Friday, March 30, 2012

Posterior Torso Muscles, Part 2: Under the Radar

You may have noticed that I keep describing the forearm as the most difficult part of human anatomy to teach (and the most difficult to understand!) This area is so difficult for some pretty straightforward reasons: 1) There are so many forearm muscles, and 2) they all look very much alike. But sometimes I forget that the posterior torso muscles, although they are large and varied, are also among the most difficult to understand. In my class, they sometimes require as many days to get through as the forearm muscles. But the posterior torso muscles are confusing for a completely different reason-- unlike most muscles that concern the figure artist, they like to live under the radar.

Like most areas of the body, the posterior torso has several layers of muscle. When rendering most areas of the body, we artists tend to think primarily about the superficial muscle layers-- the ones that are visible at the surface and contribute most to the body's outward appearance. But things are a little different on the posterior torso. Its muscles are exceptionally broad, thin, and flat, which means we sometimes can see deeper muscles showing right through them. This is pretty cool, but it does make things a little more confusing.

In fact, sometimes the deeper muscles of the posterior torso show more clearly on the surface than the superficial muscles. How can that be? This happens because the superficial muscles are often so thin and flat that they just don't look like much on the surface. But the muscles underneath them are rounder and more defined, so it's actually easier to pick out their shapes. 

There are some wonderful examples of this in the figure renderings of Brian Skol. Brian is a student at the Ravenswood Atelier in Chicago, and during his time there he has developed a solid, stunning figure drawing technique and turned out an impressive collection of work. Brian also took my Anatomy course at AAA several years back, and while he is among several top students who mastered the class, he is distinctive in that he's the only one who's ever said the Anatomy final exam was not difficult enough! This comment was unexpected and refreshing, as I'm used to hearing just the opposite.

Several of Brian's figure drawings lovingly demonstrate the posterior torso landscape, but today (and in a few upcoming posts) we'll concentrate on this one. First let's look at the subtle ridges that help define the structure of the back.

Each of the skin ridges in Brian's figure drawing subtly reveals a muscular relationship on the posterolateral torso. Ridge 1 shows the point at which the latissimus dorsi meets the lumbar sheath. Ridge 2 demonstrates the furrow at which the external oblique muscle meets the rectus abdominis muscle. Ridge 3 shows where the serratus anterior muscle travels back toward its insertion on the underside of the scapula. (It's actually under latissimus dorsi at this point, but we can still see it because latissimus dorsi is so thin.) Ridge 4 shows the teres major muscle as it extends laterally toward the upper arm, where it will insert on the anterior side of the proximal humerus. The lower edge of teres major is hidden under latissimus dorsi, but it still shows. Finally, ridge 5 shows the posterior edge of the deltoid.

I know, I know. I did say subtle, right? But part of the beauty of the human body is that so much of it is a mystery; its structures are always there, doing their job, making things happen, but many don't necessarily need to be right in your face; they'd prefer to stay under the radar and let you discover them. 

So let's take a closer look at these shy little devils. The first step in this process was to block in some overall muscle shapes and a few bony landmarks, then to identify them.

Drawing in these basic structure shapes was the starting point for exploring each of the posterior torso ridges more closely. The iliac crest is shown in yellow but unlabeled. The large posterior torso muscles are trapezius (T) and latissimus dorsi (LD). Together with deltoid (D), they form a triangular window, in which we can see infraspinatus (Inf) and teres major (TM). Teres minor is barely peeking out between infraspinatus and teres major, but I didn't label it. Just anterior to latissimus dorsi, we can see two anterior torso muscles: external oblique (EO) and rectus abdominis (RA). I've also included some structures just under the iliac crest: Gluteus maximus (Gmax), gluteus medius (Gmed),  and tensor fasciae latae (TFL) all surround the greater trochanter of the femur (GT). Distal to that we can see vastus lateralis (VL) and the iliotibial band (ITB).

Now let's take a look at the ridges that are so beautifully and accurately rendered in Brian's drawing. We'll start with ridge number 1, where the lumbar sheath meets the latissimus dorsi muscle. First, here's how these structures look from a direct posterior view.

The lumbar sheath is a flat, diamond shaped aponeurosis covering a large portion of the lower back. It serves at an origin point for the large, bilateral latissimus dorsi muscles. We can often see a ridge along the lower back where these two structures meet. Note that both latissimus dorsi and the lumbar sheath have been removed on the right side of the body in this image.

You might recall this image from my first posterior torso post. At that time we looked at the relationships of the larger back muscles and the small triangular window they form (in which we can see some smaller muscles on the posterior surface of the scapula.) Now let's use this image to look at the landscape of the lower back. Occupying most of its space is the lumbar sheath, a diamond shaped aponeurosis (which is broad, flat, tendinous muscle covering.) The lumbar sheath serves as an origin point for the large bilateral latissimus dorsi muscles that run across the lower back and sides. Because the lumbar sheath is not muscle tissue, doesn't bulk up with use. So it can appear somewhat flat in comparison to the lumbar sheath. This means we can often see a ridge where these two structures meet. That's what we're seeing on the area labeled number 1 on Brian's drawing.

In this next image, the basic shapes of latissimus dorsi and the lumbar sheath are placed on top of Brian's rendering. This shows the placement of the individual structures and their meeting point. 

The lumbar sheath (shown in white) is an origin point for latissimus dorsi (LD). Because the lumbar sheath is flat and the latissumus dorsi has some bulk, we can often see a ridge on the back where the two meet. This ridge forms one of the upper edges of the lumbar sheath. The lower edges are formed by a small corner of the external oblique that reaches around to the back and attaches to the iliac crest.

We can see here that both the latissimus dorsi muscle and the external oblique form borders around the lumbar sheath. The angles of these borders give the lumbar sheath its diamond shape. On a more muscular individual, the flat lumbar sheath would stand out more obviously against the surrounding muscles and would become even more visible.

I am looking forwarding to explaining the other four ridges that can be seen in this lovely rendering, but this is getting a little long, so I'm going to sign off for now. I will cover ridges 2 through 5 in an upcoming post, and I'm also working on the rest of the elbow joint posts. See you soon! 


  1. Perfect timing. We are covering this material right now in class!

  2. Is there a post on the front of the ribcage and abs?

  3. Nothing on abs yet, but here is a rib cage post: