Which muscle is considered a hip flexor and knee extensor?

The rectus femoris is a key muscle that serves dual roles as both a hip flexor and a knee extensor. As part of the quadriceps muscle group, the rectus femoris is unique because it crosses both the hip and knee joints. This anatomical positioning allows it to flex the thigh at the hip joint while extending the leg at the knee joint.

When the rectus femoris contracts, it raises the thigh towards the torso, thereby performing hip flexion. Simultaneously, it facilitates the straightening of the leg at the knee, which is knee extension. This functional versatility makes it essential for activities such as running, jumping, and kicking.

Other muscles listed do not fulfill both of these roles. The biceps femoris primarily acts as a knee flexor and hip extensor, the gastrocnemius primarily aids in plantarflexion of the foot and assists in knee flexion, and the adductor longus primarily functions in adducting the thigh rather than flexing the hip or extending the knee. Thus, the rectus femoris is the only muscle among the options that aligns with the definitions of both a hip flexor and a knee extensor.