Functional Anatomy of the Shoulder

Original Editor - Ewa Jaraczewska

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

Introduction

The shoulder complex is one of the most complicated regions of the body. It involves four joints, muscles and tendons, ligaments, labrum, capsule, bursa, and neuromuscular bundles, and allows for a large range of motion. The muscles are the primary mechanism in providing a stable support base for the upper extremity movements. It is known as dynamic stabilisation. The shoulder mobility, particularly its glenohumeral joint mobility, allows one to move and position the hand through a wide range of space.

The dynamic relationship between muscle forces, ligament constraints, and bony articulations allows high freedom of movement at the expense of stability. Hence susceptibility to dysfunction and instability of the shoulder joint. The shoulder injuries may include sprains, strains, tears, and various degenerative 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 a 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 comprises four bones: clavicle, sternum, scapula and proximal end of the humerus.

The clavicle connects to the acromion, part of the scapula, and the sternum, which belongs to the thoracic spine complex. It makes up the front part of the shoulder. The clavicle bone is a sigmoid-shaped long bone which lies horizontally. As a connection between the axial skeleton and the shoulder girdle, the clavicle transfers the weight from the upper limbs to the axial skeleton.

The sternum completes the anterior chest wall. It is a flat cancellous bone with a compact cortex. The sternum is slightly convex anteriorly, with multiple indentations along its lateral borders called costal notches. It consists of three parts: the manubrium, the body, and the xiphoid process:

• The manubrium is flat and four-sided. It widens superiorly.
• The body is a flat rectangular bone with facets on its lateral border for rib articulation.
• The xiphoid process is a thin bony that projects inferiorly.

The scapula is a thin, flat triangular-shaped bone placed on the postero-lateral aspect of the thoracic cage. It has 2 surfaces, 3 borders, 3 angles and 3 processes:

• Scapula surfaces: anterior and posterior. The anterior surface (the costal surface) contains the shallow subscapular fossa. The scapular spine divides the posterior surface into a supraspinous fossa and infraspinous fossa.
• Scapula borders:
  • superior with the acromion that is continuous with the scapular spine and arcs anteriorly over the humeral head
  • medial towards the spine
  • lateral with glenoid fossa located superiorly
• Scapula angles: superior covered by trapezius, inferior covered by latissimus dorsi, and lateral bearing the glenoid fossa
• Scapula processes: the acromion, the spine, and the coracoid process

The proximal end of the humerus contains the head, an irregular hemisphere. It articulates with the glenoid fossa of the scapula. The anatomical neck is the part between the head and the tuberosities, and 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. It is very prominent and palpable. The bicipital groove (intertubercular groove or sulcus) is located between the two tuberosities. The tree muscles that attach here in the direction from medial to lateral are teres major, latissimus dorsi, and pectoralis major. Also, the long head of the biceps tendon is placed here.

Bones, Articulations and Kinematics of the Shoulder

The shoulder complex comprises significant soft tissues: joint capsules, the labrum, ligaments, bursae, tendons, and muscles. Due to the high mobility of the shoulder complex, its stability is based on the coordinated effort between the static (non-contractile, such as ligaments) and dynamic (contractile such as tendons and muscles) tissues.

