Functional Anatomy of the Shoulder

Movers of the Shoulder

The bony structures and 4 articulations of the shoulder described in the first lesson form the suspensory link for the attachment of the upper extremity to the trunk. The muscles that act on these bony structures produce the most dynamic and mobile joint in the body. The minimal bony constraints allow a wide range of motion at the expense of joint stability, which is augmented dynamically by the stablilizer muscles and ligaments. Shoulder mobility is produced by complex interactions of the functional groups of the shoulder for Abduction, Flexion, Adduction, Extension, Internal Rotation, External Rotation and Scapulothoracic Motion.

There are Extrinsic and Intrinsic shoulder muscles, defined by the origin of the muscles.

8 Extrinsic Shoulder Muscles
These arise from the axial skeleton and insert on the Shoulder Girdle or Humerus.
- Anterior Extrinsic muscles: Pectoralis Major and Pectoralis Minor
- Posterior Extrinsic muscles: Trapezius, Latissimus Dorsi, Serratus Anterior (see the Dissection Window to the right) and three deeper muscles: Levator Scapulae, Rhomboid Minor, and Rhomboid Major (click here to see a labeled image of the deep muscles)
6 Intrinsic Shoulder Muscles
These arise from the Shoulder Girdle and insert on the medial or lateral aspect of the Humerus.
- Medial Insertions: Subscapularis, and Teres Major
- Lateral Insertions: Supraspinatus, Infraspinatus, Teres Minor, and Deltoid

The shoulder muscles are innervated by nerves C4 through T1 and the Spinal Accessory Nerve (Trapezius).

Contraction of a primary agonist (the 'mover') moves the arm at the Glenohumeral Joint, but also generates a dislocating force that threatens joint stability. Generation of such a force requires activation of an antagonist muscle or muscles to prevent dislocation. This is referred to as a force couple for dynamic stability and a wide range of motion.

The Two Abductors

Abduction of the arm.

Abductors move the arm away from the side of the torso laterally in the coronal plane. The Humerus can abduct up to 120 degrees (the remaining motion to elevate the arm overhead comes from 60 degrees of Scapular rotation).

Supraspinatus
- Originates from the Supraspinatus Fossa of the Scapula, passes laterally under the Acromion and the Coricoacromial Ligament to form a broad tendon that inserts into the Shoulder Joint Capsule and the Greater Tuberosity of the proximal Humerus.
- Provides the first 30° of abduction of the Humerus.
- Forms the upper part of the Rotator Cuff.
Deltoid
- The Deltoid is a multipennate muscle with 3 large components from 3 different origins and 3 different functions. The three components merge and insert on the lateral aspect of the mid-shaft of the Humerus at the Deltoid Tuberosity.
  - The Anterior Deltoid (Clavicular part) arises anteriorly from the lateral third of the Clavicle.
  - The Mid-Deltoid (Acromial Part) arises laterally from the Acromion.
  - The Posterior Deltoid (Spinal Part) arises from the Scapular Spine.
- The Mid-Deltoid (Acromial Part) is the major shoulder abductor.

Dynamic Stability:

During abduction from contraction of the Supraspinatus and Mid-Deltoid (Acromial Part) muscles, dynamic stability to prevent dislocation at the Glenohumeral Joint is produced by a force couple. The oblique Rotator Cuff muscles - the Infraspinatus, Subscapularis and Teres Minor muscles produce the force-couple for dynamic stability of the Glenohumeral Joint during abduction.

The Four Flexors

Flexion of the arm.

Flexors move the arm in front of the torso in the sagittal plane (aka Anteversion).

Coracobrachialis
- Originates on the Coracoid Process and inserts on the medial aspect of the mid-shaft of the Humerus.
Biceps Brachii
- Biceps Brachii Short Head originates at the Coracoid Process of the Scapula.
- Biceps Brachii Long Head originates at the Supraglenoid Tubercle of the Scapula, lies within the Glenohumeral Joint and descends between the Greater and Lesser Tuberosities of the Humerus.
- The two heads merge and insert on the Bicipital Tuberosity of the Radius.
- The Biceps Brachii crosses two joints and functions to flex the Humerus and to flex and supinate the forearm.
Pectoralis Major Clavicular Head
- Originates from the medial third of the Clavicle and inserts on the Intertubercular Groove of the Humerus.
Anterior Deltoid (Clavicular Part)
- Originates from the Acromion and inserts on the lateral aspect of the mid-shaft of the Humerus.

Dynamic Stability:

During forward flexion, dynamic stability occurs with the force couple of Supraspinatus, Infraspinatus, and Serratus Anterior contraction.

The Seven Adductors

Adduction of the arm.

