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Functions and anatomy of the semitendinosus muscle shown with 3D model animation.
Hey everyone! It's Nicole from Kenhub, and in this tutorial, we're going to be looking at the functions of the semitendinosus muscle which is this muscle you can see here isolated on the screen.
Now let it begin by saying that the semitendinosus is a posterior thigh muscle which means it is located in the posterior compartment of the thigh as you can see here between the buttocks and the popliteal fossa. As we know, in order for muscles to carry out their proper function, there must be a good nerve supply and the semitendinosus is no exception. It's innervated by the tibial nerve which is this nerve you can see here highlighted in green. It's the tibial division of the sciatic nerve.
The semitendinosus is actually one of three large muscles located in the posterior compartment of the thigh. The three muscles being the semitendinosus, the semimembranosus and the biceps femoris muscles. These three posterior thigh muscles are also sometimes referred to together as the hamstrings.
The fibers of the semitendinosus muscle originate or have their origin on a palpable protrusion of the surface of the ischium called the ischial tuberosity. This muscle originates from the medial aspect of the ischial tuberosity and has a shared common tendon at its origin with the long head of the biceps femoris, which as I mentioned, also happens to be one of the hamstring muscles.
The muscle belly of the semitendinosus has a fusiform shape and the muscle fibers go on top to form a tendon just below the mid-thigh. The tendon crosses the collateral ligament as you can see here and goes on to insert medially on the superior surface of the tibia.
You should also probably know that two other tendons from the gracilis and sartorius muscles attach at the attachment of the tendon of the semitendinosus. These three tendons together are referred to as the pes anserinus, because the point at which their tendons attach together looks like a goose's foot and the Latin translation of pes anserinus quite literally is "goose foot".
Likewise, the semitendinosus has an interesting backstory to its name as the muscle tapers off to form tendons at its insertion and as the words semi- means half, you can see where the name semitendinosus comes from quite literally as half of the muscle is made up of a tendon.
Now, let's look at the actions that this muscle can carry out. We've already seen that the semitendinosus is one of the hamstring muscles found in the posterior thigh. So, as you can see, along with the two other hamstring muscles, the semitendinosus acts on the hip joint and the knee joint.
When we look at the action of the semitendinosus at the hip joint, we can see from this movement here that it extends the thigh. This action is carried out in conjunction with the semimembranosus and the biceps femoris muscles. So, we can safely say that these three muscles together work in synergy to extend the thigh at the hip joint.
Let's move on and talk about the movements that the muscle is involved in at the knee joint. At the knee joint, the semitendinosus facilitates flexion of the leg so as you can see here as the leg is being flexed, the semitendinosus is one of the muscles working to facilitate this movement and the joint that is affected is the knee joint. This is a movement that we make when we are walking or running or even when you stretch after a workout by raising your legs so that your heel touches your buttocks. This movement is known as the flexion of the leg at the knee joint.
In addition to extension at the hip joint and flexion at the knee joint, the semitendinosus together with the semimembranosus muscle facilitates internal rotation at the knee joint when the knee is flexed, as you can see here. For example, in a sitting down position, when you have your foot planted on the floor and you move your leg inwards so that your toes are pointing towards the midline, this is what's called internal rotation, and the semitendinosus together with the semimembranosus acts on the knee joint to make this movement possible.
So just to recap the main points that we now know about the semitendinosus muscle, we now know that the action of this muscle produces movement at the hip joint and at the knee joint, it extends the thigh at the hip joint by raising the thigh posteriorly, it flexes the leg at the knee joints by raising the lower leg posteriorly, and finally, we also know that when the knee is flexed, the semitendinosus acts as an internal rotator at the knee joint by rotating the lower leg medially so that the toes point towards the midline of the body.
This brings us to the end of our tutorial. I hope you enjoyed this video and don't forget to subscribe to watch more short videos like this one on our website. Thanks!