Muscles of the Shoulder

The muscles of the shoulder are a complex group of muscles that control movement and maintain the stability of the glenohumeral (shoulder) joint. These muscles are categorized into two main groups: extrinsic shoulder muscles that originate from the axial skeleton and act on the scapula and humerus, and intrinsic shoulder muscles that originate from the scapula or clavicle and act directly on the humerus. Together, they enable a wide range of motions including flexion, extension, abduction, adduction, rotation, and circumduction of the arm.

Classification

The shoulder muscles are typically divided into two functional categories:

1. Extrinsic Shoulder Muscles

• Superficial group: Trapezius, Latissimus dorsi
• Deep group: Levator scapulae, Rhomboid major, Rhomboid minor

2. Intrinsic Shoulder Muscles

• Deltoid
• Teres major
• Rotator cuff muscles:
  • Supraspinatus
  • Infraspinatus
  • Teres minor
  • Subscapularis

Location

Shoulder muscles are distributed around the scapula, clavicle, and humerus. Some arise from the vertebral column or thoracic cage (extrinsic) and insert onto the scapula or humerus, while others are confined to the scapulohumeral region (intrinsic). Below is a summary of their anatomical placement and orientation:

Structure and Function

Deltoid

Responsible for abduction of the arm from 15° to 90°, along with flexion, extension, and rotation depending on fiber group. It provides the rounded shape to the shoulder and stabilizes the humeral head in the glenoid cavity.

Teres Major

Assists in adduction, extension, and medial rotation of the arm. It works synergistically with the latissimus dorsi and inserts beside it on the humerus.

Supraspinatus

Initiates the first 15° of arm abduction. It lies superior to the shoulder joint and passes beneath the acromion, making it vulnerable to impingement syndromes.

Infraspinatus

Located posteriorly, it contributes to lateral rotation of the humerus and stabilizes the shoulder joint as part of the rotator cuff.

Teres Minor

Also involved in lateral rotation and stabilization of the shoulder joint. It is the smallest rotator cuff muscle.

Subscapularis

The only rotator cuff muscle that medially rotates the arm. It forms the anterior wall of the axilla and inserts into the lesser tubercle of the humerus.

Biomechanics

Shoulder muscles coordinate with one another to provide a wide range of motion while maintaining glenohumeral stability. The rotator cuff muscles compress the humeral head into the glenoid during movement, preventing dislocation. The deltoid acts with trapezius and serratus anterior to elevate the arm beyond 90° via scapulohumeral rhythm. Teres major and latissimus dorsi function to bring the arm back to the trunk (adduction and extension).

Physiological Role(s)

• Enable complex upper limb tasks: reaching, lifting, throwing, climbing, etc.
• Maintain shoulder posture and joint alignment during motion.
• Absorb and redistribute mechanical forces across the shoulder girdle.
• Provide fine control for upper limb manipulation tasks requiring precision.

Development

Shoulder muscles originate from the paraxial mesoderm of the somites, particularly from the hypaxial portions that contribute to limb musculature. Motor innervation from the brachial plexus is established early, and the rotator cuff muscles develop in close association with joint capsule formation. Ossification of their bony attachments completes during adolescence, and full functional maturation continues into early adulthood.

Clinical Significance

• Rotator cuff tear: Common in older adults or athletes; usually involves the supraspinatus tendon. Leads to weakness, pain, and limited abduction.
• Shoulder impingement syndrome: Caused by narrowing of the space under the acromion, compressing the supraspinatus or subacromial bursa.
• Axillary nerve injury: May result from shoulder dislocation or surgical neck fracture, causing deltoid atrophy and abduction weakness.
• Subscapularis dysfunction: Leads to weakness in medial rotation and instability of the anterior shoulder joint.
• Myofascial pain syndrome: Trigger points in the trapezius or deltoid can radiate pain to the shoulder and neck.

Anatomical Variations

• Accessory slips of deltoid or supraspinatus may be present.
• Infraspinatus and teres minor may share fibers or have unclear separation in some individuals.
• Occasional absence or fusion of rotator cuff components, especially in congenital musculoskeletal disorders.

Cross-sectional and Surface Anatomy

• The deltoid muscle is easily palpable and defines the shoulder’s contour.
• Rotator cuff tendons lie deep to the deltoid and are accessible via imaging or intra-articular injection.
• Posteriorly, the infraspinatus and teres minor form the posterior axillary wall along with the long head of the triceps.