Actions: The only muscle of the quadriceps to cross both the hip and knee joints. It flexes the thigh at the hip joint, and extends at the knee joint.

Do the quads flex or extend the hip?

The function of the quadriceps femoris muscle is to extend the leg at the knee joint and to flex the thigh at the hip joint.

What quadriceps muscle helps with hip flexion?

The only part of quadriceps which acts as a hip flexor is this part, rectus femoris. Its the only part that arises from above the hip joint, which is here. Rectus femoris arises by two heads, from here and here, just above the acetabulum.