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Posterior muscles of the leg

Origins, insertions, innervation and functions of the superficial and deep flexors.

Show transcript

Hello, everyone. This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, I'm going to be talking about the posterior leg muscles. So we’re going to be looking at the different muscles that you find on the posterior side of your leg as you can see here on the image on the right side. And we’re going to go into the different attachment points, so origins and insertions. We’re going to include innervation and functions of… for all these muscles.

Now, before I go into details on each single muscle of the posterior leg, I want to divide them into groups, into layers, better said. Now, there is one superficial layer which consists of two main muscles: the triceps surae and the plantaris muscle. The deep layer consists of muscles : the popliteus, the tibialis posterior, the flexor digitorum longus, and the flexor hallucis longus. Now, these muscles are all considered part of the deep layer of the posterior leg muscles.

Now, we’re going to move on to the very first muscle here on our list. But right now, on your screen, you have two different muscles highlighted. But these are not two different muscles. These are two muscles that are part of this one here that we found on the first… the first muscle on our list, the triceps surae. Now, this is a three-headed muscle that you find on the dorsal lower leg. And the heads and body of this muscle determine the surface anatomy of the calf.

Now, the triceps surae consists of the gastrocnemius, as you can see here on the image, which has two heads, so clearly seen here—one, two heads, which have eventually two origin points, which we will look at. And the soleus is found underneath the gastrocnemius and has one head.

We’re going to start off with the gastrocnemius, here highlighted in green. And this muscle, in terms of origin points, we’re going to see that one of the heads is going to be originating from the medial epicondyle of the femur. So as you can see here, this is the medial epicondyle. And the medial head of the gastrocnemius is originating from this portion of the bone.

If you have a medial head, you would have, then, another one called a lateral head. Because there is another head, the second head of the gastrocnemius is originating from the lateral epicondyle of the femur. And as you can see clearly on this image, the gastrocnemius overlaps the soleus muscle almost completely.

Now, moving on to the insertion point of the gastrocnemius, at a distal third of the lower leg, both the soleus and the gastrocnemius muscles merge into a common tendon, famous tendon—this one right here as you can see—known as the Achilles tendon which you can easily palpate on the posterior superior portion of your foot. You can feel a tendon there, and this is the Achilles tendon formed by the gastrocnemius and also the soleus which merge at this point.

And then this tendon is going to insert in this bone right here, as you can see, known as the calcaneus. And the Achilles tendon is inserting on the posterior surface of the calcaneus, at the tuberosity of the calcaneus. So be more specific especially on your exam. Know that this is on the tubero ... it’s inserting on the tuberosity of the calcaneus.

Now, we’re going to move on to the other muscle that we’ve been talking about, the soleus. And we’re going to start off with the origin points. This muscle originates at the head of the fibula, the posterior border of the fibula, and it’s also attached to the soleal line on the tibia through a tendonous arch.

When I was talking about the insertion point of the gastrocnemius, I mentioned also the insertion point for the soleus. But it’s important to remember here that these two muscles, the gastrocnemius and the soleus, will merge into the Achilles tendon and then insert at the tuberosity of the calcaneus as well.

So now that we discussed the different attachment points for the triceps surae, it is time for us to just briefly mention its innervations, meaning that both the gastrocnemius and the soleus muscle are going to be innervated by the tibial nerve which is a branch of the sciatic nerve.

Now, let’s move on to the next topic here on the triceps surae, and I'm showing you here some images so I can talk about the different functions. And here on the right side, you see both the gastrocnemius and the soleus contracting and for that reason producing some of the functions that we’re going to be discussing. And on the left side, you see an image of the foot so you can... I can show some arrows of the actions that are going to be performed by this muscle.

Its most important function is plantar flexion on the upper ankle joint, as you can see here represented by these arrows, which enable the lift of the heel against gravity when walking or jumping. Plantar flexion fixes the lower leg when standing and for that reason prevents the upper body from falling forward.

In addition, it is important to add that this muscle, the triceps, is going to be responsible for or is the strongest supinator of the lower ankle joint. And supination of the ankle joint is when you lift the medial side of your foot and then you stand on the lateral edge of the foot. And you can see that this movement being indicated here by these arrows.

I wouldn’t finish the functions of the triceps without mentioning that the gastrocnemius is contributing or contributes to a small extent to flexion of the knee.

We’re going to move on to the next muscle on our list, this really thin muscle known as the plantaris. And in terms of origin points for this muscle, you need to remember that it’s coming from the oblique popliteal ligament and also the lateral epicondyle of the femur which you can clearly see here on this image, the muscle coming from the lateral epicondyle of this bone.

Now in terms of insertion point, you need to remember that the tuberosity of the calcaneus will serve as an insertion point via the Achilles tendon. If we look at the innervation of the plantaris, all you need to remember like the previous ones, the previous muscle that we talked about. The plantaris is going to be innervated by the tibial nerve.

Now, the deep posterior muscles of the lower leg are part of the calf musculature and we’re going to be talking about them right now. They are located within the deep posterior compartment and include these that you’re seeing right now, the popliteus, the tibialis posterior, the flexor digitorum longus, and the flexor hallucis longus.

And before we do so, I want to do a clarification here that the deep posterior muscles are supplied, in terms of innervations, by the tibial nerve. So the tibialis posterior, the flexor digitorum longus, and the flexor hallucis longus, and popliteus muscle are going to be innervated by this nerve that you can see here on the image highlighted in green, the tibial nerve.

We’re going to start off with the very first muscle of the deep layer, here, seen highlighted in green, the popliteus. And in terms of general information that you need to know, it’s a small muscle located at the knee joint, and sometimes, we consider it as a part of the posterior thigh musculature.

