For both men and women, the deltoids, also referred to as delts, are a very desirable muscle to build. Well-developed shoulders can give the appearance of a smaller waist (because the shoulders look broader). Therefore, well trained delts improve the look of the entire physique.
The deltoid has three different heads: anterior (front), lateral (middle), and posterior (rear). Each head has similar functions, but they do have some variance.
The front delt originates from the outside portion of the clavicle, and inserts into the humerus.
The anterior fibers have multiple functions including: shoulder flexion (raising your arm directly out in front of you), shoulder horizontal adduction (put your arm out to the side of you, then draw your arm forward), shoulder internal rotation (hold your arm directly in front of you, now twist your arm until your elbow is pointed toward the ceiling), and shoulder abduction (raise your arm out to the side).
The anterior delts share some functions with the pectoral muscle. In fact, the front delts are involved in any pressing movement that you do. That is why many people feel their anterior deltoids when performing chest movements.
Defined middle delts give the appearance of broader shoulders, which creates the illusion of a smaller waist (a widely desired trait for men and women).
The lateral delt originates at the acromion (one of the bones of the scapula) and inserts into the humerus.
The lateral delt only has two functions, shoulder abduction (raising one’s arm out to the side), and assistance in shoulder flexion (raising your arm directly out in front of you). So, any time you raise your arm, the lateral deltoid is involved.
Also, because of the size of the middle deltoid (it’s a relatively small muscle), the fact that it shares its functions with the front delt, and its short contraction length (the delts carry high loads through relatively short ranges of motion) the lateral delt can be difficult to isolate.
The posterior deltoid originates from the bottom lower part of the scapula and inserts into the humerus. The posterior delt pulls backward on the shoulder joint (shoulder extension), horizontally abducts the shoulder joint (stick your arms out in front of you, then draw them back while keeping your arms horizontal), and shoulder external rotation (stick your arms out in front of you, and point your elbows toward the floor).
In short, the rear delts are involved in any type of pulling movement you complete. Unfortunately, the posterior deltoid is ignored by many. Not only do the rear delts add to the appearance of a physique, but they also add to the stability and overall health of the shoulder joint.
Author Michel Gundil found that, "bodybuilders have front delts that are on average five times bigger than sedentary people. Their lateral delts are three times bigger, and their rear delts are only 10 to 15 percent bigger. That imbalance translates into supraspinatus tendon abnormalities and is a source of shoulder pain."
Balancing the training of the anterior and posterior deltoids is extremely important.
Q: Which deltoid head should I focus on to give my shoulders that 'rounded look'?
A: It depends on your other training. If you do a lot of chest work then there’s no need to do isolation exercises for the front delt. In that case, you’d need to train the lateral and posterior heads. However, if you don’t do much chest work, then divide your time up evenly among the three heads.
Q: Is there a way to isolate my pecs from my front delts? Or vice versa?
A: The pectoralis and the anterior deltoid share the same functions, so no, they cannot be completely isolated from one another.
Gundill, M. Pressing Issues: Building better shoulders with overhead presses. Ironman. August 2002, pg. 42.