Originally Posted by Choccy
Sounds like your bike doesn't fit too well. I had lower back pains on long rides but never on short rides so I lowered my saddle by 5mm and now the pain has gone. If you want a good ab exercise try hanging leg raises but do them really slow. You will only need to do 3 sets of 10 then rest your abs for 3 or 4 days.
Well I know my road bike is pretty well fitted since I tweaked it for the last year, but you might be right concerning my mtb. I can't really get around that because of performance factors i.e. I need my saddle that far back in order to me my rear from getting squirrely.