The rectus femoris is one of four muscles that make up the quadriceps group, which extends the knee. The other three muscles that synergistically work with the rectus femoris to extend the knee are:

1. Vastus Lateralis: Located on the outside of the thigh.

2. Vastus Medialis: Located on the inside of the thigh.

3. Vastus Intermedius: Located deep within the thigh, under the rectus femoris.

While the rectus femoris is the only quadriceps muscle that also flexes the hip, all four muscles work together to extend the knee.