shoulder pain and shoulder-related disability. Degenerative changes, repetitive micro-traumas, severe traumatic injuries, atraumatic injuries and secondary dysfunctions can result in rotator cuff tears. You can learn more about shoulder conditions in the shoulder programme.
7. Erb’s palsy occurs when there is an injury to the brachial plexus, specifically the upper brachial plexus, at birth. Backpackers, or people carrying heavy weights while hiking, can also experience compression of the upper trunks of the brachial plexus.

Resources

• Terry GC, Chopp TM. Functional anatomy of the shoulder. J Athl Train. 2000 Jul;35(3):248-55.
• Alashkham A, Alraddadi A, Felts P, Soames R. Blood supply and vascularity of the glenoid labrum: Its clinical implications. J Orthop Surg (Hong Kong). 2017 Sep-Dec;25(3):2309499017731632.