So this final part of the tutorial is on the muscles of the posterior compartment of the
thigh.
There's only three muscles that you need to know in this compartment and these muscles
are all innervated by branches of the sciatic nerve.
If I just remove the gluteus maximus, you can see the sciatic nerve running into the
posterior compartment of the leg.
It emerges underneath the piriformis muscle in the greater sciatic foramen and it runs
into the posterior compartment of the leg to supply these three muscles.
So these muscles all act at the hip and the knee joint apart from the short head of the
biceps femoris muscle, which I'll come on to talk about in a bit.
These muscles extend the hip joint and flex the knee joint.
So you can see these deep gluteal muscles here.
You've got the superior and inferior gemellus and the obturator internus muscles.
You know that they originate a little bit about the ischial tuberosity.
If I just remove these for the time being, you can see the origin of the muscles of the
posterior compartment.
So these muscles are collectively known as the hamstring muscles.
They all originate apart from the short head of the biceps femoris muscle on the ischial
tuberosity.
So you can see I've just isolated the hamstring muscles and you can see their position on
the sacrum.
So we're looking posteriorly obviously on the pelvic bone and you can see the ischial
tuberosity here where the hamstring muscles are inserting.
So the first muscle is this one here, the biceps femoris muscle.
This lies laterally.
It's got two heads as the name suggests.
The word 'biceps' means 'two heads' in Latin.
Remember, the biceps muscle in the arm has two heads, a long and a short head.
It's the same for this muscle except this is the biceps femoris muscle.
So the two-headed muscle of the femur.
The long head of the biceps femoris muscle originates on the ischial tuberosity and the
short head, which is here, you can just see it beneath the long head, it originates on
the lateral lip of the linea aspera on the femur.
I'll just isolate these two muscles so we can take a closer look at them.
You can see that both heads converge to form this common tendon and it inserts onto the
head of the fibula.
You can see it.
We're looking anteriorly here.
The fibula, laterally.
You can see how the biceps femoris runs obliquely across the posterior compartment originating
on the ischial tuberosity and then inserting onto the head of the fibula.
So this muscle flexes the leg at the knee joint and it also extends and laterally rotates
the thigh at the hip joint.
It can also laterally rotate the leg at the knee joint when the knee joint is slightly
flexed.
So these muscles are obviously innervated by the sciatic nerve, but the long head receives
innervation via the tibial branch of the sciatic nerve, whereas the short head receives innervation
via the common fibula branch of the peroneal nerve.
So the biceps femoris muscle lies laterally.
You've got these two muscles, the semitendinosus and the semimembranosus, which lie medially.
So a way of remembering this is that semimembranosus, m for medial.
The semimembranosus and the semitendinosus go together.
So the semitendinosus is also medial.
But semitendinosus has the letter t in it, so --tendinosus sits on top.
So you've got the semimembranosus and semitendinosus medially with the semitendinosus sitting superficially,
so it sits on top of the semimembranosus.
So again, this muscle, the semitendinosus originates on the ischial tuberosity and it
descends to insert just behind the insertion points of the gracilis muscle and the sartorius
muscle on the medial aspect of the upper tibia.
So again, what this muscle does is it extends the hip joint and it flexes the knee joint.
It's innervated by the tibial branch of the sciatic nerve.
So finally, you've got the semimembranosus muscle.
This muscle sits under the semitendinosus.
It originates again on the ischial tuberosity and it inserts down here.
So it inserts also on the medial condyle of the femur and also on the medial condyle of
the tibia.
It also blends with the joint capsule of the knee.
So the fascia that surrounds the knee joint, some of the fibers that come off this tendon
blend/join the fascia that surrounds the knee and also, it contributes to some of the ligaments
around the knee.
You can see here its insertion posteriorly on the medial condyle of the tibia and the
medial condyle of the femur.
So this muscle, it flexes the knee joint and it extends the thigh at the hip joint.
It also medially rotates at the hip and the knee joint.
So it acts together with the semitendinosus.
I forgot to write that the semitendinosus also medially rotates at the hip and knee
joint.
So these muscles work together.
So again, this muscle is innervated by the tibial branch of the sciatic nerve.
So those are the three muscles that you need to know in the posterior compartment.
It's quite simple, the posterior compartment.