Muscles of the Leg

901,661 views
Next →

Transcription

Posterior Compartment

Okay so this tutorial is on the muscles of the leg.

 

The leg is the region of the lower limb, which lies between the knee joint and the ankle joint.

I'm going to break this tutorial into two parts, so I'm going to do the first part on the posterior compartment, and then the second part will be on the muscles of the anterior and lateral compartments.

 

Just like my tutorials on the thigh and the upper limb, the muscles of the leg can be broken down into compartments. And these compartments are separated by intermuscular septa, and the interosseus membrane between the tibia and the fibula.

 

The muscles of the posterior compartment here, mainly act to plantar flex the foot, flex the digits, and invert the foot. Plantar flexion is when you, get up on your tip toes basically, so if you, so if you can see this angle between the foot and the shin, it...you extend this angle.

 

You open this angle further, so you, so it's getting up on your tip toes essentially, and dorsiflexion is bringing your toes towards your head.

 

If you imagine lying down on your back and bringing your toes up towards your head - that's dorsiflexion and plantarflexion is the opposite.  The muscles of the posterior compartment plantar flex at the ankle joint, they flex the digits, and they invert the foot. The muscles of the anterior compartment, here, do the opposite really, so they dorsiflex the foot, so bring the toes upwards. They extend the digits, and they invert the foot also.

 

As well as plantarflexion, flexion of the digits and inversion, there are two muscles in the posterior compartment which actually can flex at the knee because of their attachment on the femur, and these are the gastrocnemius and the plantaris muscle, which I'll come on to talk about.

 

The lateral compartment are the muscles here, which lie laterally, and these evert the foot.

 

This tutorial will be concerned with the muscles of the posterior compartment of the leg.

 

Briefly just a quick word about innervation. If I just bring in the nerves, you can see, so just looking here at the popliteal fossa, you can see the sciatic nerve, and it splits into two main branches, so you've got the tibial branch of the sciatic nerve, and you've got the common peroneal branch, the common fibular branch of the sciatic nerve.

 

The tibial branch of the sciatic nerve supplies muscles of the posterior compartment, and the common peroneal nerve which winds round laterally here, innervates the anterior and lateral compartments of the leg.

 

Just a quick point about the common fibular nerve, just while I've mentioned it.

 

If I just remove the muscle layer, you can see the relationship of this nerve with the head of the fibula, so this nerve winds round the...well the neck of the fibula, and this, at this point, it's very vulnerable so any impact or fractures can easily damage this nerve, and because this nerve supplies the anterior and lateral compartments of the leg, it results in foot drop.

 

It’s worth making a note of that point, so that's the common fibula nerve, which winds round laterally around the neck of the fibula and it's quite exposed in this region. Okay so now I'll just run you through the muscles of the posterior compartment, and I'll talk about the origin and the insertion and the actions.

 

I'll just get rid of the nerves and we can focus on the muscles now. Okay so the muscles of the posterior compartment can be separated into superficial and deep muscles, and in total you've got seven muscles.

 

You’ve got three muscles in the superficial layer, and four muscles in the deep layer.  I'll just work from superficial to deep and talk you through these structures.

 

Obviously we're looking at the most superficial muscle here, and this is called the gastrocnemius muscle.  This muscle has two heads, it's got a medial head here, and a lateral head. And this muscle inserts onto the femur on the medial and lateral condyles.

 

The medial head inserts on the medial condyle and the lateral head inserts on the lateral condyle. I'll just show you that in a bit more detail.  I've just isolated this muscle, so you can see it a bit more clearly, and you can see the origin of this muscle on the upper surfaces of the femoral condyles.

 

The medial, medial head originates on the upper surface of the medial femoral condyle, and the lateral head originates on the upper surface of the lateral femoral condyle, on a distinct facet. You can see on this side there's a facet here which the lateral, lateral head of the gastrocnemius muscles originates on. And then if we follow the muscle down, it's not quite shown clearly here, but it inserts onto the calcaneus via the calcaneal tendon.

