The deltoid muscle, located in the shoulder, is a large muscle group enabling diverse arm movements. It is divided into three distinct heads: the anterior (front), medial (side), and posterior (rear) deltoids. Each head contributes to the shoulder’s wide range of motion. This article focuses on the posterior deltoid, often called the “rear delt,” and its anatomical location.
Anatomical Placement
The posterior deltoid is positioned at the back of the shoulder, forming the rearmost part of the deltoid muscle. It originates from the spine of the scapula, a prominent bony ridge on the upper, posterior aspect of the shoulder blade.
From this origin, the muscle fibers extend downwards to insert onto the deltoid tuberosity, a roughened area on the lateral aspect of the humerus, the long bone of the upper arm. This attachment point is shared with the anterior and medial deltoid heads, forming a common insertion site for the entire deltoid.
It is positioned directly behind the medial deltoid and adjacent to the infraspinatus and teres minor, which are part of the rotator cuff muscles. This arrangement allows the posterior deltoid to work in conjunction with other muscles for shoulder movement.
Identifying Your Rear Delts
To locate the posterior deltoid, place your fingertips on the back of your shoulder, just below the prominent bony process known as the acromion. You should feel a distinct, rounded, and somewhat fleshy mass in this area.
To confirm you are touching the posterior deltoid, attempt a subtle muscle contraction. Gently push your elbow backward as if performing a reverse fly motion, without using external resistance. You should feel the muscle beneath your fingertips engage, becoming firmer and more defined.
This engagement confirms you have identified your posterior deltoid.
Role of the Posterior Deltoids
The posterior deltoid contributes to several shoulder movements. Its primary function is shoulder extension, which involves moving the arm directly backward, away from the front of the body. This action is used for activities such as pulling objects or reaching behind oneself.
It also contributes to the external rotation of the humerus, which means rotating the upper arm outwards, away from the body’s midline. This rotational capability is used for many daily tasks and athletic movements.
Additionally, the posterior deltoid is involved in horizontal abduction, an action where the arm moves outwards to the side while remaining parallel to the ground. This movement is characteristic of exercises like a reverse fly.
Beyond its role in movement, the posterior deltoid also contributes to the stability of the shoulder joint, helping to prevent dislocations. It assists in maintaining proper posture by counteracting the tendency for shoulders to round forward.