Deltoid Muscle – Anatomy Breakdown Video & Clinical Guide
The Deltoid muscle is the iconic, rounded shoulder muscle everyone knows, but its anatomy and function are far more complex than they appear. In this detailed video, we’ll break down its origin, insertion, function, innervation, and clinical relevance. Perfect for students, clinicians, trainers, or anyone wanting to understand shoulder anatomy on a deeper level.
Watch the full video below and read on for practical clinical insights.
Quick Hits
The Deltoid is a large, triangular muscle covering the shoulder joint, giving the shoulder its rounded contour. It has three distinct parts with unique origins and functional roles:
Anterior (clavicular) fibers: Origin from the lateral third of the clavicle.
Middle (acromial) fibers: Origin from the acromion process.
Posterior (spinal) fibers: Origin from the spine of the scapula.
All fibers insert onto the deltoid tuberosity of the humerus.
Innervation: Axillary nerve (C5, C6).
Actions:
Anterior fibers: Shoulder flexion, internal rotation, horizontal adduction.
Middle fibers: Shoulder abduction (primary abductor from ~15° to ~90°).
Posterior fibers: Shoulder extension, external rotation, horizontal abduction.
The deltoid is essential for dynamic shoulder stability and coordinated arm movement in all planes.
Clinical Relevance
Clinically, the deltoid is a critical muscle in shoulder rehab, especially after rotator cuff injuries or surgeries. It's often used to compensate for rotator cuff weakness, which can lead to altered movement patterns and overuse.
The axillary nerve is vulnerable to injury in anterior shoulder dislocations or surgical procedures, leading to deltoid atrophy and loss of abduction strength.
In practice, assessment should include isolated strength testing of all three heads, while rehabilitation should focus on balanced activation and integration into functional movement. Exercises like prone reverse flys, front raises, lateral raises, and band-resisted horizontal abduction can selectively target different fibers.
What’s Next
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