This ab move isn't just a crunch. By focusing on one leg at a time and pressing the other foot into the wall, you're building core stability that keeps your spine protected even when your limbs are moving.
Sit about one foot away from a wall, and lie down on your back. Place your feet flat on the wall so your legs are in a tabletop position.
Extend your left leg at a diagonal so your toes are just hovering off the wall. Extend your arms overhead and press your lower back into the floor to engage your core. This is your starting position.
Draw your left knee in toward your chest while simultaneously curling your shoulders off the floor. Then, draw your arms toward the wall so they're next to your hips.
Slowly release the crunch, returning to the starting position. That's one rep.