Deltoid Muscle: What to Know

Medically Reviewed by Jabeen Begum, MD on September 01, 2022
4 min read

Your deltoid muscles make up a main muscle group in your upper body. They’re connected with many other muscles and bones in your upper back, neck, and shoulder region and are extremely mobile. They keep your upper body balanced and help you perform everyday actions like lifting, pushing, and pulling.

What is the deltoid muscle? Deltoids are also known as shoulder muscles. They’re a very important muscle group that’s located where your upper arm meets your torso. 

Your deltoids are one of the most complex muscle groups in your body, and they’re made to move. They have minimal attachments to the bone, which means they can move in almost any direction without restrictions. This mobility also makes them one of the areas most prone to injury. 

Shoulder injuries are one of the most common reasons for a visit to the doctor’s office. Nearly 8 million people go to the doctor annually because of shoulder pain. Injuries to a part of the shoulder called the rotator cuff are the most common. 

About 67% of Americans will experience shoulder pain at some point. It’s usually due to injuries within the group of muscles and bones around the shoulder. This usually happens if you lift something improperly: for instance, at a strange angle. Injuries can also be caused by inflammation of the muscle tendons (tendonitis) or the growth of fluid-filled sacs near the muscle (bursitis). 

Fortunately, certain exercises and routines guided by a fitness instructor can help you keep your deltoids strong and flexible to avoid injuries. 

The deltoids give you the ability to move your arms almost any way you want. If your arm is hanging alongside your body, try moving your thumb inward toward your body, then outward. Your deltoids and surrounding shoulder muscles enable you to do that. 

Deltoids also provide stabilization to your arms and let you engage in functional movements like reaching, catching, and throwing. 

Each shoulder has three separate muscles that, together, compose the deltoid muscle parts: 

Anterior deltoids. When you move your thumb inward, you use your anterior deltoids. You also use them when you move your arm forward in front of your body. 

Lateral deltoid. If you move your arms out to the side, you are using the largest deltoid muscles: the lateral. 

Posterior deltoids. You use your posterior deltoids when you reach backward or rotate the shoulder. They’re often the smallest of the three because they can get less use than the other two. 

Your deltoid muscles are easy to find. They cover the very top of your arm. 

To locate your deltoid muscles, touch the side of your neck, then follow your shoulder line outward to the point where your arm slants downward. Deltoids are close to the surface of the skin, and most people can feel them. 

They’re attached to three different bones in your upper body: 

  • Collarbone (clavicle)
  • Shoulder blade (scapula)
  • Upper arm bone (humerus)

Your deltoid muscles start near your clavicle and scapula. The end of the muscles, or insertion point, attaches near your humerus.

There are other muscles and joints that support the deltoids, too. They all work together to support the movement of the shoulder and arm. 

Shoulder joints. Joints are points where two or more bones meet. Cartilage usually grows between the bones to absorb pressure and make motion comfortable and seamless. 

There are three major shoulder joints:

  • Glenohumeral joint (GH). This is the main shoulder joint that has a “ball and socket” and assists with deltoid muscle function. It holds the “ball” of the humerus in the “socket” of the scapula. 
  • Acromioclavicular joint (AC). Your acromion is the tip of your scapula bone. The AC joint attaches the scapula to the collarbone. 
  • Sternoclavicular joint (SC). This attaches the breastbone (sternum) and the clavicle. It helps strengthen the connection between the arm, shoulder, and the rest of the skeleton. 

Rotator cuff. The rotator cuff is a group of four muscles that assist the deltoids with moving the arm in different directions and help keep the shoulder centered. The four muscles in the rotator cuff are: 

  • Supraspinatus
  • Infraspinatus
  • Teres minor
  • Subscapularis

If your deltoid is hurting or feels strange, it could mean the area is strained or injured. Here are some signs to watch for: 

  • Limited range of motion in the deltoids or pain when you move a certain way
  • A tingling feeling
  • Numbness
  • Swelling 
  • Your shoulder hanging in an unnatural way
  • Stiffness
  • Muscle spasms 
  • Weak feeling in the muscle

All of these signs could indicate a deltoid muscle injury. You should seek medical care immediately if you have severe muscle swelling and pain or can’t move your shoulder or arm. 

Overusing or straining your shoulder can cause serious injuries. Here are some conditions that can commonly affect the deltoids. 

Strains. Deltoid muscle strains can happen from overuse. Over time, your muscles can become strained if you use them too much.  

Bursitis. Bursae are small sacs filled with fluid that stay between the muscle and bone. Bursitis is inflammation of these sacs that can cause pain or discomfort. 

Rotator cuff tear. This is one of the more common shoulder-related injuries. The deltoid and surrounding support system can also become damaged. 

Shoulder impingement. Impingement can be a very painful condition, occurring when muscle rubs against bone. 

Tendonitis. Tendons connect bones and muscles. Tendonitis occurs when that tissue becomes inflamed and painful. 

You’re more at risk for a deltoid injury if you play sports, lift weights, or perform repetitive arm movements. 

Here are some ways you can avoid injury, then, while also learning how to strengthen the deltoid muscle:

  • Warm up before exercise and stretch afterward.
  • Follow instructions when performing weight-bearing exercises.
  • Avoid using your deltoids if they’re sore from previous exercise. Give them time to recover.