When performed correctly, the leg extension targets all four heads of the quadriceps muscles. The leg extension is a straightforward knee extension motion, but there is still the potential to use improper form and fail to target the muscles appropriately. This video from Jeff Nippard provides an excellent background to the leg extension exercise, the anatomy and biomechanics of the quadriceps muscles, and a demonstration of how to perform the leg extension to maximize growth in the leg muscles.