if u want ab exercise, get some hook boots that people use to hang upside down from a chinup bar. Now add 100 pounds and get to it hanging situps w 100 pound weights. When you can do a set of 10 you're in good enough shape to kite. Typical ab exercises will not build nearly the amount of ab strength u need 4 kiting. U mite need spotter after a set, 'cause it's difficult to unhook your boots from the chinup bar sometimes
Or u can just kite more.
What a load of tosh, I did basic core exercises building up reps and difficulty and it made an amazing difference over a 6 week period. Is not all about abs and you certainly don't need to do weighted crunches from a chin-up bar, that's just asking for trouble and injury! There are some very good kite surf exercise sites that will give you great exercises you can do at home that will build your core strength combined with some major muscle groups which is what really helps with kiting. Just doing your abs on their own is not the best idea, better to build up whole body strength that way the load is spread and doesn't just affect one part. In addition better core helps with stance and proper stance will reduce the aching in your abs - although a decent session will still leave you feeling it no matter what