I just started using the ab wheel and can do a few on my knees without too much problem. however, after maybe 4 or 5 i feel a lot of tension on my lower back. I read somewehre that this happens if your core isn't strong enough to stabilize it. so my question is, should I continue to do it and go through the tension of my lower back or build up my core before I continue to do the ab wheel? thanks for any input.