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Muscles of the leg

Origins, insertions, innervation and functions of the muscles of the leg

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Transcript

Walking is a huge part of our lives. We walk to university, we walk to the grocery store, we walk our dogs, our dogs walk us – you get the idea. If you pay close attention to this video, you’ll notice that when the foot comes in contact with the ground, it does so heel first, and when the foot is lifted back up, the toes are the last to leave the ground. Two movements that are crucial to this process are dorsiflexion and plantarflexion, which are carried out by the anterior and posterior compartments of the leg, respectively. Today, we’re going to be learning more about these movements and the muscles that perform them in our tutorial on the muscles of the leg.

Before we begin, I just want to take some time to clear up an issue that can arise when switching from everyday terms to anatomical terms. So, normally, in your everyday life, your arm is your arm and your leg is your leg, whereas in anatomy, your arm or your upper limb is divided into the arm and the forearm and your leg or lower limb is divided into the thigh and the leg. So when I say we’re going to be talking about the muscles of the leg, I really mean this region here, between the knee and the foot.

Okay, so now that we’ve got that figured out, I’d like to give you a quick overview of what we’re going to be talking about in this tutorial.

So, first, we’re going to be looking at the bones of the lower limb as they form the bony framework that our muscles attach to then we’ll look at the muscles of the leg which can be divided into three compartments – the anterior compartment, the lateral compartment, and the posterior compartment. And the posterior compartment can be further divided into the superficial layer and the deep layer. And as we talk about each muscle, we’ll learn their origin, insertion, function, and innervation. And finally, we’ll conclude our tutorial with some clinical notes.

Okay, so let’s get started with the bones of the lower limb.

So here we can see the image I showed you a little bit earlier, but I’ve also dissected away the musculature so that you can see the bones a little bit better. So this bone that we can see up here is the bone of the thigh which is known as the femur, and between the thigh and the leg, we can see the patella which is more commonly known as the kneecap, and moving distally, we can see the bones of the leg which are the tibia and the fibula.

Next, we have the bones of the foot which can be divided into three groups. So first we have the tarsals and there are seven of these and they’re located in the ankle and the midfoot. Then we have the metatarsals, and we have five of these – one for each digit. And located most distally are the phalanges, which are the bones of your digits. And as you can see, the big toe has a proximal phalanx and a distal phalanx whereas the other toes each have three phalanges – the proximal, middle and distal phalanges.

So now that we’re familiar with the bones of the lower limb, let’s talk about the muscles of the leg starting with the anterior compartment.

So the first muscle of the anterior compartment that we’re going to be talking about today is the tibialis anterior muscle, and this muscle originates from the lateral surface of the tibia, the lateral condyle of the tibia, and the interosseous membrane of the leg. The interosseous membrane is a thin sheet of fibrous connective tissue linking the two bones of the leg, the tibia, and the fibula. And the tibialis anterior then goes on to insert at the base of the metatarsal of the big toe and the plantar surface of the medial cuneiform bone which is one of the tarsals.

If we add this image here which is an inferior view of the bones of the foot, we can see these points of the insertion a little bit more clearly. So here we have the metatarsal of the big toe and over here we can see the medial cuneiform bone.

So what does the tibialis anterior do? Well, when this muscle contracts, it causes dorsiflexion of the foot at the ankle joint, which as we can see decreases the angle between the dorsum of the foot and the leg, and this muscle is also responsible for inversion of the foot. And inversion of the foot involves tilting the sole of the foot towards the midline of the body like we can see here. The tibialis anterior also supports the medial longitudinal arch of the foot.

Speaking of muscle functions, if you want to know even more about the actions of each of the muscles mentioned in this video, why not take some time later on to check out our awesome 3D muscle function video series on the lower limb. Here you’ll be able to watch each muscle performing its function in isolation and visualize every single movement without missing a thing!

