Which of the following muscles does not extend the knee?

The semitendinosus muscle is primarily involved in flexing the knee rather than extending it. It is one of the muscles in the hamstring group, which is located at the back of the thigh. While the quadriceps femoris, which includes the rectus femoris and the vastus lateralis, is primarily responsible for extending the knee joint, the semitendinosus plays a crucial role in movements that require bending the knee, such as sitting and walking.

The quadriceps femoris group consists of four muscles (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius) that work together to extend the knee, making them key players in activities like running and jumping. The rectus femoris is a part of this group and also flexes the hip, but its primary function related to the knee is extension. The vastus lateralis is another component of the quadriceps that specifically helps extend the knee. Thus, the key point is that the semitendinosus is not involved in knee extension, making it the correct answer to the question.