Muscles of the Thigh

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Anterior Compartment

Okay, so I’m going to do a couple of tutorials on the muscles of the thigh. The muscles of the thigh are arranged into three compartments. You’ve got an anterior compartment, medial, and posterior compartment and these are separated by the intermuscular septum.

The anterior compartment is known as the extensor compartment, the medial compartment is known as the adductor compartment, and you’ve got the posterior compartment which is the flexor compartment.

 

These various compartments have different actions on the hip and knee joints. And the different compartments are supplied by different nerves.

 

The anterior compartment is supplied by the femoral nerve. The medial compartment is supplied mainly by the obturator nerve, and the posterior compartment of the thigh is supplied by the sciatic nerve.

 

In this first part of the tutorial I’m going to talk about the anterior compartment of the thigh.

 

This is this group of muscles here anteriorly in the thigh, obviously and these muscles are supplied by the femoral nerve. And you’ve got five muscles in this compartment. You’ve got the four large quadriceps femoris muscles, and then you’ve also got the sartorius muscle. As well as these muscles, you’ve also got the terminal ends of two muscles here.

 

The iliacus and the psoas major, you can see them superiorly here, on the anterior compartment, the terminal ends of these two muscles. I’ll show you these muscles now.

 

We’ll take a look at the iliopsoas muscles. Now the iliopsoas muscles are the psoas major and the iliacus.

 

Collectively the iliacus and psoas are knows as the iliopsoas muscles. The iliacus muscle is this muscle here. And it inserts onto the lesser trochanter of the femur together with the psoas major muscle.

 

This is the psoas major muscle here and they have this common insertion point on the lesser trochanter of the femur. The iliopsoas and the iliacus muscle are collectively referred to as the iliopsoas muscles.

 

You can see here I’ve just highlighted the iliacus and psoas major muscles so you can see the origin and the insertion a bit more clearly.

 

You can see the psoas major muscle originating on the sides of the vertebral bodies of the transverse processes and the intervertebral discs. And it originates from the vertebral bodies from L-sorry, T12 down to L5.  And you can see its insertion on the lesser trochanter of the femur. And also the iliacus muscle which sits in the iliac fossa, and then inserting on the same place on the lesser trochanter.

 

What these muscles do is they flex the hip. Flex the thigh at the hip joint.

 

Okay, so the next group of muscles in the anterior compartment are the quadriceps femo

ris muscles.

 

You’ve got four muscles in this group. You’ve got the rectus femoris muscles, and you’ve got the three vastus muscles.  You’ve got the vastus medialis, lateralis and intermedius which lies on top of the rectus femoris muscle.

 

The rectus femoris muscle is interesting. It originates on the pelvis. It has action both at the hip joint and at the knee joint whereas the vastus muscles only act at the knee joint.

 

The rectus femoris muscle is this muscle here which lies in the anterior compartment and is the most superficial muscle.

 

This muscle originates on the pelvis, on the anterior inferior iliac spine, and its also got another head which is called the reflected head which isn’t shown on this model but it’s reflected back and originates just superior to the acetabular fossa.

 

There’s two heads to the rectus femoris muscle; one originates on the anterior inferior iliac spine, and the other is reflected back and it originates on the… just superior to the acetabulum, and that’s called the reflected head. The head that originates on the anterior inferior iliac spine is known as the straight head.

 

The rectus femoris along with the vastus muscles inserts by the quadriceps tendon onto the patella bone. And the patella bone inserts onto the tibia by the patellar ligament.

 

A muscle is attached to the bone by tendons and bone is attached to bone by ligaments.

 

The quadriceps tendon is this common insertion onto the patella bone. And the patella then inserts onto the tibia by the patellar ligament so it’s attached to the tibia by the patellar ligament.

 

Because the rectus femoris originates on the pelvis, it flexes the thigh, the hip, whereas the vastus muscles don’t do that because they originate on the femur.

 

Let’s take a look at the vastus muscles.

 

I’ll just get rid of the rectus femoris muscle. And you can see this muscle here sitting laterally, this is the vastus lateralis. And again, this originates from the femur and it inserts onto the patella by the quadriceps femoris tendon. Then medially you’ve got this teardrop-shaped muscle, which is known as the vastus medialis because it sits medially. And this again inserts by the quadriceps femoris tendon by the patella. And just under the rectus femoris muscle, which I removed, you’ve got this muscle here, the vastus intermedius.

 

These three muscles all originate on the femur and they insert onto the patella by the quadriceps femoris tendon and extend the leg at the knee joint.

