What muscle stabilizes the knee on the anterior side?

Human Kinetics explains that the quadriceps muscle group stabilizes the knee on the anterior side. It further says that the rectus femoris muscle, one of the quadriceps muscle group, is the most anterior of them all. The other three quadriceps muscles are vastus medialis, vastus lateralis and vastus intermedialis.

As noted by InnerBody.com, these large muscles that originate in the ilium and femur are part of the muscular system providing protection for the knee joint while enabling joint motion. According to InnerBody.com, when the quadriceps group contracts, the leg extends at the knee and the thigh flexes at the hip.