In order for the tibialis anterior to perform its functions, it requires innervation and it’s supplied by the deep fibular nerve which is also known as the deep peroneal nerve. Note that the terms fibular and peroneal are interchangeable, but throughout our tutorial, we’ll use the word fibular. The deep fibular nerve supplies all of the muscles in the anterior compartment and it originates from the sciatic nerve. The sciatic nerve bifurcates into the tibial nerve and the common fibular nerve, and the common fibular nerve then divides into the superficial fibular nerve and our nerve of interest, the deep fibular nerve.

Located lateral to the tibialis anterior, we find the extensor digitorum longus, and this muscle originates from the lateral condyle of the tibia, the head of the fibula, the anterior border of the fibula, and the interosseous membrane of the leg. It then splits into four tendons and inserts onto the bases of the middle and distal phalanges of the lateral four toes.

The extensor digitorum longus is responsible for dorsiflexion of the foot at the ankle joint and extension of the lateral four toes at the metatarsophalangeal and interphalangeal joints. And as we can see in our image, extension of the toes pulls them towards the leg, decreasing the angle between the toes and the leg. So this muscle also causes eversion of the foot which involves titling the sole of the foot away from the midline of the body. And like the tibialis anterior, the extensor digitorum longus is innervated by the deep fibular nerve which we can see here highlighted in green.

Before we move on, I do want to mention the fibularis tertius. So, the fibularis tertius is normally connected to the extensor digitorum longus, and is therefore usually considered as part of this muscle. It originates from the anterior border of the fibula and inserts onto the base of the metatarsal of the little toe. When the fibularis tertius contracts, it causes dorsiflexion of the foot at the ankle joint and eversion of the foot, and this muscle is also innervated by the deep fibular nerve.

The last muscle of the anterior compartment of the leg is the extensor hallucis longus. So in terms of origin points, this muscle originates from the medial surface of the fibula and the interosseous membrane of the leg. It then goes on to insert at the base of the distal phalanx of the big toe, and this muscle is responsible for dorsiflexion of the foot at the ankle joint and extension of the big toe at the metatarsophalangeal and interphalangeal joints.

So as we can see in our image, extension of the big toe pulls it towards the leg decreasing the angle between the big toe and the leg. Again, this muscle is innervated by the deep fibular nerve.

So while we’re here, I just want to take a minute to talk about some retinacula. So retinacula are fibrous bands that act like seatbelts for the tendons of your muscles. They hold them in place so that they can’t move around freely under your skin. So the superior extensor retinaculum passes from the fibula to the tibia proximal to the malleoli. And it binds the tendons of the muscles of the anterior compartment of the leg before they cross the ankle joint.

Moving inferiorly, we can see the inferior extensor retinaculum, and as you can see, the inferior extensor retinaculum is y-shaped. And this structure runs from the calcaneus which is the largest tarsal bone of the foot. It then splits into two attaching to the medial malleolus and the plantar aponeurosis. And the inferior extensor retinaculum binds the tendons of the muscles of the anterior compartment of the leg after they cross the ankle joint.

Before we move on to the lateral compartment of the leg, let me just summarize some key points about the anterior compartment. So the anterior compartment is made up of four muscles – the tibialis anterior, the extensor digitorum longus, the fibularis tertius, and the extensor hallucis longus. And these muscles work together to perform dorsiflexion of the foot at the ankle joint, and they’re all innervated by the deep fibular nerve.

Okay, so let’s have a look at the muscles of the lateral compartment of the leg.

The lateral compartment of the leg consists of two muscles – the fibularis longus and the fibularis brevis. The fibularis longus originates from the head of the fibula and the lateral surface of the fibula. Its tendon passes behind the lateral malleolus and crosses the underside of the foot to insert onto the base of the metatarsal of the big toe as well as the medial cuneiform bone. So when the fibularis longus contracts, it causes eversion of the foot and plantarflexion of the foot at the ankle joint, and as we can see in our image, plantarflexion increases the angle between the dorsum of the foot and the leg and this muscle also supports the lateral longitudinal and transverse arches of the foot.

As for innervation, the fibularis longus is innervated by the superficial fibular nerve which is a branch of the common fibular nerve that we mentioned earlier.

