Use a leg curl machine to load the muscles in the back or posterior of the leg, primarily the hamstrings. The leg extension machine has you straighten your leg against resistance and thus works the muscles on the front or anterior of the leg —
Leg extensions are exercises usually done with a lever machine. You sit on a padded seat and raise a padded bar with your legs. The exercise works mainly the quadriceps muscles of the front of the thigh-the rectus femoris and the vastus muscles.