if you r getting back pain and r a seasoned and regular bocker, then Id say go see a doc or other medical professional.
4 beginners, yes its using bits of the body in new and different ways thats straining things. It would be the same if you took up, oo say, canoeing for the 1st time. You wil leave the water exhausted and pretty soon your body will be only on 'screaming terms' with your brain. So you leave a day or 2 for things to heal un tuffen. Its the same in a gym, you work legs..rest them and work chest...rest....
Warmup exercise would help a great deal, tho no1 ever seems to do um. But think of the warm up exersives 1 does be for a football or rugby match or an aerobics leason. Think about it , strapping 8-9Kg of metal un glass/carbon fibre to the legs then vigorously jumping about. Dont you think a few stretches might b in order? Un hey a few min cool down at end of meet/session? Yep I know no street cred d'oh hehe.
Basically, warm up un cool down exercises will help a great deal......but ya no gona do um