Bones and Articulations

Bones	Articulations	Characteristics	Key palpation points
Scapula Ribs	Scapulothoracic joint	It is not a true anatomic joint. It's a complex joint that articulates the scapula and the thorax. The main role includes the centralisation of the humeral head in the socket during the elevation of the arm	To palpate the scapula, position the patient in a prone. If the patient can tolerate it, position the arm of the tested side on the patient's small back (in internal rotation). This will help to visualise the scapula borders. With one hand, cup the shoulder from the front to lift the humerus. With this passive retraction manoeuvre, the scapula's borders can be clearly identified. The lateral border is difficult to palpate as it is covered with muscles. The patient is sitting or standing with their back towards you, trunk flexed, and scapula protracted. Locate the C7 vertebra by palpating its long spinous process to palpate the first rib. The first rib is about two centimetres lateral and slightly inferior to the C7 spinous process. Trapezius and levator scapulae cover the first rib. This may make it difficult to palpate the first rib. The same method is applied to the second rib. Ribs three and four are located under the medial end of the spine of the scapula. Rib seven is under the inferior angle of the scapula. The costal angle of rib nine is at the level of the spinous process of the L1.
Sternum Clavicle	Sternoclavicular joint (SC)	This is the articulation between the clavicle's sternal end, the sternum's manubrium, and the first costal cartilage. It contains an articular disc, which separates the joint into two compartments	The sternum is located on the thorax's anterior wall, in the rib cage's middle. The second costal cartilage marks the superior portion of the sternum. The xiphoid process is located on the sternum's inferior end at the seventh costal cartilage level. 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. To palpate the clavicle, start with the centre of the clavicle, as it is easy to find. Continue palpating by pinching the posterior and anterior aspects of the clavicle and by moving your fingers towards the medial end of the clavicle where the clavicle meets the manubrium of the sternum. The joint between the medial end of the clavicle and the sternum is the sternoclavicular joint. You can confirm the joint location by passively moving the clavicle up and down, which creates a small motion at the joint.
The acromion process of the scapula Clavicle	Acromioclavicular joint (AC)	This is the articulation between the clavicle's lateral end and the scapula's acromion. It forms a combined motion with the glenohumeral joint to fulfil the function of the shoulder girdle.	To palpate the acromion process of the scapula, the patient is prone. First, locate the spine of the scapula, which sits on the top of the shoulder. 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 an acromial angle and the beginning of the acromion process, which turns upwards towards the front of the shoulder. You can now palpate the acromion process's posterior, lateral, and anterior parts. The AC joint is palpated at the lateral end of the clavicle. The joint line may be difficult to palpate as the trapezius muscle covers it. Place your finger on the AC joint to feel the joint line and move the clavicle up and down passively.
Humerus Glenoid fossa of the scapula	Glenohumeral joint	Made up of the glenoid fossa of the scapula and the head of the humerus. It has the greatest 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, start with locating the coracoid process. To do it, move the patient's arm slightly to the side. Ask the patient to lift the arm above the surface to activate the anterior deltoid. Now you can palpate the medial border of the muscle. Move your finger laterally and feel for the knuckle under the skin. This 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 age. The 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. Additionally, the Body Mass Index (BMI) may have a negative effect on shoulder flexibility.

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

Other factors influencing the shoulder range of motion in the general population include 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
• range of motion declines with age
• lower mean range for all assessed movements except external rotation of the right shoulder for those who are right-hand dominant compared to those who are left-hand dominant.

Joint	Type of joint	Plane of movement	Motion	Kinematics	Closed pack position	Open pack position
Scapulothoracic joint	Synovial	Frontal Transverse Saggital or scapular (according to some resources, more in a scapular plane than the saggital) Clinical significance: "Overhead rehabilitation exercises should be performed in the scapular plane, especially in subjects with rotator cuff tendinopathy."	The joint kinematics include translations and rotations of the scapula on the ribcage. The scapula: elevation, depression, adduction, and abduction. It 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 Scapular upwards rotation (external rotation): 60 degrees Scapular downwards rotation (internal rotation): 30 degrees	Because it is not a true joint, it 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 Rotations 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 produced by maximal arm elevation and full scapular rotation.	Arms resting by the side of the body
Acromioclavicular joint	Synovial-plane type		The range of motion is difficult to measure as it combines the clavicle and glenohumeral joint. The upper limb resting in a horizontal position allows for the maximum rotation range of motion.	Axial rotation: 25 degrees	90 degrees of shoulder abduction	Arms resting by the side of the body
Glenohumeral joint	Ball and socket	Saggital 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	Approximately 50 degrees of abduction, slight horizontal adduction, and external rotation.

Shoulder Passive Range of Motion Assessment

The therapist must consider various factors to select a testing position for passive range of motion assessment, as there are no guidelines for selecting the sitting, supine, or standing testing position. These factors include:

• The therapist's height is shorter than the patient's stature, which may cause difficulty in performing the assessment
• A patient with multiple disabilities may not be able to assume a desired position

It is recommended that the testing position should be documented, and repeated assessments of the range of motion should be administered in a consistent position.

