The deltoid muscle forms the rounded contour of the shoulder. This muscle is composed of three distinct heads, which are named based on their location: the anterior (front), the medial or lateral (side), and the posterior (rear) deltoid. Each head performs specific actions, allowing the shoulder joint a wide range of motion. This article focuses on the posterior deltoid, commonly called the rear delt, detailing its precise location and function.
Anatomical Placement of the Rear Deltoid
The posterior deltoid is a triangular muscle situated at the back of the shoulder joint. To understand its location, identify its points of attachment, known as the origin and insertion. The muscle originates along the spine of the scapula, which is the prominent ridge of bone running horizontally across the back of the shoulder blade.
From this bony origin, the muscle fibers extend diagonally forward and laterally toward the upper arm bone, the humerus. The insertion point for the posterior deltoid, along with the other two deltoid heads, is on the deltoid tuberosity, a rough, raised area located approximately halfway down the side of the humerus. The muscle’s path places it directly across the back of the glenohumeral joint, the main shoulder socket.
How to Identify the Muscle
The rear deltoid can be physically located and differentiated from surrounding muscles by using simple palpation and movement. To find the muscle, place your fingers on the back of your shoulder, just below the ridge of your shoulder blade. This area is medial to the side of the shoulder and lateral to the upper back muscles, such as the trapezius.
To confirm the location, perform a gentle movement test. Ask a partner to place light resistance on your forearm while you extend your arm backward, away from your torso. As you perform this action, you should feel the posterior deltoid muscle contracting and becoming firm under your fingers. This method helps to isolate the contraction of the rear deltoid from the nearby triceps muscle.
The Function of the Posterior Deltoid
The primary role of the posterior deltoid is to move the arm backward, a motion known as shoulder extension. This action is employed during everyday movements like pulling an object toward the body or pushing yourself up from a chair.
The rear deltoid also plays a part in horizontal abduction, moving the arm away from the midline of the body in the horizontal plane, such as during a reverse fly motion. Another function is external rotation of the shoulder joint, which turns the arm outward. Working with the rotator cuff muscles, the posterior deltoid contributes to shoulder joint stability by helping to keep the head of the humerus centered within the socket.