The shoulder muscle is mainly the deltoid, a large, three-headed, thick, triangular muscle which originates from the clavicle and the scapula at the rear of the shoulder and extends down to its insertion in the upper arm and the trapezius.
Basic function: To rotate and lift the arm. The anterior deltoid lifts the arm to the front; the medial deltoid lifts the arm to the side; the posterior deltoid lifts the arm to the rear.
The trapezius, is the flat, triangular muscle that extends out and down from the neck and then down between the shoulder blades.
Basic function: To lift the entire shoulder girdle, draw the scapula up, down, and to either side, and help turn the head.
In the 1940s men wore coats with huge, padded shoulders and pinched waists, giving them an exaggerated V shape, they did this to make it look like they had shoulder muscle. Coincidentally, that is the shape that bodybuilders work very hard to develop, and a significant part of this look is wide, fully developed shoulders.
Steve Reeves was one of the first bodybuilders to develop the classic V shape with his shoulder muscle. He was able to achieve this look because he had naturally wide shoulders and a small waist. Proportions like these help create the most aesthetic physiques in bodybuilding.
Shoulder width is, to a great extent, determined by skeletal structure. That is something you are born with. A bodybuilder like Reeves, with his very wide shoulder structure, has an enormous advantage, especially when he is standing relaxed. Don Howarth, Dave Draper, and Frank Zane, all champions who began training around the time of Schwarzenegger, are other good examples of this wide, square-shouldered look. Kevin Levrone and Nasser El Sonbaty also possess wide shoulders.
There is another type of physique which is characterized not by narrowness through the shoulders, but by a "hanging" look. Reg Park was not narrow, but his traps and shoulders sloped downward. Schwarzenegger's shoulders have this same sort of hanging look, so they took much narrower when I stand relaxed than when executing a pose like a lat spread, where the real width becomes apparent. Watch Paul Dillett onstage and you'll see somewhat of the same structure.
The other factor involved in a wide-shouldered look or shoulder muscle such as the development of the side deltoids or lateral deltoids. When these muscles are fully developed, you get a very impressive display when they are flexed. Sergio Oliva and Tom Platz, for example, have tremendous shoulder development or shoulder muscle, yet do not look particularly wide and square when they are standing relaxed onstage. The ideal look for the competition bodybuilder is to have both a square bone structure and great side deltoid development. Look at Dorian Yates's shoulder muscle and you'll understand how valuable a structure like this can be.
Incidentally, bodybuilders noted for fantastic deltoid development are usually also known for enormous shoulder strength, Behind-The-Neck Presses with 225 pounds and up; Front Presses with 315, as both Sergio and Franco used to do; Ken Waller, with his powerful front deltoids, did Dumbbell Presses with 140-pound dumbbells.
But width, and the development of the side head of the deltoid is only one aspect of the total development of shoulder muscle. Shoulders also need to be thick, to show development in front and the rear, to tie in properly to the pectorals and the biceps as well as to the traps and the rest of the back.
The deltoids are extremely versatile. in order to move the arm forward, side-to-side, up and around, the deltoids have three distinct lobes of muscle called heads: the anterior (front) head, the medial (side) head, and the posterior (rear) head.
The deltoids play a prominent part in virtually every bodybuilding pose. They add to your width and size in a front double-biceps pose; to your muscularity in a most-muscular pose. The thickness and development of all three heads play an important part in poses seen from the side, such as the side chest shot or triceps pose. From the rear, the effect of a pose like the rear double-biceps is highly dependent on how much shape, separation, and definition you have achieved in the rear delts.
Your deltoid development should show definition and striations no matter what movement you make, while hitting all of the poses just cited as well as when you are moving in transition from one to another. There has to be an interconnection so that the three heads work together with all the adjacent muscles, thereby giving you a hard, muscular look.
But having complete deltoid development is also important while standing relaxed. From the front and from the back, good side deltoid development makes you look wider. In front, you should have complete separation of the deltoids and pectorals. For some people, this separation is natural; for others, it requires a lot of specialized weak point training. From the side, rear delt development gives you that "bump" in the back of your shoulders you see so clearly in great champions like Flex Wheeler or Dorian Yates, and both the rear deltoids and traps are extremely important when viewed from the back.
Of course, shoulder width and deltoid development are actually two different things. Steve Reeves, for example, was not particularly thick and massive through the delts, in spire of his great width. Conversely, Larry Scott, who in the 1960s won the first Mr. Olympia competition, exhibited thick, muscular deltoids whose massive development offset his natural rather narrow proportions. Shawn Ray's shoulder width is not exceptional, but you don't notice because his deltoids are so thick and fully developed.
Many bodybuilders with comparatively narrow proportions have been saved by great deltoid development. A good example of this is Reg Park. Reg worked very hard to compensate for relatively narrow skeletal proportions, and he ended up with enormous shoulder development. He was the first bodybuilder to bench-press 500 pounds, and this was possible only because of the size and strength of his front deltoids, which along with the chest and triceps work very hard in that lift.
One additional point worth making is that all of these champions trained very differently. Franco developed enormous front deltoid from all the pressing he did, so he had to add a lot of rear deltoid training to his workouts to achieve the correct balance. Larry Scott got his best results in shoulder training using the Stripping Method, starting with heavy weights and going lighter set after set to really burn the deltoids, 90 pound dumbbells down to 30 pound dumbbells. Dorian Yates spent a number of years doing a kind of high-intensity training that stressed relatively few sets but all kinds of intensity techniques like negatives, forced reps, forced negatives, and partial reps.
The point here, according to Schwarzenegger is that no two individuals have bodies that are exactly alike or will end up training any body part exactly the same. There is not a bodybuilder alive who has never had to adjust his training to overcome weak points in order to create a well-proportioned and balanced physique.