Keith sounds like a douchebag. But u enjoy going to the gym, though moderation is certainly key. Not exactly a limerick but the flow wasn't terrible. First line was way too long. I'm not sure if you're saying exercise has poor long term effects on the lower back if not executed properly, or, that Keith exercises so much that he's "screwing himself in the back."

I liked this though

So sorry to do this, but although I like the idea, I feel like it didn't flow as well as a limerick poem should. I hate giving negative reviews but I feel like the idea had so much potential and it was almost really good; there was just something a little off about it. Keep writing, and sorry if I come off as rude!