Muscles of the Leg – Part 1 – Posterior Compartment – Anatomy Tutorial

Muscles of the Leg – Part 1 – Posterior Compartment – Anatomy Tutorial


Okay so this tutorial is on the muscles of
the leg. So the leg is the region of the lower limb, which lies between the knee joint and
the ankle joint. So 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. So 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. So the muscles of the posterior compartment here, mainly act
to plantar flex the foot, flex the digits, and invert the foot. So 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. So 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. So if you imagine lying down on your back and bringing your toes up towards
your head – that’s dorsiflexion and plantarflexion is the opposite. So 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. So 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. So the lateral compartment are the muscles here, which lie
laterally, and these evert the foot. So this tutorial will be concerned with the muscles
of the posterior compartment of the leg. So 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. So 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. So just a
quick point about the common fibular nerve, just while I’ve mentioned it. So 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. So 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. So 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.
So you’ve got three muscles in the superficial layer, and four muscles in the deep layer.
So I’ll just work from superficial to deep and talk you through these structures. So
obviously we’re looking at the most superficial muscle here, and this is called the gastrocnemius
muscle. So 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. So the medial
head inserts on the medial condyle and the lateral head inserts on the lateral condyle.
So I’ll just show you that in a bit more detail. So 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. So 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. So 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. So I’ve just brought the muscle back in and you can
see the tendon now. So this is also known as the Achilles tendon. So 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. So that’s the gastrocnemius muscle and that’s the most superficial
muscle. So 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. So this muscle actually lies under the medial head of the gastrocnemius
muscle. So you can see the tendon emerging here and it actually lies underneath this.
So we’ll just take a closer look at this muscle’s origin. So 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. So 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. So 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. So 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. So these three muscles are innervated by the
tibial nerve – so that’s the branch of the sciatic nerve which innervates the posterior
compartment. So 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. So 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. So 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. So this muscle actually serves to unlock the knee when it’s
locked in extension. So 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. So in the standing position, the knee is fully
extended and it’s locked like this. So the popliteus functions to laterally rotate the
femur on the tibia and unlock the knee joint. So again this muscle is innervated by the
tibial branch of the sciatic nerve. So next we’ve got these three large muscles. So 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. So 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. So 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. So 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. So 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. So
I’ll try and show you that. So 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. So 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. So what
this muscle does is it obviously flexes the digits. So 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. So 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.
So the last muscle is this muscle which sits between them. So this is the tibialis posterior
muscle. So 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. So
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. So 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. So those are the muscles
of the posterior compartment of the leg, I hope that’s cleared things up a little bit.