Hamstrings that look as good from behind as the front.
Keep your hips down. Lifting them off the pad cheats the hamstrings.
Lying face down puts your hamstrings in the machine's line of pull with your bodyweight anchoring your hips to the bench, so all the pad's resistance goes into curling your heels toward your glutes. The prone position makes cheating obvious, the instant the hips rise, the hamstrings have handed off the work. It's the strictest curl variation, the one that builds the hamstring sweep you see from behind.
Both belong. Seated curls train the hamstrings at a longer hip angle, lying curls let you anchor hard and squeeze at short lengths. If your gym has one, use it fully, if it has both, alternate.
Curl up strong, squeeze for a beat, then take two to three seconds on the way down. The slow negative is where the hamstrings earn most of their growth, slamming the stack throws that away.
Hamstrings cramp when they're squeezing hard at their shortest, especially with hips popping up. Keep your hips pressed into the bench, ease the top squeeze slightly, and build up the volume over a few weeks.
