The deltoid muscle is the large, triangular muscle that forms the rounded contour of the shoulder. This muscle group is divided into three distinct heads: the anterior (front), the lateral (side), and the posterior (rear). The posterior deltoid, or rear delt, is situated at the back of the shoulder joint. Understanding its specific location and function is important for achieving balanced shoulder strength and maintaining healthy upper body posture.
Anatomy of the Posterior Deltoid
The posterior deltoid is a thick, fleshy muscle located on the dorsal, or back, aspect of the shoulder girdle. It originates from the spine of the scapula, a prominent bony ridge on the shoulder blade. The fibers converge as they travel laterally toward the humerus, the upper arm bone.
All three heads of the deltoid share a common attachment point on the humerus called the deltoid tuberosity. This insertion point is a raised area found approximately halfway down the lateral side of the humerus. The rear delt works closely with other muscles of the upper back, including the rotator cuff muscles, and contributes significantly to the shoulder’s posterior contour.
The Role of the Rear Delts in Movement
The primary functions of the posterior deltoid revolve around pulling the arm backward and rotating the shoulder joint. Its main action is shoulder extension, which involves moving the upper arm straight backward behind the body. This movement is used during activities like rowing.
The muscle also plays a major part in horizontal abduction, which is the action of moving the arm away from the midline of the body when the arm is parallel to the ground. This occurs during movements such as the reverse fly. The posterior fibers assist in the external, or lateral, rotation of the shoulder. This rotational ability counteracts the internal rotation forces generated by the chest and anterior shoulder muscles. The coordination of the posterior deltoid with the rotator cuff is necessary to ensure the head of the humerus remains centered within the shoulder socket during various arm movements.
Practical Identification and Postural Significance
To identify the posterior deltoid, you can easily palpate the muscle on yourself. Reach across your body and place your fingers on the back of the opposite shoulder to locate the muscle mass. If you gently push your arm backward against light resistance, you will feel the muscle fibers contract beneath your fingertips.
The strength and function of this muscle group hold great significance for overall shoulder health and posture. In many individuals, the anterior deltoids and pectoral muscles become overdeveloped from common activities or training, leading to a muscle imbalance. This imbalance can cause the shoulders to pull forward, resulting in a rounded-shoulder posture. The posterior deltoid acts as a counter-force, helping to retract the scapula and pull the shoulders back into proper alignment. When the rear delts are weak, the tendency to slouch is exacerbated, increasing the strain on the upper back and neck. Maintaining the strength of the rear delts is important for stabilizing the shoulder joint and preventing the forward-rolled posture.