Shoulder flexion

• The patient should be in a sitting position to avoid compression on the scapula when supine
• The patient's arm is placed with the palm facing inwards, with the forearm in mid-position
• The therapist grasps the distal humerus, close to the elbow
• The therapist moves the humerus in an anterior and upward direction to the limit of shoulder flexion
• The elbow is left in extension, if possible, to minimize stretching of the 2-joint triceps muscle, thus possibly affecting the ROM of shoulder flexion.
• Testing ends with a firm end feel
• The norm: pure glenohumeral flexion: 80-90 degrees, with clavicle and scapula moving: 180 degrees

Shoulder extension

• The patient in a sitting, standing or side-lying position
• The patient's arm is placed with the palm facing inwards, with the forearm in mid-position
• The therapist grasps the patient’s humerus as distal as possible
• The therapist moves it backwards
• Testing ends with a firm end feel
• The norm: 60 degrees

Shoulder abduction

• The patient in a sitting, standing or supine position
• The patient's arm is placed with the palm facing inwards, with the forearm in mid-position
• The therapist grasps the patient’s humerus as distal as possible
• The therapist abducts the arm
• Testing ends with a firm end feel
• Norm: 90 degrees for pure glenohumeral abduction. Further motion is expected when the clavicle and scapula are not fixated

Shoulder adduction

• The patient in a sitting, standing or supine position
• The patient's arm is placed with the palm facing inwards, with the forearm in mid-position
• The therapist grasps the patient’s humerus as distal as possible
• The therapist adducts the arm
• Testing ends with a firm end feel
• Norm: 30 degrees

Shoulder external rotation

• The patient in a sitting, standing, supine or side-lying position
• The therapist stabilises the scapula with one hand and brings the arm to 45º of glenohumeral abduction with the other hand by holding the patient by the elbow and keeping the 0 degrees of rotation
• The therapist externally rotates the arm while observing any scapular retraction
• The movement is stopped with a firm end feel or when the scapular displacement starts
• Norm: approximately 60 degrees

Shoulder internal rotation

• The patient in a sitting, standing, supine or side-lying position
• The therapist stabilises the scapula with one hand and brings the arm to 45º of glenohumeral abduction with the other hand by holding the patient by the elbow and keeping the 0 degrees of rotation
• Next, the therapist brings the patient's hand behind the patient's back and lifts it off the thorax
• Testing ends with a firm end feel
• Norm: around 100 degrees from the anatomic position
• The alternative testing position for internal and external glenohumeral joint rotation is prone with a shoulder at 90 degrees abduction and 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 his head for external or towards the patient's hip for internal rotation.

Shoulder Bursae

Shoulder bursae allow normal movement in the shoulder joint. Yet, they can become a leading cause of shoulder pathology and dysfunction. The neurovascular structures found in some bursae indicate their contribution to the blood supply to surrounding structures and their involvement in mechanoreception.

The following are the shoulder 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. It is considered a primary source of shoulder pain.
• 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 Bursa are often taken as a single bursa, the Subacromial Deltoid Bursa.

Glenoid Labrum

The glenoid labrum is a fibrocartilaginous, ridge-like connective tissue that increases the humeral head's articular surface area by deepening the glenoid fossa.  Its primary roles include:

• providing the primary attachment for the glenohumeral ligaments
• giving rise to the long-head biceps tendon, capsule, and scapular neck
• conforming to the curvature of the humeral head and contributing to approximately 50% of the depth of the shoulder joint.