 

The final muscle we have is this muscle here called the sartorius. This is a strap-like muscle which runs from the anterior superior iliac spine, all the way down on the medial surface of the proximal tibia.

 

This is the sartorius muscle and it acts to flex the thigh at the hip joint and also to flex the thigh at the knee joint.

 

Let’s take a look at it more isolated.

 

You can see you’ve got the anterior superior iliac spine here where the sartorius muscle originates and just below you’ve got the anterior inferior iliac spine where the rectus femoris muscle originates. And the sartorius winds around obliquely descending along the thigh to insert on the medial surface of the proximal tibia.

 

The insertion point here is just anterior to the insertion point of the gracilis and the semitendinosus muscle.

 

You can see you’ve got the hamstring muscle here and the gracilis, so they insert just behind the insertion point of the sartorius muscle.

 

That’s the sartorius.

 

Those were the muscles of the anterior compartment of the thigh. You’ve got the iliopsoas muscles, you’ve got the quadriceps femoris muscles and you’ve got the sartorius muscle. And the muscles that originate on the pelvic bones have actions at the hip and knee and the muscles that originate only on the femur only act on the knee joint.

 

The rectus femoris muscles and the iliopsoas muscles act on the hip, and the quadriceps femoris muscles act at the knee. And these muscles are innervated by the femoral nerve.

 

Medial Compartment

In this second part of the tutorial, I’m going to talk about the medial compartment of the thigh. There are six muscles in this medial compartment. These muscles are mainly innervated by the obturator nerve. These muscles act mainly to adduct the femur, the thigh at the hip joint – ad-duct, adduct.

 

I’m not going to talk in too much detail about the origin and insertion of these muscles because it takes quite a bit of time, so I just want you to get an idea of what these muscle are and their actions.   these muscles mainly originate on the body of the pubis and the ischiopubic ramus of the pelvic bone, so this area on the pelvis and they insert at various points on the femur and also in the case of the gracilis, on the tibia.

 

This view here just shows the medial compartment muscles of the thigh. I’ll be flicking between the two models.

 

The first muscle we’ll look at the gracilis muscle. This muscle runs vertically down from the medial aspect of the pelvis on the body of the pubis down to insert on the medial surface of the proximal tibia.   I’ve just isolated it here and you can see its path.   it runs vertically down the inside of the thigh, originating as you can see on the body of the pubis and inserting on the medial surface of the proximal tibia.

 

It inserts between the insertion points of the sartorius muscle, which inserts anteriorly. Posteriorly, you have the insertion point of the semitendinosus muscle, which is a hamstring muscle in the posterior compartment.

 

You can see this muscle here. This muscle, this is the semitendinosus muscle and you can see the tendon inserts just behind the insertion point of the gracilis muscle. And just anteriorly, you’ve got the sartorius muscle, which is a muscle of the anterior compartment that descends obliquely along the thigh.   the insertion point of the gracilis is sandwiched between these two on the medial surface of the proximal tibia.

 

This muscle is innervated by the obturator nerve and it acts to adduct the thigh and it also flexes the leg at the knee joint.

 

Next, we’ve got the pectineus muscle, which is this muscle here.   This muscle is a rectangular muscle and it’s innervated by the femoral nerve.   there are two muscles which are innervated by different nerves. You’ve got the pectineus, which is innervated by the femoral nerve and you’ve got the hamstring part of the adductor magnus muscle, which is innervated by the sciatic nerve.

 

The pectineus muscle is innervated by the femoral nerve and it acts to adduct the thigh at the hip joint and it also flexes at the hip joint. You can see its origin up here. This is the pectineal line. It inserts here on the femur.

 

This muscle actually forms part of the medial floor of the femoral triangle. I’ll do a tutorial on the femoral triangle, but this muscle forms part of the floor of the femoral triangle.

 

Just to briefly tell you, the femoral triangle is this triangle bound superiorly by the inguinal ligament. You’ve got this medial border of the sartorius forming the lateral border of the triangle and the medial border of this muscle, the adductor longus forming the medial border of the triangle.   it’s this area here. You can see the pectineus forming the floor of this triangle, the femoral triangle.

 

Next, we’ve got the adductor muscles. You’ve got adductor longus, the adductor brevis and the adductor magnus. The one you can see here is the adductor longus. The adductor brevis and magnus sit deep to this.

 

The adductor longus forms the most medial muscle of the floor of the femoral triangle and its medial border forms the medial border of the femoral triangle.   this is the adductor longus muscle.

