I get back pain sometimes, when i ride all day my back aches pretty badly afterwards. I think its because im tall, and so more leverage is put through my back? :P Anyway, its probably to do with me caning my back a few weeks ago, i was pulling a simple manual down my road and my shoddy back brake didnt work, then bang! i was in serious pain. Im pretty sure everyone experiences back pain at some point whilst riding trials, its just the nature of the sport.