The Deltoid Muscle

What does deltoids mean?

• The name deltoids comes from the Greek letter Delta, which is a triangle.  As you might guess, 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?

Since the deltoid has a very broad area of attachment, it is usually divided into three sections when we describe its attachments.

• 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 – It also 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. But, in contrast, the posterior part of the deltoid lengthens while my shoulder joint flexes.

We could even 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 also contribute to a sensation of pain in your deltoids, however the source of the pain can actually come from somewhere else. To learn more, check out this article about trigger points. Also be sure to take a look at the images and their referral patterns from the infraspinatus muscle.

In addition to trigger points, 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. While symptoms vary, you would likely 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:

Mild: 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.

More serious: 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.

Most serious: Grade 3

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

Trigger points