It’s important to always do each ab exercise correctly, especially if you want a flat tummy. In a standard crunch, don’t just pull your head up and down a hundred times, you have to lift your shoulder blades off the ground, posture straight, your head and chest moving up and forward slightly so that your abs literally CRUNCH together. Squeeze HARD at the top, making sure to exhale on each rep. They should BURN by 15 reps, if they don’t, you are either doing them wrong or you need to add resistance. Keep them SLOW and CONTROLLED. 15 correct ones are better than 50 incorrect ones.