Shoulders (deltoids)

The rounded cap of the shoulder. The three-headed deltoid lifts the arm in every direction; beneath it the rotator cuff keeps the ball centred in a very shallow, mobile socket — the trade-off for the shoulder being the body's most mobile joint.

This muscle in 3D

Drag to rotate · scroll to zoom — see the shape, origin and insertion of the shoulders (deltoids). 3D model via Sketchfab (CC-BY).

Anatomy

Muscles: Deltoid (anterior, lateral, posterior heads); rotator cuff underneath

Origin: Clavicle, acromion and scapular spine.

Insertion: Deltoid tuberosity of the humerus.

Actions:

• Shoulder abduction (lift arm to the side — lateral head)
• Shoulder flexion (arm forward — anterior head)
• Shoulder extension & horizontal abduction (arm back — posterior head)
• Rotator cuff: hold the humeral head centred and rotate the arm

How the muscle works

The three deltoid heads pull the arm in different directions, so pressing, raising to the front and raising to the rear each bias a different head. Because the socket is shallow, the rotator cuff must actively compress the joint throughout — stability here is muscular, not bony.

Fibre-type bias: Mixed; cuff is postural/stabilising.

Functional role: Reaching, lifting and pressing overhead; positioning and stabilising the arm.

Common problems

• Rotator-cuff tendinopathy / impingement
• Shoulder instability or dislocation
• Posterior-delt & cuff weakness with rounded posture

Training & stretching

Overhead press, lateral raises, front raises, rear-delt flyes, cuff work (external rotations).

Cross-body posterior-shoulder stretch; sleeper stretch for internal rotation.

Fix or train this

• Neck & Shoulder Pain
• Build Muscle (Plateau)