Lateral Deltoid: Functional Anatomy Guide

The lateral deltoid (L. latus, side ; deltoides, triangular) refers to the middle (side) head of the deltoid muscle. It is the outermost head of the deltoid and is primarily responsible for raising the arm to the side. This gesture is not very common in sports or in daily life. The role of the middle of the shoulder is mostly aesthetic given its curve and its size in relation to the torso. In other words, this head is the most impressive in terms of shoulder width. This is why the lateral part of the deltoid is highly valued.

In this post you will find out everything you need to know about the functional anatomy of the lateral deltoid – shape (appearance), function (muscle action), location, insertion, and origin.

Introduction

The shoulders refer to the deltoid muscles found on the top of the upper arm.

The roundish-looking muscle that caps the top of your upper arm is called your deltoid, and it’s the shoulder muscle you’re showing off when you wear a sleeveless shirt. It’s made up (composed) of three distinct sections (heads): front (anterior) deltoid, lateral (side) deltoid, and rear (posterior) deltoid. These heads originate on different points of the shoulder girdle but all converge on one common tendon that inserts on the humerus (upper arm bone).

Lateral Deltoid: Location, Shape, Function

The side deltoids (also known as the middle deltoids) or, as their scientific designation declares, the lateral deltoids, are located as one would guess, on the side of the shoulder when standing relaxed. In other words, this head of the deltoid muscle is situated between the anterior and posterior head, and lies superficial to the insertions of the supraspinatus, infraspinatus and teres minor. As you can see from the image, this head is also triangular in appearance.

The lateral (side) deltoid is involved in many of the same movements as the anterior deltoid: abduction (moving the arm away from the body), flexion (moving the arm upward to the front), and transverse flexion (moving the arm across the chest).

This head of the deltoid muscle handles most of the load when you push a weight straight overhead – the “vertical push” exercises, such as shoulder presses and upright rows. When you lift your arms straight out to your sides, the middle (lateral) and rear deltoids work hard, along with the supraspinatus, one of the four rotator cuff muscles, which lies beneath the deltoid.

The Origin and Insertion of the Lateral Deltoid

This head of the deltoid muscle arises (originates) from the acromion process and inserts into deltoid tubercle of the humerus.  Simply referred to as the acromium, this point forms the summit of the shoulder and offers an attachment for the medial deltoid before it comes together with all the deltoid muscles to attach to the humerus, just underneath the biceps. Its  main action is abduction of the humerus. It is the stron­gest of the three heads and is aided by the supraspinatus in abduction and limited by the tightening of the lower shoulder capsule. As the arm is abducted, the lateral deltoid contracts, while the anterior and posterior deltoid are stretched, steadying the limb and preventing any sideways motion.

Let’s summarise:

• Muscle name: Lateral (side; medial) deltoid;
• Location: On the side of the shoulder when standing relaxed;
• Shape: Tiny strip of muscle (triangular in appearance);
• Origin: Lateral border of the acromion process;
• Insertion: Deltoid tuberosity on the lateral midshaft of the humerus;
• Action (function): Shoulder abduction, flexion, transverse flexion;
• Nerve supply: Axillary nerve (C5-C6)

Other Names for Lateral Deltoid

• Side deltoid
• Medial deltoid
• Lateral head of the deltoid
• Outer deltoid

Related Muscles

• Anterior deltoid
• Posterior deltoid
• Rotator cuff muscles

Shoulder Training Tips

The shoulders are beautiful muscles when properly developed. They provide an appearance of width and also give the illusion of bigger arms. In order to achieve the three dimensional look that shoulders provide, all three heads (anterior, medial, and posterior) have to be developed.

The shoulders can be a very stubborn body part, especially if you don’t know how to train them correctly. Even a slight variation in an exercise’s proper form can mean the difference between no results and major results.

When training the deltoids, it’s important to be aware of which heads are working to produce a particular movement in order to get the desired training effect. Because the lateral deltoid is only one part of your deltoid and your shoulder is the most complex joint in your body, it is impossible to isolate the lateral delt entirely. But you can perform exercises that recruit the muscle fibers more than others. For example, when you peform lateral raises you will recruit lateral deltoids more, even though the anterior and posterior deltoids will work to.

Lateral Deltoid Exercises

There really are only two main side deltoid exercises (movements) you’ll need to include in your routine: (1) lateral raises (isolation shoulder exercise), and (2) upright rows (compoud shoulder exercise).

Keep in mind that lateral deltoids are also heavily involved in pressing shoulder exercises (such as dumbbell or barbell shoulder presses), especially if you perform them with the elbows out to the side.

Although dumbbells are classic exercise equipment, they are unfortunately not the best choice for lateral raises. We strongly recommend using machines over dumbbells to perform lateral raises. Therefore, machine lateral raise should be your new exercise of the first choice if you goal is to increase the size of your lateral deltoid heads (those on the sides of you).

Furthermore, your next best choice for lateral raises involves using a low cable pulley. The ideal scenario is to have a pulley where you can adjust the height. In this case, put the pulley a little above your knee so that the resistance provided by the cable comes well within the shoulder’s pulling axis. When the pulley is close to the ground, the resistance does not come from the side, which reduces the work of the deltoid.

Finally, we recommend skipping upright rows. They can easily cause severe shoulder impingement and wrist pain (or stiffness) in many people.

Machine and cable lateral raises will be more than enough to stimulate muscle growth in this section (head) of the deltoid muscle.

List of Compound and Isolation Exercises for Lateral Delts