Learning Objectives

By the end of this lesson, the student should be able to:

• Classify the muscles of the shoulder into intrinsic and extrinsic groups.
• Identify the attachments, actions, and innervation of the intrinsic (scapulohumeral) muscles.
• Explain the stabilizing role of the rotator cuff.
• Apply anatomical knowledge to common clinical conditions such as rotator cuff tendonitis.

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Overview of the Shoulder Muscles

The muscles of the shoulder are responsible for moving the upper limb and for forming the characteristic contour of the shoulder. They are divided into two main groups:

1. Extrinsic Muscles

• Originating from the torso
• Attach to the clavicle, scapula, or humerus

2. Intrinsic Muscles (Scapulohumeral Group)

• Originates from the scapula and/or clavicle
• Attach to the humerus

The intrinsic group consists of six muscles:

• Deltoid
• Teres major
• Four rotator cuff muscles: supraspinatus, infraspinatus, subscapularis, and teres minor.

This section focuses on the intrinsic muscles of the shoulder—their attachments, actions, and innervation.

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Deltoid & Teres Major

Deltoid

The deltoid is a large, triangular muscle that forms the rounded contour of the shoulder. It has three distinct parts:

Activity

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Teres Major

The teres major forms the inferior border of the quadrangular space.

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Rotator Cuff Muscles

The rotator cuff is a group of four muscles that surround the glenohumeral joint. Their primary collective role is to stabilize the humeral head within the glenoid fossa during movement.

Individual Muscles

Activity

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Clinical Relevance: Rotator Cuff Tendonitis

Rotator cuff tendonitis is caused by repetitive use of the shoulder joint.
The supraspinatus tendon is most commonly affected because it rubs against the coracoacromial arch during abduction.

Management

• Rest
• Analgesics
• Physiotherapy
• Severe cases: steroid injections or surgery

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Activity