Final answer:
The anterior thigh muscles that extend the leg are collectively called the quadriceps femoris group.
Step-by-step explanation:
The anterior thigh muscles that extend the leg are collectively called the quadriceps femoris group. This group consists of four muscles: the rectus femoris, the vastus lateralis, the vastus medialis, and the vastus intermedius. Together, these muscles extend and stabilize the knee.