 

I've just brought the muscle back in and you can see the tendon now.  This is also known as the Achilles tendon.  This muscle has two functions, it plantarflexes the foot, so plantarflexion is when you get up on your tip toes, so you can see how this would act if you look at the insertion, so you can see if the muscle contracts it pulls the calcaneus upwards and it would get you up on your tip toes - so that's plantarflexion. And also because of its origin on the femoral condyles, it can also flex at the knee.

 

That’s the gastrocnemius muscle and that's the most superficial muscle.

 

Next we've got this tiny little muscle here, which is called the plantaris muscle, so it’s got a very short muscle belly and then it’s got a very long tendon, which winds round down the medial side of the leg and joins the calcaneal tendon to insert onto the calcaneus.

 

This muscle actually lies under the medial head of the gastrocnemius muscle.

 

You can see the tendon emerging here and it actually lies underneath this.  We’ll just take a closer look at this muscle's origin.

 

This muscle originates on the lower part of the supracondylar ridge. You can't see very clearly here, but it originates on the lower part of the supracondylar ridge, which comes down off the femur here, and it also originates on, it joins with the oblique popliteal ligament and then like I mentioned before, it winds down medially, this long thin tendon and then it joins onto the calcaneal tendon with the soleus and gastrocnemius muscle.

 

Again looking at its origin and its insertion, you could work out that this muscle acts to plantar flex at the ankle joint, and also because it originates on the femur, like the gastrocnemius muscle does, it can also flex at the knee joint.

 

The third muscle of the superficial group of the posterior muscles is this large muscle here, which lies underneath the plantaris muscles and the gastrocnemius, so this muscle originates, as you can see, on the proximal ends of the tibia and fibula, and it again joins the calcaneal tendon, to insert onto the calcaneus.

 

This muscle doesn't originate on the femur, so it can't flex the knee, so this muscles primary function is to plantar flex at the ankle joint.

 

These three muscles are innervated by the tibial nerve - so that's the branch of the sciatic nerve which innervates the posterior compartment.

 

Next we have the muscles of the deep layer, so I'll just remove the soleus and the plantaris, and we can now see the four muscles of the deep layer of the posterior compartment.

 

The most superior muscle is this little muscle here called the popliteus, and this muscle originates on the posterior surface of the proximal tibia, and it winds round laterally to insert onto the lateral femoral condyle, and it actually penetrates the joint capsule of the knee, passing between the lateral meniscus and the fibrous membrane, to insert laterally on the lateral femoral condyle.

 

I'll just fade away the muscle layer, and you can see how the, see more clearly the origin of the popliteus, so you can see it lies on the posterior surface on the proximal tibia, originating here, and it inserts infero-laterally on the lateral femoral condyle.

 

This muscle actually serves to unlock the knee when it’s locked in extension. It does this by laterally rotating the femur, so when it contracts, so its origin here, insertion up here - so when it contracts, it brings the femur round, laterally rotating it and unlocking the knee.

 

In the standing position, the knee is fully extended and it's locked like this. The popliteus functions to laterally rotate the femur on the tibia and unlock the knee joint.  Again this muscle is innervated by the tibial branch of the sciatic nerve.

 

Next we've got these three large muscles.  Laterally here, we've got the flexor hallucis longus, medially we've got the flexor digitorum longus, and lying between these two muscles we've got the tibialis posterior muscle.

 

Starting with the most lateral muscle - the flexor hallucis longus, you can see this originates on the posterior surface of the lower fibula, and also it originates on the adjacent interosseus membrane, which you can't quite visualise here, and it inserts on the base of the proximal phalanx, so the base of the big toe, sorry the distal phalanx, base of the...the distal phalanx of the great toe.

 

We'll just follow this muscle down, and you can see the tendon passing through a groove on the talus, which is this bone here, and then it passes under this shelf of bone - so this is the...this little shelf of bone is on the calcaneus and it's called the sustentaculum talus.

 

You can see the flexor hallucis longus tendon passes underneath this shelf and it runs down to the base of the distal phalanx of the great toe.

 

Next we've got this muscle here which is called the flexor digitorum longus, and this muscle as you can see sits on the posterior surface of the medial tibia, so the tibia bone here. And this muscle as you can tell by the name flexes the digits, so it inserts onto the bases of the lateral four distal phalanges, and on the plantar surface.

