The Posterior Torso Muscles: Let's Go *Back* in Time. Get It?

Hello, all! My apologies for the long wait on this back muscle post, but my search for suitable posterior torso photos has been frustrating to say the least. Let's just say it's not easy to get friends, male or female, to pose topless for me. They're just not those kind of people. At least not since college. But my quandary serendipitously coincided with a massive office clean up during which I came across a dusty old hardbound book. The kind that you know used to have a paper jacket but now has only a cloth cover with gold embossed lettering. The title? Atlas of Anatomy for Artists. Copyright 1947, by Dover Publications, Inc. A true relic of the past!

You know the type of archaic artist's anatomy book I'm talking about-- one color printing on thick creamy paper, formal diagrams labeled entirely in Latin, black and white photos in which the models are haunted, anemic looking creatures with little black triangles covering up their genitalia. Or in which the genitalia are awkwardly airbrushed out so the model, no matter which sex, has the anatomy of a Ken doll. Or in which the model, wishing his or her identity to be hidden, is wearing some sort of blindfold, giving the whole thing a vague interrogation quality. This awkward modesty may seem quaint to us now, but back then it was a necessity. That type of anatomical detail would have earned a book a place in the XXX section of the local paperback grotto.

Is it an art anatomy book or a an advertisement for Interrogation Barbie and Firing Squad Ken? In 1947, it wasn't easy to tell the difference.

The quotes in this book are priceless. One reads "The anatomy of the female differs from that of the male in that the fatty tissue is better developed in the thighs and buttocks." Um, I guess that's one way of putting it. Another gem: "Plate 100 shows a well-built female with almost normal proportions." I'll bet that description made the model's day. Oh, Mr. Author, she might look completely normal if she didn't have one of your old gym socks tied around her eyes.

But I haven't gotten to the best part. Inside the front cover is the handwritten name of the original owner and her art school: Helen Davis, American Academy of Art, Chicago, Ill, 1950 – 51. My mom! Yep, my mom not only went to art school in the early 50s, but she went to the same school at which I teach anatomy today. I find this pretty cool.

My mom's name written in her first art anatomy book. Must ask her who this Buck person was. Did my father know about this?

In any case, the unearthing of this book from my 12 years of accumulated office junk came just in time. I've been wanting to do a post about back muscles, but I've been having a very difficult time finding photos that will show you all the visible surface landmarks. It turns out this book had several nice, clear (albeit archaic looking) posterior torso shots. I don't think they're quite enough, but we can at least begin here.

Shall we start with a quick overview?

On the left side of this figure, only the larger back muscles, trapezius and latissimus dorsi, are shown. We can also see the detoid, a large shoulder muscle on the side. The borders of these three muscles form a little triangular window through which three small muscles peek. Those muscles are not shown on the left side, however. On the right side we see them completely exposed. The blue dashed line indicates the triangular window through with they peek.

The posterior torso, like most areas of the body, is covered by several layers of muscle. But the surface landmarks her are a little more complicated than in other areas because the most superficial back muscles are so thin... and this means many of the deeper back muscles show at the surface as well. It's going to take more than one post to explain this, so we'll start today by looking at the superficial muscles only.

In order to become oriented on the back, it helps to find the largest muscles first and then move on to their smaller counterparts. The two largest muscles in the back (that, in fact, together cover most of the back) are the trapezius muscle and the latissimus dorsi. 

The trapezius is a diamond shaped muscle that covers most of the upper back. It's long midline origin attaches to nineteen vertebrae!-- C7 all the way down to T12. Its two lateral points attach to either scapula (the spine of each scapulae, to be specific,) its upper point attaches to the occipital bone on the posterior cranium, and its lower point attaches to the spinous process of the T12 vertebra.

The latissimus dorsi is a bilateral muscle that covers most of the lower back. It originates at the lumbar sheath, which attaches to the lumbar vertebrae and part of the sacrum. The latissimus dorsi's fibers converge as it travels upward toward its insertion on the proximal anterior humerus. As it reaches for the humerus it forms the back wall of an underarm depression called the axilla (which is more commonly known as the armpit.)

