For machine leg curls, ditch the ego in pursuit of greater gains by doing them with your feet in plantar flexion on the way down. In other words, on the eccentric or lowering phase of the movement, point your toes. This makes it much tougher... and much tougher leads to more muscle growth.
To really ramp it up, attach a band to the machine so that the resistance increases as your raise your legs up, then hold that top position for a second before lowering under control. Don't just drop your legs back down: resist!
Originally coined "The Bear" by John Davies, this series of five exercises in one teaches the body to move with great ROM in a fluid, powerful, and fast manner. It's fantastic as a total body warm-up, improving overall power production and motor skill coordination in a short period of time.