Which muscle is involved in both hip flexion and knee extension?

The rectus femoris is the muscle that plays a dual role in both hip flexion and knee extension. It is one of the four muscles that make up the quadriceps group, and its unique position allows it to cross both the hip and knee joints. When the rectus femoris contracts, it can flex the hip by pulling the femur upwards while simultaneously extending the knee by acting on the patella through the quadriceps tendon. This dual functionality is essential in movements such as running or kicking, where both actions are needed in a coordinated manner.

In contrast, the sartorius primarily aids in hip flexion and external rotation of the thigh without directly contributing to knee extension. The gracilis is mainly involved in hip adduction and assists with knee flexion, whereas the vastus medialis primarily focuses on knee extension without any role in hip flexion. Thus, the rectus femoris stands out for its combined action on both the hip and knee joints, making it the correct answer.