On this figure's right side, we can see the three posterior scapula muscles that peek through the triangular window formed by the larger muscles. These muscles are infraspinatus (whose name indicates its location inferior to the spine of the scapula) teres minor, and teres major. Teres minor, the smallest and deepest, tends to get squeezed out of the picture more often than not.

 The borders of the trapezius, the latissimus dorsi, and the deltoid (the large muscle on the shoulder) form a nice little triangular window on the posterior torso. Through this window peeks three smaller back muscles, infraspinatus, teres minor, and teres major. These muscles lie on the posterior scapula and are mostly used to adduct and rotate the upper arm. The key to finding these three posterior scapula muscles is first locating the triangular window formed by deltoid, trapezius, and the latissimus dorsi, and then looking inside it. 

Of these three posterior scapula muscles, the most superior (infraspinatus) and the most inferior (teres major) show most clearly, and they are the largest and most superficial. Teres minor, the smallest and deepest of the three, tends to get squeezed out of the picture. As a surface landmark, it often reads as a small crease or dimple between the other two.

Now let's try placing these on one of our 1947 models...

Man, just think. This is someone's grandpa now. Notice the long, diamond shaped tendinous floor of the trapezius, which itself can often be seen clearly as a surface landmark, particularly when you're as buff as this gentleman. Notice also that the lower tip of trapezius attaches to the spinous process of the T12 vertebra. Visible bony landmarks include the lateral end of the clavicle, the acromion process, spine, medial border, and inferior angle of the scapula,  and the iliac crest.

The muscles shown in the previous diagram are all shown bilaterally in this image. So we can see the larger muscles with their triangular window on both sides, as well as the posterior scapula muscles peeking through. Notice that the latissimus dorsi overlaps teres major a little bit. Keep in mind, though, that it does not obscure teres major, because it's so thin. If you're unable to locate the muscle windows in the above image, take a look at this:

In this image, only the muscle window has been given a color overlay. Infraspinatus, teres minor, and teres major peek through this window.

The blue area indicates the window formed by deltoid, trapezius, and latissimus dorsi. It's within this window that we can see infraspinatus, teres minor, and teres major. We will look at this more closely in our next post, as well as quite a few other visible muscles that I haven't even mentioned yet. Also, there is so much variation in the appearance of back muscles that I'd like to show a lot more photos! And, after all, there are lots more 1947 nudes to peruse. See you next time.

Sternocleidomastoid: Don't Forget the Cleido!

Just a quick landmark sighting today, and this time on an illustration instead of a photograph. We've already had a close look at the sternocleidomastoid muscle and its surrounding structures in an earlier post, The Anterior Neck: Theme and Variations. But this wonderful rendering by an old pal made me want to revisit Mr. SCM briefly. Shawn Campbell is a talented and prolific artist whose seemingly endless turnout of illustrations have been a great source of inspiration to me ever since we met in Ms. Brackman's seventh grade art class. Thirty-plus years later, Shawn and I continue to share our friendship, our ideas, our rants and, of course, our art, with one another.

One thing I like to stress in my anatomy class is that knowledge of human skeletal and muscular structure is not only useful when drawing realistically but also when drawing a human (or humanoid) with stylized or exaggerated features. Shawn's drawing below demonstrates this beautifully. Let's take a look:

This head drawing, and all of Shawn's figure and portrait studies, show his knowledge of the human form and his ease and comfort in rendering it. Even in this exaggerated head study, it's clear that Shawn understands the structure of the skull, the shape relationships of the external ear, and the nuances of the anterior neck muscles. One anterior neck muscle in particular, the sternocleidomastoid, caught my eye here.

The sternocleidomastoid muscle is named for its points of origin and insertion; its name has three parts (sterno-cleido-mastoid) because this muscle has two origin points and one insertion point. The sternocleidomastoid muscle's origin points are the superior edge of the sternum (sterno-) and the medial end of the clavicle (cleido-) and its insertion point is the mastoid process (-mastoid) which is a bony lump on the temporal bone than can be felt just posterior and inferior to the ear. The bilateral sternocleidomastoid muscles grab the mastoid processes and, among other actions, allow us to turn our head from side to side.

