everything of course depends on the wind. But with a good harness, the most comfortable place on a cat is without a doubt, on the wire. I haven't had alot of strouble with my back, I'm young (22), but I'm also tall, so my back has always been bad. I actually have the most trouble with the occasional leg cramp. And If I hit my knees on anything i'm in for a bad day. Cats do require quick movents and sometimes minor acrobatics to get under the boom/sail. It sounds like maybe you should try it out a little before you get all hell bent on buying your own boat. Hmm, i would say though that if you have no gtrouble on your force 5, you should be fine, you can always bend your knees up a litte if you need too.