the deltoid muscle anterior view

The Deltoid Muscle

David Keil Anatomy, Upper Limb 3 Comments

What Does Deltoids Mean?

  • The name deltoids comes from the Greek letter Delta, which is a triangle.  The name refers to the triangular shape of the deltoid muscle.
  • The muscle was previously called the deltoidius before being shortened to deltoid.
  • In common slang the name deltoids is often shortened further to the “delts”.

Where Does the Deltoid Muscle Attach?

The deltoid has a very broad area of attachment as the deltoid muscle is divided into three sections.

  • Anterior: attaches onto the outer (lateral) third of the collarbone (clavicle).
  • Lateral or Middle: attaches onto the little shelf and edge of the shoulder blade (scapula) called the acromion process.
  • Posterior: attaches onto the ridge along the scapula called the “spine of the scapula”.
  • All of these fibers converge at the other end and attach onto what is called the deltoid tuberosity. (Tuberosity means big bump.)

What Are The Actions of the Deltoid Muscle?

The three sections of the deltoid are responsible for different actions.

  • Anterior: flexes the arm at the shoulder, internally rotates the arm at the shoulder joint, and assists in taking the arm out to the side (abduction).
  • Lateral or Middle: abducts the arm at the shoulder joint.
  • Posterior: does extension/hyperextension of the arm at the shoulder joint, externally rotates the arm at the shoulder joint, and assists in abduction of the arm at the shoulder joint.
  • When all parts of the deltoid muscle contract together it is an extremely powerful abductor of the arm.

The movement of the deltoid is especially interesting because the anterior portion functions in opposition to the posterior portion. For instance, If I raise my arm in front of me (flexion of the shoulder) the anterior part of the deltoid contracts to help lift my arm up. The posterior part of the deltoid is lengthening while my shoulder joint is flexing.

We could say that the deltoid is antagonistic to itself.

Postures Where the Deltoid Muscle Contracts




Postures Where the Deltoid Muscle is Lengthened


Deltoid Injury

Trigger points can contribute to a sensation of pain in your deltoids however the source of the pain is actually coming from somewhere else. Check out this article to learn more about trigger points. Also note the images and their referral pattern from the infraspinatus muscle.

One of the more common injuries to the deltoid muscle is deltoid strain. Deltoid strain is characterized by sudden and sharp pain at the location of the injury. One would experience intense soreness and pain when lifting the arm out to the side, and/or tenderness and swelling of the deltoid muscle. The grades of deltoid strain are described below:

Grade 1

  • Tightness in the muscles.
  • Mild pain in the deltoid
  • General function is not a problem.
  • Increase in pain when weight bearing with the arms.

Grade 2

  • Occasional twinges of pain during activity.
  • You may notice swelling.
  • Pressing into the deltoid itself can cause pain.
  • Lifting your arm up to the front, side or back against resistance can cause pain.

Grade 3

  • Unable to move your arm.
  • Severe pain.
  • Swelling.
  • Contracting the muscle will be painful and there may be a bulge or gap in the muscle.

Additional Images


deltoid trigger points

deltoid trigger point

Comments 3

    1. Post

      All muscles that attach to the shoulder girdle interact with one another. The simplest way to put is that together they create a balance around which the shoulder can function optimally. If certain muscles end up out of balance, it is possible for other muscles to be pulled out of balance and the shoulder girdle is more likely to dysfunction in some way.

  1. I may have an injured deltoid. I’m not sure if it’s due to too much shoulder stretching or too much weight bearing. What should we avoid with an injury like this and what should we do to counteract the injury? Thanks

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