 

I'll try and show you that. I've just rotated the model so we can look at the plantar surface of the foot, and I'm just going to remove the other muscles and tendons.

 

You can see the...this muscle here is the flexor hallucis longus, and you can see the tendon winding round, and it inserts onto the base of the distal phalanx of the great toe, and you've got the flexor digitorum longus muscle which I just showed you, so it winds round behind a shallow groove in the medial malleolus and then it passes inferiorly to the flexor hallucis longus tendon, and it inserts onto the bases of the lateral four distal phalanges.

 

What this muscle does is it obviously flexes the digits.

 

Just taking another look at these tendons as they pass behind the distal tibia and the tarsal bones, so you've got the flexor hallucis longus passing behind a groove in the talus bone and then underneath this shelf of bone - the sustentaculum talus on the calcaneus, and you've got the flexor digitorum longus, which passes behind a shallow groove in the medial malleolus.

 

The medial malleolus is this part of the tibia, distal part of the tibia and then it runs down onto the plantar surface of the foot, passing inferiorly to the tendon of the flexor hallucis longus.

 

The last muscle is this muscle which sits between them.  This is the tibialis posterior muscle.

 

This muscle as you can see here, originates between the tibia and fibula on the interosseus membrane, and also on the adjacent surfaces of the tibia and fibula, and this muscle runs down passing underneath the tendon of the flexor hallucis, sorry flexor digitorum longus, and you can see its insertion point... just trying to get a good look - so it inserts on the tuberosity of the navicular and also on the medial cuneiform bone.

 

If I just show you the navicular, this is this tarsal bone, so the tibialis posterior inserts onto the tuberosity of the navicular and also the cuneiform bone.

 

What this muscle does is that it plantar flexes the ankle joint and it also inverts the foot and it also supports the medial arch of the foot, so a few functions there. And again, all these muscles are innervated by the tibial branch of the sciatic nerve.

 

Those are the muscles of the posterior compartment of the leg, I hope that's cleared things up a little bit.

 

Anterior and Lateral Compartments

Okay so this is the second tutorial on the muscles of the leg, so in this tutorial I'm going to cover the muscles of the anterior and lateral compartments.

 

These compartments are supplied by the common peroneal branch of the sciatic nerve.

 

The sciatic nerve splits into two branches at the popliteal fossa. The tibial branch supplies the posterior compartment, and then you've got the common fibular, or common peroneal branch, which winds round laterally, over the head of the...the lateral head of the gastrocnemius, over the neck of the fibula, where it's vulnerable to impact injuries and fractures, and then it splits into two branches which supply the anterior and the lateral compartments.

 

I'll just fade away the muscles and then you can see how this nerve splits.

 

You've got the common fibular branch coming around laterally, and then it splits into these two branches: you've got the superficial branch, which supplies the muscles of the lateral compartment, and you've got the deep branch, which supplies the anterior compartment.

 

The muscles of the anterior compartment mainly act to dorsiflex, extend to the toes, and to invert the foot, and there are four muscles in this compartment, and this compartment is supplied by the deep branch of the common fibular nerve. The lateral compartment only has two muscles, and these muscles mainly act to evert the foot. And this compartment is supplied by the superficial branch of the common fibular nerve. Okay so you have two muscles in the lateral compartment of the leg, you've got the peroneus longus, or the fibularis longus, which is this muscle here. And you've got the fibularis brevis, which lies deep to the fibularis longus, and it attaches a bit lower down on the fibula.

 

I'll just show you the fibularis longus muscle.

 

The fibularis longus muscle, as you can see its origin, attaches on the upper lateral surface of the fibula, and also on the lateral side of the head of the fibula, and it has an interesting tendon, because it actually crosses over underneath the foot to insert medially at the distal end of the medial cuneiform bone, and at the base of the proximal, sorry, the base of the first metatarsal here.

 

If we just follow this tendon round, you can see it runs down behind the lateral malleolus and then it descends and curves forwards on the lateral side of the calcaneus and then passes under the foot. And it then passes under the cuboid bone, and in this bone there's a groove for the...for the tendon.

