To properly execute a leg curl, always start with a lighter weight. You don't want to force your body to overcompensate by lifting the hips and flexing your lower back. This fails to isolate the calves and hamstrings and can cause injury to your back. Select a weight that allows you to do 8 to 12 repetitions with reasonable effort and good form.

A quick and straightforward way to determine which weight you should use is to try the “one-rep max” test. The one-rep max test involves determining how much you can lift in one rep, then lowering your exercise weight by 60-80% during the workout. From there, add weight in 10-lb increments as needed.

There is no universal correct weight for leg curls. Even between variations, the weight you use might change! Aim for a weight that you can carry out curls with correct form for between 8-12 reps if your goal is hypertrophy, and 3-6 reps if your goal is strength. If you can perform over the top range of your reps easily, increase the weight.