Crunch

This is a small-range, ab-isolated move, not a full sit-up, and that's the point — by not swinging your hips or hip flexors into it, almost all the work stays on your abs instead of leaking into your lower back. Lie on your back with your knees bent and feet flat, hands lightly supporting your head without pulling on your neck. Curl your shoulder blades up off the floor by squeezing your abs, pause briefly at the top, then lower back down under control without letting your head drop and bounce off the floor.