 

I'll just show you that.  If we just look, I've just rotated the model and we're looking at the underside of the foot - the plantar surface of the foot, so this is the cuboid bone, which is one of the tarsal bones that sits laterally.

 

The tendon of the peroneus longus actually runs in a groove in this cuboid bone. I think it's a little bit out of place here, but you can see this groove here, so the tendon runs underneath the foot, through the groove on the cuboid bone and attaches to the base...base of the first metatarsal and also distally on the medial cuneiform bone.

 

You can just see that tendon coming across, and it inserts distally on the medial cuneiform, and at the base of this first metatarsal.

 

What this muscle does is that it actually everts the foot and it can also assist in plantarflexion. You can just imagine so seeing the insertion point here, you could just visualise if this muscle were to contract it would pull this, pull the foot round into eversion.

 

Eversion is when the soles of the feet face away from each other, and inversion is when you bring the soles of the feet to face each other.

 

Another point about the peroneus longus is that it actually provides support for the arches of the foot.

 

It mainly supports the lateral and transverse arches, and if you remember in my last tutorial, the tibialis posterior also provided support to the arch, due to its insertion on the medial aspect of the foot, and there's another muscle which I'll come on to talk about later in this tutorial, called the tibialis anterior, which also inserts medially on the foot, and contributes to arch support.

 

I've just brought back in all the muscles, and this muscle here is the tibialis anterior, which I'll talk about later, and you can see its tendon which winds round and inserts also on the base of the first metatarsal. And if I just remove this muscle here, you can see the tibialis posterior tendon.

 

You’ve got these three tendons coming in to attach to the underside on the medial aspect of the foot, and this sort of act to support the arches.  You’ve got the tibialis anterior tendon here, you've got the tibialis posterior tendon, and you've got the peroneus longus tendon, or the fibularis longus tendon coming in from this side.

 

These all function to support the arches of the foot.

 

Next we've got the peroneus brevis muscle, the fibularis brevis, so if we just remove the...this muscle.  This lies deep to the fibularis longus, and this lies on the lower two thirds, on the lateral surface of the shaft of the fibula, and you can see this tendon here, it winds round behind the lateral malleolus, just like the fibularis longus, and it curves round and inserts onto the base of the fifth metatarsal.  What this muscle does is it everts the foot.

 

The peroneus brevis and peroneus longus, or fibularis brevis/fibularis longus if you want, are innervated by the superficial branch of the common peroneal or common fibular nerve.

 

Next we've got the muscles of the anterior compartment of the leg, and there are four muscles in this compartment, you've got the tibialis anterior, the extensor hallucis longus, the extensor digitorum longus, and the fibularis tertius.

 

These muscles mainly act to dorsiflex, extend the toes, and to invert the foot.   I'll start with this muscle here, the tibialis anterior, which is the most superficial muscle.

 

I've just isolated this muscle and you can see its attachment on the tibia, on the lateral side of the tibia, and on the adjacent interosseus membrane.

 

This muscle forms a tendon which runs down the front of the leg and inserts medially on the foot.

 

This, this muscle actually provides support for the arch of the foot, as well as the tibialis posterior and the fibularis longus.  You can see its attachment on the foot here, so it inserts on the medial, the sort of lower surface of the medial cuneiform bone and on the adjacent base of the first metatarsal.

 

You can see by this medial attachment on the foot, that if this muscle contracts, it would pull the foot up, so that it will invert the foot, so again inversion is when you bring the foot up so both the Soles face each other. And it will also dorsiflex the foot, so it will pull the...pull the toes up in this direction towards the head.

 

That's the tibialis anterior, and it's innervated by the deep branch of the common peroneal nerve, so the deep, or the fibular nerve, the deep fibular nerve - so all the muscles in the anterior compartment are innervated by the deep fibular nerve.

 

Next we've got the two extensor muscles of the digits and of the great toe.

 

You’ve got the extensor digitorum longus, and the extensor hallucis longus.

 

Just like in the posterior compartment, where you've got the flexor of the toes and the flexor of the big toe, in the extensor compartment, the anterior compartment, you've got the extensor of the big toe - extensor hallucis longus  and the extensor of the digits - so extensor digitorum longus.