But or some reason, the poor clavicular origin point of the sternocleidomastoid muscle is too often neglected by figure and portrait artists. We all seem to know about the sternal attachment-- most likely because it's more visible-- and we tend to draw it very clearly (sometimes even too clearly.) But very often we completely leave out the clavicular attachment. Which is why I loved this rendering of Shawn's. He didn't forget the clavicular attachment! 

Let's look at the figure again below, but on this one (with Shawn's permission) I've added a little diagram:

The sternocleidomastoid muscle is a bilateral structure, meaning there are two of them-- one on either side of the body's midline. The two sternal attachments of the muscle connect to either side of the jugular notch at the superior edge of the manubrium of the sternum. (The jugular notch is also known as the suprasternal notch, as suprasternal means above the sternum.) This is the attachment we almost always remember to draw. 

The clavicular attachment, however, is often overlooked. As you can see here, it connects to the clavicle and it is generally wider and flatter than the sternal attachment. So from now on, don't forget to draw this lovely little portion of Mr. SCM!

One last note: Your common carotid artery runs right through the little split between the sternal and clavicular attachments of the sternocleidomastoid muscle. So press your finger in there if you want to feel your carotid pulse. Sometimes you can even see your carotid pulse on this spot. That's pretty cool.

Thanks so much to Shawn for letting me use his illustration. If you'd like to see more of Shawn's work, take a look here. I have another old school buddy helping me out with what I hope is the the next post, back muscles. See you then!

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!

Anatomical Terminology 101: Baby Needs Some Direction

This blog is seven months old now and has slowly and imperceptibly transitioned from its infancy to its... um... toddlerhood? Not sure how to complete that metaphor except to say that my baby is beginning to stand on its own two feet and get around by itself. It's getting hits from all over the world without my having to hold its hand! Ah, now I'm all misty. (ahem) OK, in any case, now that we're standing on our own, maybe this is a good time to go over some basic anatomical terminology that describes overall  direction and location on this newly upright body. 

The first day of my Anatomy course at the American Academy of Art is typically spent going over direction and location terminology-- words that allow us to describes positions of anatomical structures and their relationships to one another without having to rely on pointing. This can be useful when visuals aren't an option. In addition, it's more accurate (not to mention more eloquent) to describe a structure as residing "on the distal end of the ulnar forearm" than "way down on the end of the arm, on the side by the pinky finger."

Let's start with a look at the body's midline, which is an imaginary line that runs down center of the body from either a front or back view; it's a line on either side of which we are basically symmetrical. While no one's body is perfectly symmetrical, most of us have the same basic form mirrored on either side of the midline: We have two ears, two eyes, two arms, etc. As such, there is no midline running down the side of the body, as the front of our body is different from the back.

The above image is an anterior view of the human body. The anterior side of the body is basically the front-- everything from the forehead down to the toes. The back of the body is known as it posterior side.  The posterior side includes everything from the back of the head down to the heels.

Our next two words, medial and lateral, are based on the concept of the midline. Medial and lateral are comparative terms that define a structure's location relative to the midline. A structure that is more medial is closer to the midline, and a structure that is more lateral is farther from the midline. So we might say that the corner of the mouth falls about two centimeters lateral to the midline. (Of course, every structure is lateral to the midline, because the midline is as medial as we can get.)

These terms occur more commonly in anatomical structures that come in pairs, such as the medial and lateral epicondyles of the humerus bone. The epicondyles of the humerus are bumps on its distal end, which is the end of the humerus near the elbow. (The terms distal and proximal will be explained later in this post!) The medial and lateral epicondyles are named as such because they are similar structures that need to be distinguished from one another. This happens a lot in human anatomy-- two or more structures will be very similar but not quite the same, so their names will be similar, too, but with one distinguishing qualifier. In the case of the epicondyles of the humerus, the qualifiers are medial and lateral. We have a medial epicondyle, which is the one closer to the midline, and a lateral epicondyle, which is the one one farther from it. 

These terms are used elsewhere in the body as well, such as the medial and lateral malleoli on the ankles (bumps on the tibia fibula) and the medial and lateral canthi (corners) of the eye. Now that you've read the last paragraph, you should be able to tell which is which.

