Deltoids (Anterior, Middle, and Posterior Divisions): Learn Your Muscles

When you rub your shoulder with your hand, those superficial muscles that you feel are your deltoids. There are three muscles in the deltoid group–the anterior deltoid, middle deltoid, and posterior deltoid. The deltoids are one of the main muscles responsible for moving your arm at the shoulder joint. Here’s more about these three important shoulder muscles.

Location

deltoids

Thanks to Ground Up Strength for the image.

The origin for the anterior deltoid is at a spot called the anterosuperior border of the lateral third of the clavicle. The anterosuperior border is a combination of the directions anterior and superior. Anterosuperior means that the origin is on the top (superior) front (anterior) edge of the clavicle. For the anterior deltoid, the point of origin is on the top edge of the lateral third of the clavicle.

The origin for the middle deltoid is the superior surface of the acromion process. The acromion process is a bony extension of your scapula (shoulder blade). This little slip of bone is essential for shoulder socket stability. So, the superior surface of the acromion process is the top edge of the little piece of bone that comes off the side of your shoulder blade.

The posterior deltoid originates on the inferior (bottom) margin of the spine of the scapula. On the back of your scapula there is a bony ledge. This is called the spine of the scapula. So, the origin of the posterior deltoid is along the bottom part of the spine of your scapula.

The insertion for all three deltoid muscles is on the deltoid tuberosity of the humerus on the anterolateral surface just above the humerus’s midpoint. When we look at the word anterolateral, it looks like a combination of anterior and lateral. This means that the insertion point is about half way up your humerus on the part of the bone that is between the very front and the side.

Function

All three deltoid muscles do different things. This isn’t surprising since the anterior muscle faces the front, the middle one faces the side, and the posterior one faces the back. Basically, whenever your humerus moves at the shoulder joint, your deltoids are one of the movers.

The anterior deltoids perform abduction, horizontal flexion, and medial rotation of the humerus at the shoulder. In other words, the anterior deltoids help lift your arm straight to the side, reach your arm straight across your body, and rotate the head of your humerus toward the midline of your body.

The middle deltoids perform abduction of the humerus at the shoulder. With its location right on the side, it makes sense that this muscle’s main job is to lift your arm straight to the side. We already know that the supraspinatus is responsible for about the first 10 degrees of lift. The middle deltoid is responsible for the rest to get your arm lifted to 90 degrees.

The posterior deltoids perform abduction, horizontal extension, and lateral rotation of the humerus at the shoulder. In other words, the posterior deltoids help lift your arm straight to the side, reach your straight arm behind your body, and rotate the head of your humerus away from the midline of your body.

Common Dysfunction

According to Yoganatomy.com, “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.”

Symptoms of deltoid strain involve everything from muscle tightness to slight pain when weight bearing with the arms to a complete inability to move your arms.

Restoring or Maintaining Health

If you suspect that you have injured or torn your deltoids, contact your doctor. Your doctor can order all of the appropriate imaging, therapy, and medicine necessary to help you recover.

However, if you’re looking for exercises to help keep your deltoids strong and healthy, here are some suggestions.

  • Almost any time you are weight-bearing with your arms you will strengthen your deltoids. Some great anterior strengtheners are Plank, Crow pose, Handstand, and Downward facing dog. Try Side plank to strengthen the middle deltoids, and flip it around and try a Reverse plank or a Crab position to strengthen the posterior deltoids.
  • To stretch your deltoids, practice Eagle pose. Remember that all you have to do to stretch a muscle is to move it the opposite way of its action. So, whatever the function is that’s listed above, do the opposite to stretch that muscle.

More Information

I consulted The Concise Book of Muscles by Chris Jarmey. Recently, the book was revised and is in its third edition. I love this book as a quick go-to guide for easy to understand anatomy.

For those who are more interested in technical terminology and smaller muscles, I recommend Flash Anatomy Muscles Flash Cards. Any time a client comes to me with pain, I use these flash cards.

Also, I consulted David Keil’s article on the deltoids at Yoganatomy.com.

What’s your favorite way to strengthen your deltoids? Let us know in the comments below.

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About Sarah Stockett

Hi! I'm Sarah, and I'm a certified Pilates and yoga instructor with a passion for pain relief. When I'm not working with clients, I'm researching the best ways to get rid of pain. Do you want to learn how to practice yoga and Pilates safely in your own home? Or, do you want to know all my tips and tricks for pain relief? Join my mailing list and receive free goodies to help you.

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