I use kneeling, seated, prone. They all have benefits. Maintaining optimal form can be of any context. Making sure to keep the pelvis down to keep tension on the hamstrings during the prone leg curl is crucial but that is not to say you can never let it[pelvis, butt] raise up or have your back arch. Context is everything.
Gregory R. Taper
If I have the option, I’ll use them both. I only really have that option with a few clients that I train in a gym, since I work with most of my clients at their homes. I’ll use a stability ball for leg curls also – I’m not a big fan of those, but they’re fine for my older clients (which is most of them!). I understand that isolation exercises such as this are “out of vogue” these days but I’m not buying it. If you’re wondering if you should be including them at all, I would say yes.