Shoulder Joint Capsule

The joint capsule surrounding the glenohumeral joint is an important passive stabilizer of the shoulder joint. The glenohumeral joint capsule is thickened at the front of the capsule and is twice the size of the humeral head. It provides most of its extensibility anteriorly and inferiorly, and it “winds up” during abduction and external rotation. The joint capsule and glenohumeral ligaments function as stabilizers 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 The adjacent part of the first costal cartilage	Reinforcement to the anterior aspect of the joint	The patient is supine. To palpate the clavicle, start with the centre of the clavicle, as it is easy to find. Continue palpating by pinching the posterior and anterior aspects of the clavicle and by moving 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	The sternal end of the left clavicle	The sternal end of the right clavicle	Superior reinforcement of the joint capsule Resists excessive depression or downward glide of the clavicle
Costoclavicular ligament	The first rib and costal cartilage	The inferior surface of the clavicle.	Limits the elevation of the pectoral girdle Acts as a fulcrum for elevation-depression and protraction-retraction Checks the clavicular elevation and superior glide of the clavicle	The patient is supine with scapula depression. To palpate the first rib, find the clavicle first. Place your finger on the lateral end of the clavicle and roll it behind it. Next, with the finger, push the belly of the trapezius muscle back and apply downward pressure to feel the bony prominence. This is the first rib. To confirm your palpation 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: the coracoid process of the scapula TL: the coracoid process of the scapula	CL: conoid tubercle of the clavicle TL: the trapezoid line of the clavicle.	Prevents scapula superior dislocation Resists scapular rotation Supports the weight of the upper limb suspended from the clavicle.	To palpate the coracoid process of the scapula, start by moving the patient's arm slightly to the side. Ask the patient to lift the arm above the surface to activate the anterior deltoid. Now you can palpate the medial border of the muscle. Move your finger laterally and feel for the 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 joint posterior axial rotation	First, 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	The summit of the acromion of the scapula	The 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	The lesser tubercle of the humerus	The 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 with finding the anterolateral portion of the acromion. Just below, you will find the head of the humerus with its greater tubercle. 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 arm moves into lateral rotation. Further shoulder movement into the lateral rotation will palpate the lesser tubercle under your fingers.
Superior glenohumeral ligament	Glenoid labrum	The 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	The anterior surface of the lesser tubercle	Provides anterior stability when the shoulder is abducted at 45 and 60 degrees.
Inferior glenohumeral ligament	The glenoid	Beyond the lesser tuberosity.	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 muscles, and the scapular muscles, which control the scapulohumeral rhythm. For optimal shoulder stabilization, the dynamic stabilizers must function efficiently and synergistically. Dynamic stabilization results in a wide range of mobility for the shoulder complex and provides adequate stability when the complex is functioning normally.  The rotator cuff muscles provide the stability of the glenohumeral joint, primarily through compression of the head of the humerus against the glenoid during movement. There are supraspinatus, infraspinatus, teres minor, and subscapularis.

The following sections will include muscles according to their role in moving the scapula and the humerus. Other functions of these muscles not related to the scapula and shoulder motion are not included (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	The lateral third of the clavicle	Suprascapular Nerve C5-6	Produces scapular elevation

Scapular Depression

Muscle	Origin	Insertion	Innervation	Action
Pectoralis minor	Ribs three to five	The medial border and coracoid process of the scapula	Medial Pectoral C8-T1	The primary actions of this muscle include stabilization, depression, abduction or protraction, internal rotation and downward rotation of the scapula.
Lower trapezius	The spinous processes of the thoracic vertebrae (T4–T12).	The lateral third of the clavicle, the acromion, and the spine of the scapula	Axillary Nerve C5-6	Shoulder depression and scapula upward rotation

Scapular Protraction

Muscle	Origin	Insertion	Innervation	Action
Serratus anterior	Ribs one to nine	The costal surface of the medial scapula	Long Thoracic C5-C6-C7	The main actions are protraction and upward rotation of the scapulothoracic joint. It’s also a key scapular stabilizer, keeping the shoulder blades against the ribcage at rest and during movement. "Boxer's muscle", as it protracts the scapula during a punch.
Pectoralis minor	Ribs three to five	The medial border and coracoid process of the scapula	Medial Pectoral C8-T1	The primary actions of this muscle 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: The 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)	The lateral third of the clavicle, as well as the acromion and scapular spine of the scapula.	Spinal Accessory Motor CN XI Cervical Nerves C3-C4	Scapula retraction

Scapular Upward Rotation (Lateral Rotation)