Adductors bring the arm in close to the torso. Adduction of the arm is produced by contraction of both anterior and posterior muscles.

Anterior Adductors:
- Coracobrachialis
  - Originates on the Coracoid Process and inserts on the Humerus.
- Pectoralis Major
  - Pectoralis Major Clavicular Head
    - Originates on the medial third of the Clavicle.
  - Pectoralis Major Sternocostal Head
    - Originates from the Manubrium, Sternum, and upper Costal Cartilage, then it merges and inserts on the Intertubercular Groove of the Humerus.
- Anterior Deltoid (Clavicular Part)
  - Originates from lateral aspect of the Clavicle and inserts on the Humerus.
Posterior Adductors:
- Latissimus Dorsi
  - (The largest muscle in humans.) Originates from the posterior aspect of the back from T6 to the Iliac Crest in four parts (Vertebral, Iliac, Costal and Scapular parts) and inserts with Teres Major on the crest of the Lesser Tubercle of the medial Humerus.
- Teres Major
  - Originates on the Scapula and inserts on the Intertubercular Groove of the Humerus.
- Long Head of the Triceps
  - Originates from the Infraglenoid Tubercle of the Scapula so it crosses two joints and produces Extension at both the Shoulder and the Elbow.
- Posterior Deltoid (Spinal Part)
  - Arises from the Scapular Spine and inserts on the lateral aspect of the Humerus.

The Four Extensors

Extension of the arm.

Extensors move the arm posteriorly behind the torso in the sagittal plane (aka Retroversion).

Latissimus Dorsi
- (The largest muscle in humans.) Originates from the posterior aspect of the back from T6 to the Iliac Crest in four parts (Vertebral, Iliac, Costal and Scapular parts) and inserts with Teres Major on the crest of the Lesser Tubercle of the medial Humerus.
Teres Major
- Originates on the Scapula and inserts on the Intertubercular Groove of the Humerus.
Triceps Brachii
- The Triceps Brachii has three heads - the Long, Medial and Lateral heads. They merge and insert on the Olecranon of the Ulna and together function as the extensor at the Elbow.
  - Triceps Brachii Long Head originates from the Infraglenoid Tubercle of the Scapula, so it crosses two joints and produces Extension at both the Shoulder and the Elbow.
  - Triceps Brachii Medial Head originates at the Groove on the Humerus for the Radial Nerve.
  - Triceps Brachii Lateral Head originates from the dorsal lateral surface of the Humerus.
Posterior Deltoid (Spinal Part)
- Arises from the Scapular Spine and inserts on the lateral aspect of the Humerus.

Dynamic Stability:

Forces generated during extension would tend to cause anterior dislocation of the Humeral Head. Dynamic stability is provided by contraction of the Supraspinatus and Subscapularis muscles producing a force couple to resist anterior dislocation.

Rotators of the Shoulder

Rotation of the Shoulder refers to axial rotation of the Humerus with the arm held in adduction. Internal and external rotation of the arm occurs primarily by the Rotator Cuff muscles with assistance of the superficial muscles. (Note the vector of forces produced by contraction of these muscles as seen in the highlighted corresponding transverse sections.)

The Five Internal Rotators

Internal Rotation of the arm.

Internal (medial) Rotation occurs from contraction of both anterior and posterior muscles. The two anterior muscles insert on the lateral aspect of the Humerus. The three posterior muscles insert on the medial aspect of the Humerus.

Anterior Muscles
- Pectoralis Major Sternocostal Head
  - Originates at the Manubrium, Sternum and upper Costal Cartilage and inserts on the lateral aspect of the Humerus.
- Anterior Deltoid (Clavicular Part)
  - Originates from the lateral Clavicle and inserts on the lateral Humerus.
Posterior Muscles
- Subscapularis
  - Originates on the Costal surface of the Scapula and inserts on the medial aspect of the Humerus at the Lesser Tubercle.
  - Forms the anterior portion of the Rotator Cuff and opposes anterior displacement of Humeral Head.
- Teres Major
  - Originates from the Inferior Angle of the Scapula and inserts just below the Subscapularis Tendon on the medial aspect of the Humerus.
- Latissimus Dorsi
  - (The largest muscle in humans.) Originates from the posterior aspect of the back from T6 to the Iliac Crest in four parts (Vertebral, Iliac, Costal and Scapular parts) and inserts with Teres Major on the crest of the Lesser Tubercle of the medial Humerus.

(Rotate the torso to further explore the internal rotators.)

Dynamic Stability:

During internal rotation, dynamic stability is provided by contraction of the Mid-Deltoid (Acromial Part) and Posterior Deltoid (Spinal Part).

