I've had numerous episodes over the years; nothing too severe fortunately.
Once I was running on a treadmill and my back went into spasm; the doctor explained this as a momentary pressure on a nerve causing everything to lock up. He prescribed painkillers and valium to relax the muscles, which worked a treat.
A few years back I was suffering from dull aching in my R hip and thigh which made driving extremely miserable. I visited a consultant who didn't really help. Some weeks later I discovered a lump of knotted muscle in my lower back; just giving that a bit of vigorous manipulation a couple of times a day had it sorted in no time.
I've had a mild recurrence of that recently after doing a long ride on my 'sporty' (low front end) bike. Again, knotted muscle in a similar place which I dealt with in the same manner.
Obviously this won't be helpful in a lot of instances but it's just my experience.