You'll notice I also pointed out the terms anterior and posterior in the image caption above. The anterior side of the body is basically the front-- everything from the forehead down to the toes. The posterior side of the body is the back-- everything from the back of the head down to the heels. Both images in this post show anterior views of the human body.

Two more terms, superior and inferior, define relative position on the body in a different way. The superior end of the human body is the top of the head, and the inferior end is the bottoms of the feet. So a structure that is more superior is closer to the head, and a structure that is more inferior is closer to the feet. One example of the use of these terms is in the structure names superior vena cava and inferior vena cava. The vena cavae are the largest (and most cavernous) veins in the human circulatory system. They are named superior and inferior because one drains into the top of the heart (superior to it) and the other drains into the heart's underside (inferior to it.) We also use the roots supra- and infra- within anatomical terms to define relative structure positions. For example, the supraclavicular fossa is a depression in the skin just above the clavicle, and the infraorbital foramen is a hole in the maxilla just inferior to the orbit.

The last two direction and location terms we'll cover today are proximal and distal. These words describe relative location on a limb. We can't use superior and inferior for this, because that would change depending on the position of the limb. So we use a terms that don't depend on that: No matter what the position of the limb, there is always one end closer to the torso and one end farther from it. The proximal end of a limb is that closer to the torso. (The root prox- means near, and we see it in other terms such as approximately and proximity.) The distal end of a limb is farther from the torso. (The root dist- means farther or more distant.) So the proximal end of the arm is up by the shoulder, and the distal end of the arm is the fingertips. The leg's proximal and is up by the hip, and its distal end is at the toes.

We can also apply proximal and distal to individual structures in the limbs. For example, the femur has a proximal and and a distal end. And the tibia has a proximal end and a distal end. And we'd describe their relationship by stating that the distal and of the femur articulates with the proximal end of the tibia. 

Well, I've had enough of this, how about you? There are plenty more direction and location terms to cover, but let's save it for later. I'm thinking about getting a babysitter and skipping ahead to a shoulder post next... which would be the proximal end of the arm, right? See you next time.

Landmark Sightings, Part 1: Bruce Lee

We just finished the arm portion of my fall anatomy classes, so I thought I'd get Bruce to help me with a little recap. First the image with some muscle overlays. Not a great deal of detail here-- just the basic shapes.

Behold Bruce, a fine source of landmark sightings. Click for a full view if necessary.

Now the image with labels but without the muscle overlay:

Worth noting: Muscle striations can be seen in the deltoid muscle. The extensor carpi radialis longus muscle bulges out more than any other on the forearm, so it's casting a deeper shadow than the rest. Extensor digiti minimi, a very thin muscle that extends the pinky finger, can be seen clearly between extensor digitorum and extensor carpi ulnaris. On the anterior upper arm, the cephalic vein can be seen popping out on the biceps brachii muscle, which it runs over just before entering the deltoid furrow, a crease between the deltoid and pectoralis major muscles. On the posterior upper arm, the division between the lateral and longs heads of the triceps muscle also shows clearly. This isn't usually the case, but Bruce is quite defined!

Image courtesy CompleteMartialArts.com. Thanks!

Anterior Leg, Part 2: It's Lonely at the Top

Now that we've covered the skeletal foundation of the lower leg, we're free to move on to its musculature with reckless abandon. First, some general information: Muscles in the lower leg (and in all limb sections) are grouped into compartments, each separated from one another by an enclosing layer of fascia. Within each compartment is a specific muscle group. Muscles compartments are typically named by location (anterior, posterior, medial, lateral, etc.) while muscle groups are named for their function (adductors, flexors, extensors, etc.) It makes sense that muscles within the same functional group fall within the same physical compartment because muscles would have to have similar origins and insertions (and thus similar locations) to have similar functions.

The lower leg has three muscle compartments-- the anterior, the posterior, and the lateral. In each of these fall muscle groups, each with its own functional purpose: In the anterior compartment we find the foot extensors and dorsiflexors; in the posterior compartment we find the foot plantarflexors, and in the lateral compartment we find the foot everters. A later post will elaborate on these movements. 

