Origins, insertions, innervation and functions of the posterior thigh muscles.
Hello, hello, everyone. This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, I'm going to be talking about the posterior thigh muscles. And this is a group of muscles that you find on, of course, the posterior or the dorsal side of your thigh. And these muscles are very important for your mobility. And we’re going to be discussing the different origins, insertions, innervation, and functions associated to these structures.
Now, these muscles are important for mobility, as I mentioned, because they are the extensors of the hip joint and also flexors of the knee joint. So, for that reason, they pull your leg when you’re trying to swing back to kick a ball for example. This is a kind of movement that these muscles would be associated to.
Now, one reminder here is that let’s first list them before we talk about these muscles. What you need to know is that the posterior thigh muscles are comprised or is a group comprised of three muscles, and these are the biceps femoris, the semimembranosus, and the semitendinosus.
Now, in general, the posterior thigh musculature runs from the dorsal side of your pelvis towards the lower leg and then shapes the dorsal thigh or your posterior thigh, and you can clearly see here on this image. Even though we have here still the gluteal muscles, which is a different group of muscles that we’re going to... that we have here a tutorial here at Kenhub that you can learn about, and also these muscles, these are the lower leg muscles. They are a different group of muscles also covered on tutorials here at Kenhub.
Let’s start with the very first muscle here on our list, this seen here highlighted in green in both of these images. On the left side, you see the muscle in a relaxed position, and on the right side, it’s contracted. And this muscle is known as the biceps femoris.
Now, the biceps femoris is the largest and strongest of the three muscles that we just listed before. And it consists of two parts: the long head and the short head. And you can clearly see here on this image where it’s contracted, the long head here, going all the way up, and then you have the short head here. And we’re going to learn the origin points for both of these heads and also the insertion points right following this slide.
Now, let’s start with the origin point for the long head. And what you need to know is that the long head of the biceps femoris is going to originate from the ischial tuberosity, now, as well as the caudal part of the sacrotuberous ligament. And you can clearly see here on this image, this is the ischial tuberosity right about here. And you can clearly see the origin point for the biceps femoris or for the long head of the biceps femoris. And as you see here on this list, the ligament, the sacrotuberous ligament is also going to serve as an origin point for the long head.
Now, moving on to, now, the short head of the biceps, and in terms of origin point, you need to know that this part of the muscle is going to originate from the linea aspera, which is a rough line at the dorsal side of the femur. And the muscle, the short head, is also going to originate from another point, bony structure that is known as the lateral supracondylar line of the femur.
Now, let’s take a look at the insertion point for the biceps femoris. And both heads of the biceps unite at the popliteal fossa to a common tendon inserting at the lateral side of the head of the femur, and you can clearly see here on this image. So this is the lateral side of your leg. And the fibula is here. This is the head of the fibula and will serve as insertion point for the biceps femoris.
Also, important to mention that this is the lateral limitation of the popliteal fossa, and you can feel it if you touch your popliteal fossa and go laterally. You can feel this insertion point or this tendon.
It’s time for us to move on to the different functions associated to the biceps, and now, you have an image here on your right side that shows the muscle in action. And we’re looking at it from a lateral view, and you can clearly see here moving: the hip joint and also the knee joint. And for that reason, let’s add that the long head of the biceps is going to contribute to extension of the leg at the hip joint and also is going to be responsible for stabilizing the hip joint. And you can see here an arrow that shows extension of the thigh.
Now, when the long head and the short head work together, what’s going to happen is these are going to be able to flex the knee joint as you can clearly see here on this image as well.
Now, also the biceps femoris is the only muscle from that list that we talked about, the posterior thigh muscles that can perform a little bit of external rotation of the knee joint, indicated here by this arrow.
