The Dorsal Forearm, Part 2: Which Side Are You On, Anyway?

Now that we’ve established a method for identifying the two compartments of the human forearm, let’s look at the dorsal compartment a little more closely. The dorsal side of the forearm is the "top" side-- the side that usually faces upward and is generally darker and hairier (due to more melanin and a greater number of hair follicles.) The muscles on the dorsal side of the arm (unlike those on the ventral side) are arranged in a single layer, so they all lie directly under the surface of the skin. This means most of them can be seen pretty clearly on a well defined individual, although their relative visibility will depend on the position of the hand. The muscles in this compartment are all part of the extensor/supinator muscle group, which means they either extend the wrist or fingers or supinate the forearm, or both.

When considering which muscles will show in a given position, we want to keep in mind what the muscles in a specific area tend to do. We are looking at the extensor/supinator group today, so it's safe to assume that these muscles can be seen more clearly when we are supinating the forearm (which means turning the palm upward) or extending the wrist or fingers (which means opening them up.) It's also helpful to remember that muscles on the radial (thumb) side of the arm tend to pull the hand in the direction of the radius (or abduct it) and muscles on the ulnar (pinky) side of the hand tend to pull the hand in the direction of the ulna (or adduct it.) As such, radial side muscles tend to show more when the hand is abducted and ulnar side muscles tend to show more when the hand is adducted. Knowing these four movements will help us to remember which hand positions will make which muscles stand out.

So... here is an overview of what we'll cover today:

When identifying muscle shapes on the forearm (or anywhere else on the body, for that matter) it’s best to locate the most obvious structures first and then find the other structures based on their relationships to the former. Today we'll look at the ulnar (pinky) side muscles, because I think they're easier to find first. Once we've established their location, we'll go on to the radial (thumb) side in the next post.

When becoming oriented on the dorsal forearm, I usually begin with what I like to call the twin muscles-- two muscles the seem to look more alike than any of the others. They are often the first to make themselves evident when looking at the dorsal forearm. The twin muscles are extensor digitorum and extensor carpi ulnaris. They both originate at the lateral epicondyle of the humerus (the bony bump on the lateral side of the elbow) they are about the same width, and they both run straight down the dorsal side of the forearm, toward the hand. Find them below in both the anatomical rendering and the photograph, and notice how similar they are.

The dorsal forearm "twin" muscles, extensor digitorum and extensor carpi ulnaris, look very much alike. We can distinguish one from the other by remembering that extensor carpi ulnaris is closer to the ulnar (pinky) side of the arm. 

Please note that the elbow position in the photo does not exactly match that in the diagram. But for our purposes, we can see what we need to see: The twin muscles and the furrow where the two muscle groups meet (shown with a dashed line.)

OK, so now that we’ve found the twin muscles, the next step is to distinguish them from one another. This is easy. The twin muscles are extensor digitorum and extensor carpi ulnaris. It makes sense that, of the two, extensor carpi ulnaris is closer to the ulnar side of arm. It actually lies right next to the ulna (whose location is designated by the furrow on the forearm, marked with a dashed line in the photo.) Once we've positively identified extensor carpi ulnaris, we can assume the other twin must be extensor digitorum.

The next muscle we'll locate is extensor digiti minimi. As its name tells us, this muscle extends the small finger (digiti minimi is Latin for smallest finger.) Extensor digiti minimi is very easy to locate because it's very narrow and lies right between the twin muscles. On the anatomical diagram, you can see that the tendon of this muscle is headed straight for the smallest digit, where it will insert on its dorsal side. This tendon is often visible on the dorsal hand. You can see an example of this in an earlier post called The Dorsal Hand: The Dorsal Foot's Better Looking Sibling. The image below demonstrates the location of both extensor digiti mimimi and the last muscle we'll cover today, anconeus.

Location of extensor digiti minimi and anconeus. 

Anconeus is somewhat easy to remember, because it's the shortest muscle in the extensor/supinator group and has the shortest name. It is triangular in shape and can be found running from the lateral epicondyle of the humerus to the proximal end of the ulna. As mentioned in the last post, it can also be found by following the crease along the ulnar side of the arm (the one that divides the two forearm muscle groups) proximally (which means upward, toward the body) until it suddenly ends. When it ends, you'll see the triangular shape of anconeus.

Here is one last image to help you visualize more clearly the four muscles we covered today. This is a repeat from part 1 and shows outlines of all the muscles in the dorsal compartment. Look closely at those we covered today: extensor carpi ulnaris, extensor digitorum, extensor digiti mimini, and anconeus. Ignore the rest for now; we'll get to them soon.

Next time we'll find the last three muscles in the dorsal forearm, those that run along the radial side. They are a little more difficult to spot, but if were start by locating the twin muscles, everything else falls into place pretty clearly.

