Final answer:
The prime mover for knee flexion is the hamstrings muscles located in the posterior compartment of the thigh.
Step-by-step explanation:
The prime mover for knee flexion, also known as the knee flexors, are the hamstrings.
When we want to flex the knee joint, the hamstrings muscles are activated. These muscles include the biceps femoris, semitendinosus, and semimembranosus. They work together to flex the knee and are located in the posterior compartment of the thigh.
For example, when we want to sit cross-legged, the hamstrings are responsible for flexing the knee.