Similar to the triceps, there are 3 heads to the deltoids; the anterior or front (right, in red), the middle (right, in green), also known as the lateral, the outer, or side, and the posterior (right, in blue) or the rear.
The anterior deltoid originates on the anterior surface of the clavicle (collarbone)
The middle or lateral deltoid starts on the acromion and spine of the scapula/shoulder blade (seen below),
The posterior deltoid starts on the spine of the scapula (seen below).
All 3 heads then come together and attach on the humerus (upper arm).
The deltoids are involved in nearly every single thing you do, including things like deadlifts. Although the shoulder does things like abduct the arm (humerus), rotate the humerus, aid in pressing and throwing, etc. the shoulder also has to work hard to keep the humerus from dislocating from the humeral socket. The shoulder is unique in that it allows for the greatest range of motion out of all joints in the body, however, this can also increase the risk of injury.
Every workout that involves the shoulders should begin with a thorough warmup to get the soft tissues that make up the stabilization and strength of the shoulder warm, as well as activating the muscles to ensure stability. Even on days when you are not directly working the shoulders, it is beneficial to do a routine like the following to increase the strength and durability of the entire shoulder girdle.
Take deadlifts, for example. Although you are not abducting your arms or pressing weight overhead, your deltoids need to work hard to engage the head of the humerus and prevent it from literally being pulled from its socket.
- Primarily used to abduct the shoulder when the arm is externally rotated
- Flexes the arm at the shoulder
- Internally rotates the arm
- Primarily used to abduct shoulder when the arm is internally rotated
- Primarily used to the extend the arm at the shoulder