If you are not overweight (and it appears you are not), consider doing a well-rounded abdominal routine. Research shows that Pilates is great at getting all of the abdominal muscles in the ab group. Planks, the "double leg stretch," Pilates "hundred" are a few good ones. If you have a healthy spine, the "roll up" and "criss cross" will hit the other muscles nicely. These are done with precision. So, a few reps usually just 6-10 or each exercise are very effective and saving on calories.