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.