The Deltoid Area: Soft Shoulder and Varied Terrain

It's often assumed that the tissue landscape of the human shoulder is entirely muscle. When we draw a muscular individual, we cover his or her shoulders with lumps and bumps that represent lots of rippling beef. It's easy to forget that the terrain in this area is actually more varied; if you look closely (and palpate, if the muscular individual doesn't mind!) you'll observe a wide flat area where bone comes right up to the surface. What we're seeing (and feeling?) there is the acromion process of the scapula, a flat horizontal process that, while serving as both origin and insertion points for muscles, is not obscured by those muscles. This bony landmark is even more apparent when the arm is abducted; the deltoid muscle is primarily responsible for arm abduction, and as it bulges out during this action, its contracted form around the flat acromion process makes the latter stand out even more clearly.

Let's take a look at the appearance of the acromion process in the photo below. Note how it remains flat while muscle tissue bulges out around it.

This individual's deltoid and trapezius muscles are contracted because he is raising his arm over his head. In between these muscles, we see the flat acromion process, which serves as a partial insertion point for trapezius and a partial origin point for deltoid. When the these two muscles are contracted, the acromion process of the scapula (labeled A.P.) becomes quite pronounced.

Now lets look at a simple diagram in which the scapula landmarks and the basic shapes of the surrounding muscles are shown:

Bony and muscular landscape of the shoulder in a raised arm.
A.P = acromion process; E.C.R.L. = extensor carpi radialis longus.

This image displays two exposed bony portions of the scapula, the acromion process and the spine. It also displays the surrounding visible muscles, including those that attach to these scapula landmarks, the trapezius and the deltoid. 

Varied Terrain: The trapezius muscle is primarily a back muscle and most of it cannot be seen in this image. But its upper fibers, those that attach to the scapula, can be seen here. This portion of the trapezius inserts directly onto the spine of the scapula, the acromion process of the scapula, and also the lateral half of the clavicle. It does not, however, obscure any of these bony landmarks. The deltoid, coincidentally, originates at all these bony landmarks-- and it doesn't obscure them either. So these bony features remain visible just under the skin. As we move from the neck, over the shoulder, and down onto the arm, there is varied terrain: First we have the softly curved sweep of the trapezius along the neck, then we have the acromion process of the scapula forming a hard flat area on the shoulder, and then we have the long convex curve of the deltoid forming the rest of the shoulder.

Merge: As we see above, the deltoid's origin is very wide; it originates under the entire length of the scapula's spine, the acromion process, and the lateral half of the clavicle! Its insertion on the lateral humerus, however, is very narrow. So the deltoid's fibers converge together and it narrows to sort of a point before it inserts. The wide origin and narrow insertion of the deltoid form a triangular shape-- hence the name deltoid, which means delta-like in shape (as in the Greek letter delta.)

Three Lanes: You'll also observe in the diagram above that the deltoid muscle has three distinct sections, each named for its origin point. The anterior portion of the deltoid originates under the lateral half of the clavicle, the acromial portion originates at the acromion process of scapula, and the posterior portion originates under the spine of the scapula. Remember, none of these bone features are obscured by the muscle, which is why we can see them on the surface the body.

This Way: The pointed insertion end of the deltoid is a nice orientation landmark because it helps us find another muscle-- the brachialis. The brachialis is fairly easy to locate anyway, because it lies directly under the biceps brachii muscle, which is easy to locate on the anterior surface of the upper arm. The brachialis is shorter than biceps brachii, but a little wider, so it can be seen peeking out on either side. It's easier to spot on the lateral side because a) it shows more clearly there, and b) as mentioned above, the insertion end of the deltoid points right to it. 

No Turns: The brachialis muscle is, like biceps brachii, an arm flexor, but it does not supinate the arm like biceps brachii. Biceps brachii can supinate the arm as well as flex it because part of it inserts onto the radius, the bone responsible for forearm supination and pronation. Brachialis, however, inserts onto the coranoid process of the ulna and pulls it proximally, thus flexing the arm but not supinating.

The only other muscle that can be seen on the lateral arm is the triceps. Triceps is a three-headed muscle (tri = 3, and ceps = heads) meaning it has three distinct sections that come from three different origin points. While these origin points are obscured by other muscles, the three heads of the triceps muscle are still easy to distinguish. As expected, the lateral head is visible in this view of the lateral arm. One last note: The triceps muscle is the widest upper arm muscle, so some portion of it can always be seen, even from a straight on anterior view. In the lateral view above, we can see the triceps most posterior, the biceps brachii most anterior,  and the brachialis sandwiched in between them.

We'll look at the triceps muscle more closely in a future post, not to mention the back, including the entire trapezius muscle. This is my last post of 2011, and I'd like to wish you all a happy new year! If you're headed out tonight, please travel safely and obey those road signs! See you in 2012!