 This is a quick demonstration of the POPRITIUS muscle. The POPRITIUS muscle is the deepest of the deep compartment of the back of the leg. We can see the POPRITIUS inserts on the back of the DBR. This is the solial line, below the solial line is the solius and then the muscle goes up and laterally the origin is from two places. POPRITIUS muscle is by fit. One portion goes inside the knee joint here. It goes to the capsule of the knee joint and gets attached to the lateral meniscus and the other portion goes outside and gets attached to the lateral condyle of the femur and it separates the lateral meniscus of the knee from the febular collateral ligament and it also separates the capsule of the knee joint. This POPRITIUS it also forms the lower part of the floor of the POPRITIUS muscle and it is covered by the POPRITIUS fascia. This POPRITIUS muscle has got a bursa between it and the knee joint which communicates with the knee joint and that is called the POPRITIUS bursa. And this POPRITIUS also gets an expansion from this muscle that we see above it. This is the semi-membranosis and the semi-membranosis gives an aponeurotic expansion which reinforces the POPRITIUS fascia. What is the action of this POPRITIUS muscle? The POPRITIUS muscle helps to lock the knee joint and unlock the knee joint both during extension and during flexion. If it acts only when the leg is extended, when the leg is extended and not weight bearing then it rotates the tibia 5 degrees medially. When the leg is extended and weight bearing tibia cannot rotate then it rotates a few more 5 degrees laterally to unlock the knee joint. So, that is one very important use of the POPRITIUS muscle. The POPRITIUS muscle is supplied by branch of the tibial nerve which has got a unique course. It runs on the surface of the POPRITIUS and then it goes inside the POPRITIUS and supplies the POPRITIUS from its deep aspect. So, these are some salient points about the POPRITIUS muscle which is the deepest muscle in the posterior compartment. So, that is all for now about the deep muscles of the POPRITIUS compartment. Thank you very much for watching. If you have any questions or comments please put them in the comment section below. This is Sanyal Sanyal. Have a nice day.