Here's how to do it: Lie faceup on a bench with a resistance band wrapped around bottom of bench and handles under arches of feet. Extend legs straight up and keep lower back pressed into the bench (no arching). Bend left leg, lowering foot so your knee creates a 90-degree angle, then straighten. Do 10 reps. Repeat with right leg.
Don't let your legs fold into your stomach. Continue the movement, doing 8 reps per leg, then 6 reps, and so on until reaching 2.
Try this move: Lying Leg Extension