Hey, rather infrequently (every few months perhaps), my lower back locks up. It's not really painful - but I will have to sit down, or rather lay down because sitting alone doesn't do much good. It passes after a few moments, typically, and then I feel alright again. It tends to happen: * after some physical efforts, like a long/hard run or something (>1 hours and with some intensity) * when changing position. the other day I got up from the toilet So typically, I lay down for a few moments, then I get up again and feel fine. When I feel locked up, again it's not really painful, maybe more like a discomfort, but I feel like I cannot bend or use my back muscles. I'm mid 30s, pretty fit in general and I don't really have back problems history. Any ideas what I should do? I don't necessarily do a whole lot of back stretches or anything, althought I am pretty active in general and reasonably fit/flexible...