Today we'll be discussing the anterior compartment of the lower leg, but only its muscles that actually appear on the lower leg. Some muscles in the anterior compartment, while they lie in the lower leg, don't show up on its surface. Their tendons may show, but they don't surface until they've already reached the foot. Those particular muscle tendons are discussed in The Dorsal Foot: How Do I Love Thee? Let Me Count Your Tendons. The tibialis anterior, it turns out, is the only muscle whose body can clearly be seen on the anterior surface of the lower leg. It's on top of everything else, and it stands completely alone. 

DO YOU KNOW WHAT THIS MEANS??? It means that after this long, long, boring introduction, we're only going to cover one muscle today-- the tibialis anterior. As you may remember, I was planning to cover the tibialis anterior muscle last time but quickly realized it was impossible without first going over the lower leg bones; although tibialis anterior stands alone muscularly, its relationship with the tibia is the key to its identification.

So... let's start with an overview of the muscles in this area and their relationship to the bones covered last week in The Anterior Leg, Part 1: The Supporting Cast.

The entire medial surface of the tibis is exposed, but the lateral surface is obscured by the tibialis anterior muscle.

As we saw in the last post, the tibia is the larger and more medial of the two lower leg bones. There is a long ridge down its anterior side known as the anterior crest. One either side of the anterior crest are two long, flat surfaces. The medial of these (called, um, the medial surface) is completely exposed. It comes right up to the surface of the body, and it's what we colloquially refer to as the shin.

The lateral surface of the tibia is not a surface landmark because it is almost entirely obscured by, YES, the tibialis anterior muscle! This lovely little structure moves the whole foot, and is the only muscle in the anterior compartment to do so. (The other muscles in this compartment move the toes.) Because tibialis anterior is in the dorsiflexor group, it dorsiflexes the foot, or points it upward. This is not typically a very strong or pronounced foot movement, but it is important in making sure our foot is lifted up enough with each step so that we don't drag our toes. Yes, this little muscle keeps us from stubbing our toes. (Well, most of the time, anyway.)

Let's take a look at the tibia and the tibialis anterior's appearance on the lower leg:

The tibialis anterior muscle and its tendon can be seen very clearly on the surface of the leg when the foot is dorsiflexed and inverted. We can also see the vast surface of the medial tibia, as well as several other bony landmarks.

Notice how the lateral side of the lower leg appears soft and rounded, while the medial side appears flat and smooth. This is because the lateral side is soft tissue (in the form of the tibialis anterior muscle) and the medial side is the long, wide medial surface of the tibia.

Notice also how the tibialis anterior tendon shows clearly from just below the muscle body all the way down to the medial foot. It's most prominent just over the ankle. Notice also that the tendon of the extensor hallucis longus muscle runs parallel with that of tibialis anterior on the dorsal foot. We can tell one from the other, though, because the tendon of tibialis anterior is wider and more medial, and it surfaces more proximally than the tendon of extensor hallicus longus.

I have also pointed out a few other surface landmarks in the photo above, including certain features of the tibia and some dorsal foot tendons that come from anterior leg muscles whose bodies we cannot see up in the leg. 

In case the basic muscular and bony shapes need to be clarified, take a look at this very simple diagram, in which the basic bone and muscle shapes are overlaid onto the photo:

One last thing: Did you notice there is no medial compartment in the lower leg? Although we can invert our foot (turn its sole inward) there is no specific compartment whose function is only this. It makes sense that muscles on the medial side of the leg would invert the foot-- or pull it medially-- but alas, there is no medial compartment. But it turns out a medial compartment is not necessary here, because two other muscles on the lower leg take care of inversion. Gastrocnemius (in the posterior compartment) helps with inversion of the foot, and so does our friend tibialis anterior. 

This means tibialis anterior and its tendon really show when we are both dorsiflexing and inverting at the same time (or pointing the foot upward and inward at the same time.) Notice the foot in the photos is held in that position to ensure the best possible view for the camera.

