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!