There are also three muscles at the distal end of the dorsal forearm, but those move the thumb around, and most of what we see of them is their tendons, which show up on the wrist. So we'll cover those in a wrist post. Thanks again to my model, Shannen. See you next time!

The Dorsal Forearm, Part 1: Compartment Search

While I'm very happy to get to the first dorsal forearm post, let me first apologize for the long hiatus I've inadvertently taken. In addition to teaching at the American Academy of Art, I am also on the part time faculty at the Biomedical Visualization program at the University of Illinois at Chicago, a fabulous graduate program for future medical illustrators, animators, and anaplastologists. Although it's kept me from my blog for the past few weeks, I've very much enjoyed reading and editing many compelling research papers that this year's graduating BVIS students are finishing up. I can't give away their content, but let's just say it's amazing to see what can be done with MRI data and 3D modeling and animation applications!

The dorsal forearm is one of my favorite lectures, as its intricate and complex musculature is unmatched anywhere else in the body. At the Academy, we devote three entire classes to just the forearm; it deserves a slow and thorough explanation. I can't fit even the dorsal side into one post here, so I'll be dividing it up into a few.

Muscles in the limbs are categorized into groups (which are defined by function) that reside in muscle compartments (which are defined by location.) The forearm has two muscle compartments, the dorsal compartment (in which the extensor/supinator muscle group resides) and the ventral compartment (in which the flexor/pronator muscle group resides.) I wish I could say the dorsal and ventral compartments were placed perfectly dorsally and ventrally on the arm, but alas, they are not. Each spills over onto the other side a bit. This means while most extensor/supinators are seen on the dorsal side of the arm, a few can be seen peeking over onto the ventral side, and while most flexor/pronators are seen on the vental side of the arm, a few can be seen peeking over onto the dorsal side. 

Have I lost you yet? Agh, I know this verbal explanation is really dry, so lets try a diagram. Click for an enlarged view if necessary!

Above we see dorsal (top side) and ventral (underside) views of the right forearm. We also see the location of the extensor/supinator muscle group (shown in blue) and the flexor/pronator muscle group (shown in green.) Notice how we see mostly extensor/supinators on the dorsal side, but a few flexor/pronators creep over there. Similarly, we see mostly flexor/pronators on the ventral side, but a few extensor/supinators creep over there. It's like they each know where their home is, but they can't resist peeking around to the other side.

Since the two forearm muscle groups are not clearly divided between the ventral and dorsal sides, we use other landmarks to find their borders. On the dorsal side of the arm, the crest of the ulna (the distal end of the bone that can be seen and felt on the surface of the arm) is the border between the two muscle groups. On the ventral side, the biceps brachii tendon (coming from the biceps brachii muscle on the anterior upper arm) is the border between the two muscle groups.

When observing the muscles of the forearm, the best way to become oriented is to first figure out where the two muscle groups (and the borders between them) are. As such, when observing the dorsal forearm, the first structure we want to identify is the ulnar crest. This actually doesn't look like much on the surface, but it's easy to find A) because we can usually feel it, and B) because it merges with a nice little furrow at its proximal end.

In the above photograph, the arrow is pointing to the crease along the ulnar crest. Everything above this in the photograph is the extensor/supinator group, and everything below this is the flexor/pronator group.

In this photo, the dashed line follows most of the furrow along the ulna distally until we reach the head of the ulna (the little bump on the pinky side of the wrist. The furrow stops suddenly at its proximal end because at that point we run into the anconeus muscle, a small triangular muscle between the lateral epicondyle of the humerus and the olecranon process of the ulna (what we normally think of as the elbow.)

The area in blue (above the red dashed line) is the extensor/supinator group. The area in green (below the red dashed line) is the flexor/pronator group. Muscles are outlined and labeled:  BRR (brachioradialis); ECRL (extensor carpi radialis longus); ED (extensor digitorum); EDM (extensor digiti minimi); ECU (extensor carpi ulnaris); Anc (anconeus); and FCU (flexor carpi ulnaris) which is the only muscle from the flexor pronator group that can be seen from this view.

In the above image, we can still see the division between the two muscle groups (shown with a red dashed line) but we can also see outlines of muscles within each compartment. Notice how the division line runs right along the ulnar crest and ends distally at the head of the ulna. Notice also how the proximal end of the division line ends at the triangular anconeus muscle.

Now that things are a little more clarified, lets revisit the unembellished arm photo and take another look at the muscles shapes.

So I think that's all we'll cover for today. Keep in mind that these early posts tend to show the body in odd positions because we are still covering straightforward anatomy, and these positions are meant to help pronounce all structures in a given area. Later on we'll observe the arm (and the rest of the body) in more natural poses and note what structures are seen then. 

Dorsal forearm part 2 to come soon! At that point we'll look more closely at the individual muscles and learn to tell them apart. Thanks to my forearm model, Shannen!