You've been doing: Machine leg curls
Switch to: Body-weight leg curls
Machine leg curls have you lifting only your lower legs. Leg curls done on a bench with your feet anchored have you lifting almost your whole body. Now take a wild guess as to which one builds more muscle. You could leg curl on a machine for the rest of your life and never expose your hamstrings to the load they'll get with just one rep of the body-weight leg curl-an exercise that also builds core and glute strength.
HERE'S WHAT YOU DO
Find a bench with a leg station. A flat bench is ideal, but a decline bench will work, too. Kneel on the foot of the bench facing the top of it and anchor your heels behind the pads of the leg station. Tighten your glutes and abs-your body should be nearly straight from your thighs to your head throughout the movement. Straighten your legs a bit until you feel tension on your hamstrings, and hold your hands out in front of you to catch yourself if you lose position . Continue straightening your knees, lowering your body as far as you can without losing control (you'll need to bend your hips a bit as you go down) . Now reverse the motion, using only your hamstrings to pull your body back up. (You can bend at the hips a bit again to make it easier if you need to.) That's one rep. For maximum benefits, perform three to four sets of four to six reps, resting 60 seconds between sets.