The deltoid muscle is the large, rounded muscle that forms the cap of the shoulder, giving it shape and allowing for a wide range of arm movement. This muscle group is composed of three distinct sections, often called “heads”: the anterior (front), the medial (side), and the posterior (back). This article focuses specifically on the posterior deltoid, commonly referred to as the rear delt, to clarify its location, function, and how to feel it working.
Anatomy: Where the Rear Delts Attach
The posterior deltoid is situated on the back side of the shoulder joint, completing the triangular shape of the overall deltoid muscle. A muscle’s location is defined by its origin (fixed attachment) and its insertion (moving attachment). The rear delt originates along the spine of the scapula, which is the prominent ridge of bone running horizontally across the shoulder blade.
From the scapula, the muscle fibers travel across the shoulder joint to their common insertion point. All three heads of the deltoid muscle insert onto the deltoid tuberosity, a slightly raised area located approximately halfway down the humerus (upper arm bone). This arrangement places the posterior fibers directly over the back of the shoulder socket, allowing the muscle to pull the upper arm bone backward relative to the body.
The Role of the Posterior Deltoid
The primary function of the posterior deltoid is shoulder extension, which moves the arm backward. This action is noticeable during pulling motions, such as rowing, or when the arm is driven behind the torso. The rear delt also contributes significantly to horizontal abduction, which involves moving the arm away from the midline while parallel to the ground.
A third role is external rotation, where the muscle helps turn the arm outward at the shoulder joint. These combined functions make the muscle a stabilizer for the shoulder, especially during movements involving lifting or carrying objects. Strengthening the rear delts is associated with improving overall posture, as they help counteract the forward-pulling force of the chest muscles.
Practical Guide to Feeling the Muscle
Identifying the contraction of the posterior deltoid requires focusing on the specific movements it performs. One effective way to isolate this muscle is through a reverse fly motion, using dumbbells or a cable machine. The goal is to move the arm backward and out to the side simultaneously, emphasizing horizontal abduction.
To ensure the rear delt is activating, use a lighter resistance and concentrate on driving the elbows up and back. A common mistake is allowing larger upper back muscles, such as the trapezius, to take over the movement. This can be minimized by keeping the torso stable and focusing on the contraction felt directly on the back of the shoulder. Movements like face pulls or a seated reverse machine fly are beneficial because they engage the external rotation component while minimizing the involvement of other muscles. A strong mind-to-muscle connection will improve your ability to feel the rear deltoid working.