I've already began a greasing the groove regimen of pushups and pullups and decided that I'd do some work on my abs. I have virtually no fat on my stomach and have visible muscle, but would like to make them stronger and more defined. I know situps aren't good for your back, which is OK because it takes 50-100 situps to actually cause them to burn. I'm doing some bicycle kicks but they mess with my hip. Is their any exercises I can do to add on to my abdominal muscles?