What is the primary function of the semitendinosus muscle?

The semitendinosus muscle is one of the hamstring muscles located at the back of the thigh. Its primary functions are indeed to flex the leg at the knee and to extend the thigh at the hip. When the semitendinosus contracts, it shortens, allowing the knee to bend, which is the flexion of the leg. Additionally, because it originates from the ischial tuberosity and inserts into the tibia, it also assists in extending the thigh at the hip joint when the hip is in a flexed position.

This dual action is essential during various movements, such as running or jumping, where both knee flexion and hip extension are required. The semitendinosus plays a critical role in stabilizing the pelvis and controlling movements, highlighting its importance in both dynamic and static activities.

The other options focus solely on either knee extension or hip extension, which does not capture the complete function of the semitendinosus muscle. By addressing both actions, the correct option encompasses the full role of the muscle.