Monday, November 28, 2011

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.

Sunday, November 20, 2011

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.