The Anatomy of the Deltoid Muscle

The Main Muscle of the Shoulder

The deltoid muscle is the main muscle of the shoulder. It extends from the ridge of the scapula (shoulder blade) to the end of the clavicle (collarbone). The deltoid consists of three parts, also known as heads:

  • The anterior deltoid is located at the front of the shoulder.
  • The posterior deltoid is located at the back of the shoulder.
  • The lateral deltoid is sandwiched between the anterior and posterior deltoids.

The deltoid muscle helps move your upper arm and stabilizes the shoulder joint.

This article describes the anatomy of the deltoid muscle, including the location of the heads and their distinct functions. It also lists different health conditions that affect the deltoid muscle and what you can do to prevent or recover from injuries.

What Is the Function of the Deltoid Muscle?

Deltoid muscles are a type of skeletal muscle involved in body movements. Unlike smooth muscles, which are involuntary, skeletal muscles are voluntary (meaning you control them).

Each deltoid muscle is located where the ball-and-socket joint of your shoulder connects your humerus (upper arm bone) to your trunk.

The primary function of the deltoid muscle is to move the humerus and the shoulder joint simultaneously. The muscle movements are referred to as:

  • Abduction: This is where you lift your arm outward to the side of your body.
  • Flexion: This is where you lift your arm anywhere from the side to over your head.
  • Extension: This is where you move your arm behind you.
  • Rotation: This is where the humerus rotates in the socket of the shoulder.

The place where a muscle attaches to a bone is called an insertion point. Each of the three deltoid heads has its own insertion point. Together, they help stabilize the shoulder joint and allow for a full range of motion of the upper arm.

What Are the Deltoid Heads?

Each of the three deltoid heads—anterior, lateral, posterior—enables different movements either on their own or together.

Anterior Deltoid

The anterior deltoid located at front of the shoulder rotates the shoulder joint and moves the arm forward. It does so with a combination of flexion and medial rotation (rotating of a joint inward).

The insertion point of the anterior deltoid is on the clavicle alongside an insertion point of the pectoralis major muscle of the chest. The adjacent insertion points help stabilize the shoulder.

Lateral Deltoid

The lateral deltoid located in the middle of the shoulder rotates the shoulder joint laterally (to the side) to lift your arm sideways from your body. During abduction of the lateral deltoid, the shoulder joint moves downward to accommodate the arm's outward movement.

The insertion point of the lateral deltoid is at the outer tip of the scapula known as the acromion process.

Posterior Deltoid

The posterior deltoid located at the back of the shoulder also rotates the joint laterally to move the arm both outward and backward. During the extension of the posterior deltoid, the shoulder joint will roll downward and inward.

The insertion point of the posterior deltoid is on a part of the scapula known as the spine. This is the prominent ridge that serves as the insertion point for several other muscles. The attachment both stabilizes and prevents the overextension of the shoulder joint.

Woman in a yoga pose, stretching the deltoid muscle
Hero Images / Getty Images 

What Problems Affect the Deltoid Muscles?

The deltoid muscles are vulnerable to injuries that can affect not only the muscle but also connective tissues called tendons that connect muscles to bones. Underlying nerves or blood vessels can also be injured.

Rotator Cuff Injuries

The rotator cuff is a group of muscles and tendons that surround the shoulder joint, keeping the head of the humerus firmly situated within the socket of the shoulder.

A rotator cuff tear is a common injury in which tendons are torn either partially or completely. The destabilizing effect of a tear can place mechanical stress on the deltoid muscle, leading to a deltoid strain.

In some cases, treating a rotator cuff tear involves cutting through the deltoid muscle and then stitching it back together. This means you will need to rehabilitate the deltoid muscle along with the repaired rotator cuff.

Axillary Nerve Injury

The axillary nerve lies immediately below the deltoid muscle and controls both the deltoid muscle and other muscles of the upper arm. The nerve can be damaged by surgery or traumatic injuries, leading to symptoms such as:

  • Shoulder or arm muscle weakness
  • Pain, numbness, or tingling
  • The loss of feeling in the arm or hand
  • Trouble lifting your arm or gripping your hand
  • Shoulder or arm muscle wasting

Surgery may be needed to repair the damage along with extensive rehabilitation to regain shoulder, arm, and hand strength.

Vascular Injuries

The cephalic vein is a blood vessel that runs beside the deltoid muscle and assists with circulation and fluid management within the muscle tissues. Any injury to the cephalic vein can result in fluid buildup in the upper arm, known as peripheral edema.

If not addressed promptly, peripheral edema caused by a cephalic vein injury can lead to nerve damage and the loss of muscle strength.

The thoracoacromial artery is another blood vessel that directly services all three deltoid heads. The artery typically runs through a groove between the deltoid and pectoralis muscles, but, in some people, the vessel tunnels through the deltoid muscle itself.

In such cases, any injury to the deltoid muscle can also damage the thoracoacromial artery, causing nerve injury, pain, spasms, and muscle atrophy (wasting) if not promptly treated.

What Is Involved in Rehabilitation?

If you have surgery or a severe injury involving the deltoid muscle, rehabilitation would typically involve an immobilizing shoulder brace to keep the arm still for at least two to three weeks. This gives the muscle ample time to heal and also prevents re-injury.

While the arm is immobilized, you would be encouraged to keep the elbow, wrist, and finger joints moving. Not doing so can lead to longer rehabilitation times.

You will likely start doing strength and range-of-motion exercises six weeks after surgery or injury under the guidance of your orthopedic surgeon or a physical therapist. An occupational therapist may be recruited to help you manage or regain abilities such as dressing, writing, driving, or throwing.

This rehabilitation timeline varies by your age, general health, the seriousness of your injury or surgery, the absence or presence of vascular or nerve damage, and how adherent you are to the rehabilitation plan.

How Do You Keep Deltoid Muscles Healthy?

Not surprisingly, exercise is the best way to keep deltoid muscles healthy and reduce the risk of injury. This may involve resistance and weight training or, in older people or those with physical limitations, stretching and rotational exercises to keep joints and tendons flexible.

Exercises beneficial to the shoulders and deltoid muscles include:

In addition, there are certain habits that can prevent repetitive stress injuries to the shoulder (including tendinitis and bursitis). This includes improving your general posture and not overdoing it with heavy weights at the gym or work.

Summary

The deltoid is the main muscle in the shoulder. It is made up of three heads called the anterior, lateral, and posterior deltoids. All three heads help move the arm and stabilize the shoulder joint. Each head also has different insertion points and enables different movements known as abduction, flexion, extension, and rotation.

The deltoid muscle can be injured or impacted by surgery either directly or indirectly. In such cases, rehabilitation may be needed to restore flexibility, range of motion, and strength.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. National Institutes of Health, U.S, National Library of Medicine: StatPearls. Anatomy, shoulder and upper limb, deltoid muscle.

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By Brittany Ferri
Brittany Ferri, MS, OTR-L, CCTP, is an occupational therapist, consultant, and author specializing in psychosocial rehab.