Quadriceps
The muscle being used in leg extension is the quadriceps.
Quads and hamstrings
quadriceps: vastus medialis and lateralis, intermedius, and rectus femoris. If you are talking about hip extension: glut max are the prime movers, the hamstrings For knee extension, the quadriceps
The 3 major muscles groups worked are the Quadriceps (front of thigh), the Hamstrings (back of thigh) and the Gluteals (backside).