A lot of people these days are very down on leg curls, and not without good reason. Basically, the argument goes that leg curls just are not functional because the hamstrings never function without the gluteal muscles in everyday life, so the supine hip extension with leg curl exercise, which uses the hamstrings and glutes together, is much more useful. This is all accurate, and anybody training for athletic activity looking for functional routines should use SHELC variations (pull-throughs, single-leg Romanian deadlifts, etc.).
But at the same time, it’s also true that any exercise that allows you to add increasing amounts of tension to a muscle will over time lead to muscle growth. And if you are going to do leg curls, you want to make sure you’re doing them right.
There are two ways to do leg curls. The first is where your ankles are dorsi-flexed and your toes are pulled towards your shins. In this position, the calves are free to assist the hamstrings in flexing the knees, and you are able to lift much more weight. However, if you use the calves to plantar-flex the ankle, your hamstrings are required to do all the work, and you cannot lift as much.
While the second way is correct, many people are used to the first way, and they tend to get cramps when they first make the switch. If this happens to you, this is because the neural pathways for the exercise have been established in your brain, and your body trying to get your calves to help out like usual. Rest assured, this cramping will decrease over time as you get used to the new form. A good way to ease yourself into the new form is to start by doing the concentric with your ankles still dorsi-flexed, and then the eccentric with ankles plantar-flexed. Then as time goes on, you can slowly start to do the concentric with the ankles plantar-flexed as well.