Functions: -Hip Flexion
-Knee Extension The rectus femoris is the only muscle in the quadriceps group that is involved in hip flexion, since it is the only one that originates in the pelvis and not the femur. The rectus femoris is a weaker hip flexor when the knee is extended because it is already shortened and thus suffers from active insufficiency. In essence: the action of raising a straightened leg will recruit more iliacus, psoas major, tensor fasciae latae, and the remaining hip flexors than it will the rectus femoris. Similarly, the rectus femoris is not dominant in knee extension when the hip is flexed since it is already shortened and thus suffers from active insufficiency. In essence: the action of extending a leg from a seated position is primarily driven by the vastus lateralis, vastus medialis, and vastus intermedius, and less by the rectus femoris. The rectus femoris is considered a direct antagonist to the hamstrings. The hamstrings oppose the rectus femoris at the hip joint through extension and at the knee joint through flexion. The rectus femoris can be torn which can be very painful. | 