Okay, so it’s time to take a look at the fibularis brevis which originates from the lateral surface of the fibula. It then passes behind the lateral malleolus to insert at the tuberosity of the metatarsal of the little toe. And this muscle is responsible for eversion of the foot and it also assists in plantarflexion of the foot at the ankle joint. And like its big brother, the fibularis brevis is innervated by the superficial fibular nerve which we can see highlighted in green.

Again, it’s time to talk about some retinacula. So here we have the superior fibular retinaculum which we can see here from a posterior perspective, and this retinaculum extends from the lateral malleolus to the calcaneus, and the superior fibular retinaculum binds the tendons of the muscles of the lateral compartment of the leg. Inferior to the superior fibular retinaculum we, of course, have the inferior fibular retinaculum, and the inferior fibular retinaculum is continuous with the inferior extensor retinaculum and attaches to the calcaneus. Again, this structure binds the tendons of the muscles of the lateral compartment of the leg.

So before we move on to the posterior compartment of the leg, let’s just summarize some key facts about the lateral compartment.

So, the lateral compartment is made up of two muscles – the fibularis longus and the fibularis brevis – and these muscles work together to perform eversion and plantarflexion of the foot and they’re innervated by the superficial fibular nerve.

Okay so now that we’re familiar with the muscles of the anterior and lateral compartments, let’s take a look at the posterior compartment. And as we mentioned before, it’s got two layers, so let’s start with the superficial one first.

So, the first muscle in this layer that we’re going to be talking about is the gastrocnemius. The gastrocnemius is the most superficial muscle of the posterior compartment and it’s got two heads which have different points of origin. The lateral head originates from the lateral epicondyle of the femur and the medial head originates from the medial epicondyle of the femur. The gastrocnemius then inserts onto the calcaneal tuberosity via the calcaneal or the Achilles tendon and it’s worth noting that all three superficial muscles of the posterior compartment have the same insertion point.

When the gastrocnemius contracts, it causes plantarflexion of the foot at the ankle joint and flexion of the leg at the knee joint. And as we can see from our image, flexion of the knee decreases the angle between the thigh and the leg at the knee joint. And the innervation of the gastrocnemius is supplied by the tibial nerve which is a branch of the sciatic nerve.

The next muscle we’re going to be talking about is the plantaris muscle, and this muscle originates from the lateral supracondylar line of the femur and inserts onto the calcaneal tuberosity via the calcaneal tendon. And this muscle assists with plantarflexion of the foot at the ankle joint and flexion of the leg at the knee joint. Like the gastrocnemius, the plantaris is innervated by the tibial nerve which we can see here highlighted in green.

Lastly, we have the soleus muscle, which originates from the head of the fibula, the posterior border of the fibula, and the soleal line of the tibia. And this muscle then inserts onto the calcaneal tuberosity via the calcaneal tendon. And when the soleus contracts, it causes plantarflexion of the foot at the ankle joint, and again, this muscle is innervated by the tibial nerve.

So before we move on to the deep layer of the posterior compartment of the leg, let me just summarize some key points about the superficial layer. So this layer consists of three muscles – the gastrocnemius, the plantaris, and the soleus. And all three muscles of the superficial compartment have the same point of insertion and insert onto the calcaneal tuberosity via the calcaneal tendon. And these muscles work together to perform plantarflexion of the foot at the ankle joint and they’re innervated by the tibial nerve.

Alright, so now it’s time to dig a little bit deeper and look at the deep layer of the posterior compartment of leg.

So the first muscle of this layer that we’re going to be talking about is the popliteus muscle, and this muscle originates from the lateral condyle of the femur and the lateral meniscus. It then inserts onto the posterior surface of the tibia above the origin of the soleus muscle. So when you transition from standing still to walking, this muscle causes lateral rotation of the femur on the tibia which unlocks the knee joint so that flexion can occur, and this muscle also causes flexion and medial rotation of the leg at the knee joint and stabilizes the knee joint.

As for innervation, the popliteus is innervated by the tibial nerve, and it’s worth noting that all the muscles in the deep layer are innervated by this nerve.

