Maybe sprain is the wrong word, but you can DEFINATELY injure it where it seems like a sprain. It's only happened to me once and it was where the ex-wife was on top, hurt like a motherfucker and I couldn't get hard for days. It got better though.
It's definately sprainable... Step 1) Find a woman who really likes the cowgirl (woman on top, facing you) position. Step 2) Let her rock front to back instead of riding up and down. Step 3) Watch your lifelong friend and companion take a real beating ><