So now we're familiar with our first lower leg muscle compartment. We'll move on to the posterior and lateral compartments in upcoming posts, but I think we might first take a short break from the leg and spend a little time going over the basic terminology of direction and location on the human body. This will help define a great deal of the words used over and over again in these posts. Until then, be sure to thank lonely little tibialis anterior next time you walk without stubbing your toe.

Anterior Leg, Part 1: The Supporting Cast

Since we've covered some lateral knee and thigh structures and are due for more leg posts, I thought we'd dip down and visit the lower leg today-- specifically the anterior side. While this our the first visit to this area, it's not the first time we've mentioned the most prominent muscle here-- the tibialis anterior. This muscle is a star! Its body sweeps down the lateral surface of the tibia and stands out strikingly in foot dorsiflexion. Its tendon is even more visible on the antero-medial ankle as it courses down to the medial side of the foot. We've actually observed the tibialis anterior tendon before, in The Dorsal Foot: How Do I Love Thee? Let Me Count Your Tendons.

The tibialis anterior muscle was going get top billing in this post until it occurred to me that describing it was next to impossible without a thorough explanation of its supporting cast, the lower leg bones. So we'll examine those today and move on to a more detailed explanation of tibialis anterior next time.

The two bones of the lower leg are the tibia and the fibula. (Not fibia!) It's easy to distinguish these two bones from one another: The tibia is the wider of the two and lies more medial. It's the second longest and strongest bone in the human body (after the femur.) The fibula is the narrower of the two and lies on the lateral side. The tibia supports most of the weight placed on the lower leg, but the fibula breaks more often-- usually at its distal end-- because it's so thin. 

In the above diagram, the structures labeled in green are bony surface landmarks, which means they come right up under the surface of the skin and are often visible and/or palpable there. Notice that everything on the medial side has earned landmark status. Have you ever noticed that the medial side of your lower leg is much bonier than the lateral side? This is because the entire medial side of the tibia is completely exposed; nothing covers it other than skin, a little adipose tissue, and a thin layer of connective tissue.

The tibia, unlike other long bones, is not cylindrical in form. If cut transversely across its middle, its cross section would look more like a rounded triangle than a circle. The point of this triangle that faces anteriorly forms the anterior crest of the tibia, a long ridge down its anterior side. There are flat surfaces on either side of this crest, one lateral to it and one medial to it. The lateral surface of the tibia is not a surface landmark because it is almost entirely obscured by the tibialis anterior muscle. The medial surface of the tibia, however, is completely exposed. The tibia's medial surface and its anterior crest together form what we think of as the shin. And its complete exposure is what makes shin bumps so painful.

As we can see in the diagram above, the tibia's anterior crest and medial surface come right to the surface of the body, while its lateral surface is covered by the tibialis anterior muscle. Note also that the fibula is entirely embedded in muscle at this point (and along most of its length.) The only parts of the fibula that show on the surface of the human body are the head (at its proximal end) and the lateral malleolus (at its distal end.)

The above photo shows the appearance of these bony structures (among others) on the surface. Notice the entire medial tibia shows on the medial leg. (This will be more clear in the next post when we'll observe photos of the medial tibia juxtaposed against the tibialis anterior muscle.) We can also see the tibial tuberosity, a small bump just inferior to the patella, and the patellar ligament, which runs from the patella to the tibial tuberosity. We can also clearly see the medial malleolus of the tibia, which appears as a bump on the medial side of the ankle. Notice also that all we can see of the fibula on the lateral side are both ends of it-- the head proximally and the lateral malleolus distally.

Incidentally, this photo also shows lateral knee tendons (those of the iliotibial band and the biceps femoris muscle) which were discussed in The Lateral Knee: A Change of Scenery, and the lateral ankle tendons that were discussed in A Lateral Ankle Tendon: Peroneus Longus or Peroneus Brevis? Please visit these links for further information.

The tibia and the fibula provide the structural foundation for the muscular anatomy of the lower leg. Most of the lower leg muscle tissue is posterior to these bones, and we'll get to that soon. But next time, we'll take a good thorough look at the leading lady of the anterior leg, the lovely tibialis anterior! There might even be a sneak preview on the Human Anatomy for the Artist Facebook page. I'll get the popcorn and save you a seat down front.