The next muscle we’re going to be talking about is the flexor digitorum longus, which originates from the posterior surface of the tibia. It then splits into four tendons and inserts onto the plantar surfaces of the bases of the distal phalanges of the lateral four toes. The flexor digitorum longus is responsible for plantarflexion of the foot at the ankle joint and flexion of the lateral four toes at the metatarsophalangeal and interphalangeal joints. And as you can see in our image, this is the movement you perform when you’re curling your toes. And this muscle also performs inversion of the foot.

Like the popliteus, the flexor digitorum longus is innervated by the tibial nerve.

Located lateral to the flexor digitorum longus, we have the flexor hallucis longus, and in terms of our origin points, this muscle originates from the posterior surface of the fibula and the interosseous membrane of the leg. It then goes on to insert at the plantar surface of the base of the distal phalanx of the big toe. When this muscle contracts, it causes plantarflexion of the foot at the ankle joint and flexion of the big toe at the metatarsophalangeal and interphalangeal joints. And as we can see in our illustration, this is the movement you perform when curling your big toe. And this muscle is also responsible for inversion of the foot.

Again, the flexor hallucis longus is innervated by the tibial nerve.

So, the last muscle of the deep layer of the posterior compartment that we’re going to be talking about today is the tibialis posterior. In terms of its origin points, this muscle originates from the posterior surface of the tibia, the posterior surface of the fibula, and the interosseous membrane of the leg. Its tendon passes behind the medial malleolus and inserts onto the tuberosity of the navicular bone and three cuneiforms. And these bones are tarsals of the foot. The tibialis posterior is responsible for plantarflexion of the foot at the ankle joint and inversion of the foot. And this muscle also supports the medial longitudinal arch of the foot when walking. And this muscle is innervated by the tibial nerve.

Before we move on, let’s just talk about the last retinaculum of this tutorial which is the flexor retinaculum of the foot. And this structure extends from the medial malleolus to the calcaneus. It binds the tendons of the flexor digitorum longus, the flexor hallucis longus, and the tibialis posterior which are deep muscles of the posterior compartment of the leg.

Okay, so let’s summarize some key facts about the deep layer of the posterior compartment of the leg.

So this layer is made up of four muscles – the popliteus, the flexor digitorum longus, the flexor hallucis longus, and the tibialis posterior. And these muscles excluding the popliteus work together to perform plantarflexion of the foot at the ankle joint and they’re innervated by the tibial nerve.

Okay, so now that we’re familiar with the muscles of the leg, let’s move on to talk about some clinical notes that are relevant to these muscles.

So, foot drop is a term used to describe difficulty lifting the front part of the foot or, in other words, performing dorsiflexion. And this causes the foot to drag along the ground and increases the risk of falls. So individuals with this problem may compensate by lifting their leg higher than normal and this is often referred to as steppage gait.

So, it’s important to note that foot drop isn’t a condition, but rather it’s a sign of the underlying problem, and it’s usually caused by weakness or paralysis of the muscles of the anterior compartment of the leg. The most common cause of foot drop is compression of the fibular nerve which gives off branches that innervate the anterior and lateral compartments of the leg and it can also be caused by muscular dystrophy which results in progressive muscle weakness as well as conditions that affects the brain and the spinal cord such as stroke.

So treatment depends on the course and includes braces which hold the foot in its normal position, physiotherapy which strengthens the muscles, and electrical stimulation of the common fibular nerve. Severe or long term foot drop may require surgery and if the cause is successfully treated, foot drop may improve or disappear, however, it can also be permanent.

Okay, so before we bring our tutorial to a close, let’s quickly summarize what we’ve learned today.

So we started our tutorial with talking about the bones of the lower limb which are the femur, the patella, the tibia, the fibula, the tarsals, the metatarsals, and the phalanges. Then we moved on to talk about the muscles of the leg which we divided into three compartments – the anterior compartment, the lateral compartment, and the posterior compartment. And the posterior compartment was further divided into the superficial layer and the deep layer. And finally, we concluded our tutorial with some clinical notes about foot drop.

So that brings us to the end of our tutorial on the muscles of the leg. I hope you enjoyed it. Thanks for watching and happy studying!

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