Hey everyone! This is Nicole from Kenhub, and in this tutorial, we're going to be looking at the functions of the semimembranosus muscle. Before we start talking about this muscle's functions, first let's introduce the semimembranosus muscle.
The semimembranosus is one of the three muscles found in the posterior compartment of the thigh known as the hamstrings. One of the key aspects to the proper functioning of a muscle is innervation. Like the semitendinosus, the semimembranosus is innervated by the tibial nerve, which is the tibial branch of the sciatic nerve.
As you can see here, this muscle is flat-shaped. It has its origin on the lateral impression of the ischial tuberosity as a flat tendon. This tendon from which the muscle originates descends deep to the biceps femoris and semitendinosus muscles and forms an aponeurosis. The fibers of the semimembranosus muscle form a rounded tendon around the level of the mid-thigh and go on to insert onto the posterior aspect of the medial tibial condyle.
Now that we're finished talking about the innervation, origin and insertion of this muscle, let's now talk about the actions that this muscle facilitates. The semimembranosus muscle acts on both the hip and the knee joint in the same way as the other two muscles of the posterior compartment of the thigh. It's also an antagonist of the quadriceps femoris. Let's start by looking at the action of the semimembranosus at the hip joint.
The contraction of the muscle acts to extend the thigh which is this movement you can see here now. This action is carried out with the help of the other two hamstring muscles namely the semitendinosus and the biceps femoris. So, these three muscles are working together or in conjunction to effect this movement.
The semimembranosus enacts two movements at the knee joint. Firstly, when the muscle contracts, it causes the flexion of the leg at the knee. So, as you can see here, the leg is being flexed as the muscle contracts. This is a movement that is made, for example, when walking, running, swimming, and so on. Once again, the semimembranosus works in synergy with the other two posterior thigh muscles to flex the leg at the knee.
The second action that is carried out by the semimembranosus at the knee joint is internal rotation. This action on the knee joint by the muscle can occur when the knee is flexed at a ninety-degree angle or greater. As you can see here now, the action of this muscle helps to rotate the knee joint medially in this position.
Now, let's recap the actions of the semimembranosus.
So, in this tutorial, we've seen that the semimembranosus along with the other hamstring muscles acts on the hip joint and the knee joint. We've also seen that at the hip joint, the semimembranosus extends the thigh which is essentially the raising of the thigh posteriorly. At the knee joint, the muscle facilitates flexion where the lower leg is raised posteriorly. And, finally, we've also seen that it helps facilitate internal rotation at the knee joint when the knee is flexed by rotating the lower leg medially so that the toes point forward towards the midline of the body.
And this brings us to the end of our tutorial. Thanks for watching and don't forget to check out more videos like this one at kenhub.com.