If you look at the origin points for this muscle, it originates at the lateral condyle of the femur, as you can see here. And also the lateral meniscus of the... that you find on the knee.

If we’re going to look at the insertion point for this muscle, you need to remember that after running mediocaudally towards the tibia, it inserts above the origin of the soleus muscle at the posterior surface of the tibia, and you can clearly see also here on this image.

As I mentioned before, but just as a reminder in terms of innervation, this muscle is going to be innervated by the tibial nerve.

We’re going to move on and talk about the different functions of the popliteus. And a general task for this muscle is to stabilize the dorsal knee region, and also, it’s going to be causing internal rotation as you can see here. It is responsible for reversing the so-called terminal rotation. This refers to the locking of the tibia and femur during knee extension by slight external rotation about five degrees. And even though the popliteus anatomically ranks among the flexors of the thigh musculature, its ability to flex the knee is truly negligible. So it’s important to add here on the functions of the popliteus.

We are going to move on to another muscle here that we need to discuss on the deep layer. This is the tibialis posterior. And in terms of origin points, it’s going to originate from the interosseous membrane of the leg, and also the posterior surface of the tibia, and the posterior surface of the fibula. And you can clearly see here, by the location of the muscle, that it’s going to be inserting both on the fibula that you can see here, and the tibia, and of course, the membrane that stands between these two bones.

If we look at the different insertion points for the tibialis posterior, it inserts at the medial border of the foot at the tuberosity of the navicular bone and the cuneiform bones, which include the medial cuneiform, the lateral cuneiform, and the intermediate cuneiform. So remember this. The… all the intr… the cuneiform bones are going to serve as insertion point for the tibialis anterior... posterior, and you can clearly see here that it’s quite expanding to different bones. The insertion point is expanding or the tendon is expanding to different bones here. And distally, it forms the tendon sheet extending up to the lateral foot border.

We’re going to move on and talk about the different functions associated to the tibialis posterior. It is involved in plantar flexion of the upper ankle joint. Another function associated to the tibialis posterior is going to be supination in the lower ankle joint, represented here by this arrow, which basically is when you lift the medial side of your foot while standing on the lateral side or using the lateral side of the foot as support. Now also to add here in the functions is that this muscle has, what is known to be, a tendon sheet which will support the transverse and longitudinal arches of your foot.

Now, let’s move on to the next muscle seen here, highlighted in green. This is known as the flexor digitorum longus. In terms of origin points, it is easy. You need to remember that it is coming or this muscle arises on the posterior surface of the tibia. And you can clearly see here, located on the medial side, and for that reason, you can notice that this bone here—notice a little bit of the tibia here and a bit more here—this is going or the posterior surface of the tibia is serving as origin point.

Now, if you look at the insertion point for the... for this muscle, it curves laterally around the navicular bone, as you notice here on this image clearly. And there, the tendon fans out into four smaller tendons which insert at the bases of distal phalanges of the second to fifth toes.

Now, let’s take a look at the different functions associated to the flexor digitorum longus. What you need to know about this is that this muscle is involved in plantar flexion in the upper ankle joint, as seen here indicated by these arrows. And another thing that is important to add here is that it’s going to be also able to cause supination in the lower ankle joint or inversion, indicated here by this arrow, which is basically when you lift the medial side of your foot and you stand at the lateral side or use the lateral side of the foot as support.

Now, the other function that is important to mention here and that you can clearly see illustrated here in this image is flexion of the toes, and seen here, the arrow indicating the movement. And as I mentioned on the previous slides about the different tendons here of the flexor digitorum longus inserting on the different toes, this will enable to pull when the muscle is contracting. It’s going to pull the toes down or towards the plantar side of the foot. And this is what we call flexion of the toes.

Now, we’re going to move on to another muscle on our list, this one seen also here, highlighted in green, and this is the flexor hallucis longus. Like the name indicates, this is the flexor. This muscle is going to be flexing the big toe—“Hallucis,” Latin for “big toe”—and is the long flexor of the big toe.

And in terms of origin point, you need to remember that this muscle is going to come from the posterior surface of the fibula, as you can clearly see on this image. It’s located a little bit more laterally, leading it to then originate from this bone here, as you can see on the image, the fibula.

Now, another part that is… or another structure that is going to serve as origin point is going to be the interosseous membrane of the leg. If we look at the insertion points for the flexor hallucis longus, it runs under the sustentaculum tali of the calcaneus and finally inserts at the base of the first distal phalanx or the fir… or the base of the distal phalanx of the big toe, and you can clearly also see here on this image. So it’s inserting on the base of the distal phalanx of the toe. Now, this muscle forms a tendon sheet at the foot sole as well.

Now, if we’re looking at the different functions of the flexor hallucis longus, like all deep posterior muscles, it is… of the leg, it is involved in plantar flexion of the upper ankle joint and also supination or inversion of the lower ankle joint, seen here indicated by this arrow as well.

Now, we’re still on the different functions associated to the flexor hallucis longus, and we have this image here to illustrate the next one. And this one is called flexion of the big toe. So this muscle is going to be able to flex your big toe. So when you pull your big toe down, this muscle is going to be definitely involved in this movement.

Now, what happens is that if you’re flexing your big toe, usually you may flex a bit the second and third toe and this happens due to the fact, if you notice here on this image clearly illustrates here, crossing of the tendon of the flexor hallucis longus and also the flexor digitorum longus that we talked about. They’re crossing. So once this one is contracting, the flexor hallucis longus, this tendon may affect also the nearby tendons and cause flexion of the second and third toe. This place is what we call the plantar chiasm.

Furthermore, the tendon sheet of the flexor hallucis longus supports the transverse and longitudinal arches of your foot.

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