We have covered the attachment points and the functions for the biceps. It is time for us to now cover logically the innervation of this muscle. Now, we’re looking at the highlighted nerve here on the image, which is the tibial nerve. And what you need to know is that the biceps femoris muscle is innervated by the sciatic nerve that, on its way from... or the nerve comes all the way from the pelvis to the leg, and the sciatic nerve then splits up to be the tibial and also the peroneal. It has a peroneal part that we’re going to look at. And the long head is going to be, then, innervated by the tibial nerve. While the short head is going to be innervated by the other portion of the sciatic nerve, which is the common fibular nerve or also known as the common peroneal nerve.
Moving on to the second muscle on our list of the posterior thigh muscle seen here, highlighted in green. On the left side, you see the muscle relaxed, while on the right side, it’s contracted as well performing some of its actions. And this is the semimembranosus.
Now, just like the long head of the biceps femoris, the semimembranosus will originate at the ischial tuberosity, and we can clearly see here. So this is the ischial tuberosity, the insertion point for the biceps and then the semimembranosus also using this part of the hip bone to originate from or to arise from. And then it goes all the way down to insert at the medial condyle of the tibia, and you can also see clearly on this image. And you can also add that the oblique popliteal ligament will serve as an insertion point for the semimembranosus. So write this down in case you’re asked on your exam.
Now, let’s take a look at the different functions associated to the semimembranosus. And we have an image of the muscle in action on the right side. And this is the lateral view of the hip, and thigh, and also your knee. And you can see how it’s contributing to the movement of all these structures.
Now, as I mentioned, the semimembranosus and like the biceps femoris, this muscle is going to contribute to extension of the hip joint, as you can see here, or extension of the thigh at the hip joint and also flexion of the knee joint, as you can see here indicated by this last arrow.
Now, another function that is important to remember here about the semimembranosus, this muscle is going contribute to internal rotation of the knee joint represented here by this last arrow.
If you ask me, “What is the innervation of the semimembranosus?” This is it, the highlighted or the nerve highlighted in green. This is the tibial nerve which will serve as the innervation for the semimembranosus.
Moving on to the last muscle on our list, with a similar name, is known as the semitendinosus. And this muscle in terms of origin points, just like the other posterior thigh muscles, or the semitendinosus will originate at the ischial tuberosity, as you can also clearly see here on this image, the origin point or the ischial tuberosity serving as origin point for the semitendinosus. And another thing that you need to remember is that the sacrotuberous ligament is going to serve as origin point for the semitendinosus as well.
In terms of insertion point, the muscle is going to go all the way down to insert at the medial aspect or surface of the tibia as you can see here on the pes anserinus or the superficial part of the pes anserinus, which is found next to the tibial tuberosity.
Let’s take a look now at the different functions associated to the semitendinosus. Like the other posterior thigh muscles, it’s going to contribute to extension of the hip joint, extension of the thigh at the hip joint. You can say that as well. And also, this muscle is going to help flex the knee joint as you can see on this last arrow. Important to remember here that another function of the semitendinosus is going to include the internal rotation of the knee joint, seen here represented by this arrow on the bottom of the image.
Another question in your head might be, “What is the innervation of the semitendinosus?” And again, you just need to remember that this muscle is going to be innervated by the tibial nerve just like the semimembranosus.
Now, before I end this tutorial, there is a term here that I would like to clarify that I mentioned a few times known as pes anserinus. Now, this is commonly is used as an exam question, and I want to clarify before we complete this tutorial.
And pes anserinus is an anatomic name that we give to conjoint tendons at the medial aspect of the knee. Now, “pes anserinus” is a Latin term that means “goose’s foot,” and what happens is that these muscles are going to form this tendon that will then insert on the medial proximal surface of the tibia. And these muscles include the semitendinosus—if you remember correctly, I mentioned this on the previous slides—and another one is going to be the gracilis—it’s going to contribute to the pes anserinus—and the sartorius. So these three muscles will come together to form this goose foot and then insert on the medial proximal surface of the tibia.
Another fact that is important to add here so you can remember this for the next exam is that, sometimes, the insertion of the semimembranosus is referred to as the pes anserinus profundus. So keep that also in mind.
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