The Three External Rotators

External Rotation of the arm.

External (lateral) Rotation occurs with the contraction of three posterior muscles that insert on the posterolateral aspect of the Humerus.

Infraspinatus
- Originates from the inferior aspect of the Scapula and Scapular Spine and inserts on the posterior aspect of Greater Tubercle of the Humerus.
- It forms the upper posterior portion of the Rotator Cuff.
Teres Minor
- Originates from the Lateral Border of the Scapula and inserts on the posterior aspect of the Greater Tubercle of the Humerus just below the Infraspinatus Tendon insertion.
- Forms the lower posterior portion of the Rotator Cuff.
Posterior Deltoid (Spinal Part)
- Originates from the Spine of the Scapula and inserts at the lateral midshaft of the Humerus on the Deltoid Tuberosity.

Dynamic Stability

During external rotation, dynamic stability is produced by the force couple with contraction of the Subscapularis.

Scapulothoracic Motion

The Scapula has a wide range of gliding motion on the thorax that enhances the movement of the Humerus in the Glenoid Fossa. Movements of the Scapula include:

Elevation
Depression (from the weight of the limb, protraction- moving the Scapula laterally and forward around the thorax, aka abduction of the Scapula)
Retraction (aka adduction of the Scapula)
Upward and Downward Rotation (movement is primarily at the Acromioclavicular Joint)

Movement at the Scapulothoracic Articulation allows the Deltoid to maintain optimal power regardless of arm position. During forward flexion of the arm, the Scapula rotates anteriorly for the first 90 degrees, then posteriorly to increase stability at the Glenohumeral Joint. Elevation of the arm beyond 90 degrees of abduction is produced by contraction of the Serratus Anterior and Upper Trapezius, which rotate the Scapula as much as 60 degrees and provide anterior stabilization.

Trapezius
- As a whole its function is to retract and stabilize the Scapula. However it has three parts that extend from the occiput to T12 that act as three muscles.
  - Trapezius Desceding Part (upper fibers)
    - Arises from the Superior Nuchal Line and Ligament.
    - Inserts on the lateral third of the Clavicle.
    - It elevates the shoulder (shoulder shrug), pulls the Scapula toward the Vertebral Column (retraction), and rotates the lateral angle of the Scapula upwards.
  - Trapezius Transverse Part (middle fibers)
    - Arises from C7 to T3 Vertebrae.
    - Inserts on the Acromial end of the Clavicle, the Acromion, and the lateral part of the Scapular Spine.
    - It is a strong stabilizer because it retracts the Scapula toward the Vertebral Column as the shoulder is braced backwards and fixes the Scapula during abduction of the arm.
  - Trapezius Ascending Part (lower fibers)
    - Arises from the T3 to T12 Vertebrae
    - Inserts on the medial part of the Scapular Spine.
    - It pulls the Scapula toward the Vertebral Column (retraction) and rotates the Scapula upwards during elevation of the arm beyond 90 degrees.
Serratus Anterior
- This is a large and powerful muscle that arises by 9 slips from 1st to 9th Ribs, and covers the lateral aspect of the thorax.
- It inserts in three parts (Superior, Intermediate and Inferior) along the entire medial margin of the Scapula.
- Its chief function is to draw the Scapula forward (protraction) and produce upward rotation of the Scapula for full range elevation of the arm (beyond 90 degrees of abduction). It also draws the medial border of the Scapula into the thorax preventing "winging" of the Scapula.
Pectoralis Minor
- Originates at the 3-5th Ribs.
- Inserts on the Coracoid Process.
- Its chief action is to depress the lateral angle of the Scapula (downward rotation) and to draw the Scapula forward for stability.
3 Deep Posterior Muscles (click here to see a labeled image)
- Rhomboid Minor
  - Originates at the Ligamentum Nuchae and the Spinous Processes of C7 to T1.
  - Inserts on the medial margin of the Scapula at the base of the Scapular Spine.
  - It retracts the Scapula toward the Vertebral Column and elevates the medial border of the Scapula, thereby elevating the shoulder and producing downward rotation of the Scapula.
- Rhomboid Major
  - Originates at the Spinous Processes of 1-4 Thoracic Vertebrae.
  - Inserts on the medial margin of the Scapula below the Scapular Spine.
  - It retracts the Scapula toward the Vertebral Column and elevates the medial border of the Scapula, thereby elevating the shoulder and producing downward rotation of the Scapula.
- Levator Scapulae
  - Arises from the Transverse Processes of C1 to C4.
  - Inserts on the Superior Angle of the Scapula.
  - It elevates the Superior Angle of the Scapula while rotating the Inferior Angle medially (ie downward rotation of the Scapula).