Lacrosse is Canada's summer sport. That's why.
Okay not quite. I have scoliosis, which is a curve in a my spine. It's minor, but it's
enough that I get minor chronic pain in my midback, especially if I'm sitting still all day.
I bought a lacrosse ball a few years ago now, and it was the best purchase I have ever made with regard to my back pain. Just 30 seconds rolling my back on it at night does wonders to loosen everything up. I mostly focus on my back, but it's great for any other muscle group you want to stretch out or get some more circulation to. I think it cost me 4 or $5 and it has paid for itself hundreds of times over now.
I remember I tried tennis balls and hockey balls before buying, and I didn't like either as much. The tennis balls didn't 'dig in' quite enough, and the hockey ball was too hard. For me the lacrosse ball is great because it's hard enough to dig in but it's so a tiny bit of squishiness to it as well. So any of you with back pain out there, this may help!