 

I've just removed the tibialis anterior, and we'll just have a look at this muscle here, the extensor hallucis longus.

 

If I just isolate that, so you can see it now here, it sits, it's not quite shown here, but it actually sits medially on the fibula, on the middle half of the fibula and it also attaches to the bit of the interosseus membrane that's adjacent to the fibula, so it attaches here - its origin, and it descends along, anteriorly on the leg and crosses over medially to insert here, at the base of the distal phalanx of the great toe.

 

Because of this muscle attachment, when this muscle contracts it extends the big toe and because of its...because it's anchored here at the ankle, it can also dorsiflex the ankle, so when it contracts it brings the toes up.

 

If I just bring the other muscles back into view, you can see, well you can see these two retinaculum, these are the extensor retinaculum, so you've got a superior and inferior retinaculum.

 

These extensor retinaculum, they hold the extensor tendons in place, so I'll just remove them for now, and you can look at the...well we'll look at the other side, we'll look at the left side because I haven't removed the tibialis anterior.

 

You can look at the relationship of these tendons, so the tibialis anterior tendon is most medial, then you've got the extensor hallucis longus tendon in between the tendon of the extensor digitorum longus, which I'll just talk about now, so it sits between the extensor digitorum longus tendon and the tibialis anterior tendon on the anterior aspect of the distal leg, so just worth noting that relationship.

 

Next we've got the extensor digitorum longus muscle, so the name gives away its function - it extends, it's the long extensor of the digits.  It attaches superiorly to the extensor longus muscle, so it's this muscle here, I'll just isolate it.

 

If we just take a look at the attachment, so we can see the extensor hallucis longus muscle a bit inferiorly with its origin, and the extensor digitorum longus attaches higher up on the proximal, medial surface of the proximal tibia, and it's also got this attachment on the lateral condyle of the tibia - so that's its origin, and if we follow the muscle down we can see that it splits to form four tendons which then attach to the digits.

 

The insertion point of these, of the extensor digitorum longus is on the bases of the intermediate and the distal phalanges.  I'm not sure if it's actually shown on this model...no it's not, but the, this tendon attaches to the bases of the middle and the distal phalanges.

 

What this muscle does is it extends the lateral four digits, and it can also dorsiflex at the ankle. Okay, so the final muscle that I'm going to talk about is the, a small little muscle called the fibularis tertius, or the peroneus tertius.

 

I'll just remove away the other muscles, so we can take a look at this.

 

I've just zoomed in a little bit and you can see this muscle here, this is the fibularis tertius, or the peroneus tertius, and this muscle originates as you can see distally on the medial surface of the fibula, so it's not that clear on this model here, but the peroneus tertius, or fibularis tertius is often considered a part of the extensor digitorum longus, and it's sometimes actually joined together with this muscle, so it runs down into the foot and this tendon inserts medially on the base of the fifth metatarsals.

 

What this muscle does is that it everts the foot and assists in dorsiflexing the foot.  Those are the muscles in the anterior compartment of the foot, sorry the leg.

 

You’ve got the tibialis anterior, which you can see here. You've got the extensor of the four toes, called the extensor digitorum longus, the extensor of the big toe, the extensor hallucis longus and you've got the little peroneus tertius, which is often thought of as part of the extensor digitorum longus.

 

The way I remember the muscle of the anterior and posterior compartments is that the deep layer of the muscles in the posterior compartment is quite similar to the muscles of the anterior compartment.

 

You’ve got the tibialis muscles.  In the anterior compartment you've got the tibialis anterior, and in the posterior compartment you've got the tibialis posterior. And then you've got the muscles which act on the toes, the four toes, the four digits, so you've got the extensor digitorum longus in the anterior compartment and in the posterior compartment you've got the flexor digitorum longus. And then you've got the muscle which acts on the big toe, so you've got the extensor hallucis longus in the anterior compartment, and correspondingly you've got the flexor hallucis in the posterior compartment. And then you've got a little small muscle, so in the anterior compartment, the little small muscle is the peroneus tertius, and in the posterior compartment the little small muscle is the popliteus.

 

That’s just one way of thinking of these things, to help you remember different muscles and how they act.