Muscle	Origin	Insertion	Innervation	Action
Serratus anterior	Ribs one to nine	The costal surface of the medial scapula	Long Thoracic C5-C6-C7	The main actions are protraction and upward rotation of the scapulothoracic joint. It’s also a key scapular stabilizer, keeping the shoulder blades against the ribcage at rest and during movement. "Boxer's muscle", as it protracts the scapula during a punch.
Upper trapezius	Occipital bone and the nuchal ligament	The lateral third of the clavicle	Suprascapular Nerve C5-6	The upper and lower fibres work concurrently to produce the rotation on the scapula.
Lower trapezius	The spinous processes of the thoracic vertebrae (T4–T12).	The lateral third of the clavicle, the acromion, and the spine of the scapula	Axillary Nerve C5-6	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: The 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	The medial border and coracoid process of the scapula	Medial Pectoral C8-T1	The primary actions of this muscle 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	The medial surface of the humeral shaft.	Musculocutaneous nerve	Assists with shoulder flexion and adduction
Biceps brachii: Short head(SH) Long head(LH)	SH: the coracoid process of the scapula LH: the 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: the 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	When acting alone, the clavicular head flexes the shoulder

Shoulder Extension

Muscle	Origin	Insertion	Innervation	Action
Latissimus dorsi	Spinous processes of T7 to T12 The 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: 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 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: 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 lateral deltoid abducts the shoulder.

Shoulder Adduction

Muscle	Origin	Insertion	Innervation	Action
Subscapularis	Subscapular fossa of scapula	Lesser tuberosity of humerus	Upper and lower subscapular nerves	Internal rotation and adduction Assists with 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 with 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 should 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 The 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 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 with 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 with 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 should 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 The 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 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	Brunches	Motor fibres	Sensory fibres
Accessory nerve	Upper spinal cord( C1-C5/C6 ) Lateral aspect of the medulla oblongata.	Internal branch External branch	Purely somatic motor function innervating the trapezius	none
Dorsal scapular nerve	Anterior ramus of spinal nerve C5	none	Rhomboid major, rhomboid minor and levator scapulae muscle	none
Long thoracic nerve	Spinal nerves C5, C6, and C7		Serratus anterior muscle	none
Subclavian nerve	Anterior ramus of spinal nerves C5-C6		Subclavius muscle.	none
Suprascapular nerve	Superior trunk of brachial plexus	Motor muscular branches Sensory articular branches	Supraspinatus muscle Infraspinatus muscle	Acromioclavicular joint Glenohumeral joint
Medial pectoral nerve	Medial cord of brachial plexus (C8, T1)	Muscular branches	Pectoralis major muscle Pectoralis minor muscle	none
Lateral pectoral nerve	Lateral cord of brachial plexus (C5, C6, C7)		Pectoralis major muscle Pectoralis minor muscle	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)		The superior portion of the subscapularis
Lower subscapular nerve	Posterior cord of the brachial plexus (C5-C6)		The 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 muscle, teres minor muscle, lateral head of the triceps brachii muscle	Glenohumeral joint 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 Infraspinatus
Superior thoracic artery	Axilary artery	Collateral and terminal branches	Pectoral muscles, serratus anterior muscle, serratus anterior muscle, subclavius muscle
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 muscles 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 muscle 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, and deltoid muscles, the head of the humerus. PC: Glenohumeral joint, deltoid muscle, teres major muscle, teres minor muscle, and long and lateral heads of the triceps muscle

Clinical Relevance

1. Forward head posture increases tension on the levator scapulae muscles to co-contract for cervical extension in an elongated position. 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 involved in neck injuries, including high-velocity accidents, e.g. motor vehicle accidents.
3. Scapular winging occur when the muscles of the scapula are too weak or paralysed, resulting in a limited ability to stabilise the scapula. Damage to the long thoracic nerve, which provides a sole innervation to the serratus anterior muscle, can lead to the scapula-winging phenomenon.
4. Shoulder instability is defined as loss of shoulder comfort and function due to undesirable translation of the humeral head on the glenoid fossa.
5. 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 cause them. You can learn more about shoulder conditions from this shoulder programme.
6. Erb’s palsy occurs when there is an injury to the brachial plexus, specifically the upper brachial plexus, at birth. Backpackers can damage the nerve by compression of the upper trunks of the brachial plexus when carrying heavy weights while hiking.

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.