 

You can see its origin here on the pubis and it inserts on the middle third of the shaft of the femur.   what this muscle does is it adducts the femur and it also can medially rotate. And again, it’s innervated by the obturator nerve.

 

I’ll just get rid of some of these muscles of the anterior compartment, so we can just get a clearer look at these medial muscles.   you’ve got the pectineus, you’ve got the adductor longus muscle. And underneath the adductor longus and pectineus, you can see the adductor brevis muscle.

 

Again, this muscle has its origin on the pubis and it inserts a little bit higher up on the femur, the upper third of the femur. What this muscle does is it adducts the thigh at the hip joint and again, it’s innervated by the obturator nerve.

 

I’ll just remove the adductor brevis and we can see this big muscle lying underneath this. This is the final adductor muscle. This is the adductor magnus. ‘Magnus’ means great in Latin, so it’s the big adductor muscle. It’s got two muscle bellies.   you’ve got the adductor part and you’ve got the hamstring part, this little bit down here.

 

This lateral part of the muscle is the adductor part and the medial part is this hamstring part. It’s this hamstring part of the adductor magnus that’s innervated by the sciatic nerve, the tibial branch of the sciatic nerve.

 

In the medial compartment, everything is innervated by the obturator nerve apart from this hamstring part of the adductor magnus and the pectineus muscle, which is innervated by the femoral nerve.

 

Just taking a quick look at the origin of this muscle, you can see it’s got quite a broad origin. It’s got two origins for the hamstring part and the adductor part.   the hamstring part originates on the ischial tuberosity and the adductor part originates on the ischiopubic ramus.

 

You can see where this muscle inserts. The hamstring part inserts on something known as the adductor tubercle on the medial condyle of the femur, the distal femur. You can see that there. The adductor part inserts on the posterior surface of the femur. This muscle adducts and medially rotates.   there, it’s just important to know about these two different parts.

 

And then you’ve got this little circular opening in the muscle. This is known as the adductor hiatus. This allows the femoral artery and veins to pass through. If I just bring those in, you’ll be able to see how they pass through this hiatus in the adductor magnus muscle to run into the popliteal fossa.

 

The final muscle is the obturator externus muscle. We’ll just show you that. If you remember from the tutorial on the gluteal region, you’ve got the obturator internus which originates on the medial surface of the obturator membrane.   the obturator externus is this muscle which originates on the lateral or external surface of the obturator membrane.

 

Unlike all the muscles of the medial compartment, this one actually laterally rotates the femur. It doesn’t adduct.

 

If we just take a look at this muscle, this is the obturator foramen and it’s covered by an obturator membrane. You’ve got the obturator externus sitting on top of it. It’s this fan-shaped muscle and you can see how it extends behind the hip joint and it inserts into this little – you can see this little depression, this oval depression, in this trochanteric fossa. This is where it inserts and it laterally rotates the femur.

 

You’ve got the obturator internus sitting on the medial side. This is the obturator externus sitting on the medial compartment. And again, it’s innervated by the obturator nerve.

 

Those are the six muscles of the medial compartment of the thigh. You’ve got the gracilis most medially. You’ve got the pectineus, which is one that’s innervated by the femoral rather than the obturator nerve. You’ve got the three adductor muscles with the adductor longus the most superficial, the adductor brevis behind that and the big adductor magnus with its two heads, one of which is innervated by the sciatic nerve (the hamstring part). And then you’ve got the obturator externus muscle.   those are the six muscles of the medial compartment of the thigh.

 

Posterior Compartment

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.

 

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.

 

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.

 

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.

 

You can see I’ve just isolated the hamstring muscles and you can see their position on the sacrum.   We’re looking posteriorly obviously on the pelvic bone and you can see the ischial tuberosity here where the hamstring muscles are inserting.

 

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.   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.

 

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.

 

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.

 

The biceps femoris muscle lies laterally. You’ve got these two muscles, the semitendinosus and the semimembranosus, which lie medially.   A way of remembering this is that semimembranosus, m for medial. The semimembranosus and the semitendinosus go together.   The semitendinosus is also medial. But semitendinosus has the letter t in it, so –tendinosus sits on top.   You’ve got the semimembranosus and semitendinosus medially with the semitendinosus sitting superficially, so it sits on top of the semimembranosus.

 

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.

 

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.

 

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.   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.   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.

 

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.   It acts together with the semitendinosus. I forgot to write that the semitendinosus also medially rotates at the hip and knee joint.   These muscles work together.

 

Again, this muscle is innervated by the tibial branch of the sciatic nerve.   Those are the three muscles that you need to know in the posterior compartment. It’s quite simple, the posterior compartment.