They don't prescribe bed rest for back pain any more, but they used to.
These days, if you go to your local GP complaining about back pain, he (or she) is more likely to prescribe moderate exercise than bed rest. But it hasn't always been like that -it wasn't that long ago where bed rest was the standard prescription for back pain.
The problem with bed rest is that your muscles aren't doing anything. And if they're not busy, they get bored. And when they get bored, they get weak. Which leads to increased risk of back problems as weak muscles aren't up to the job of looking after your spine.
(I apologise at the lack of precise medical terminology there - as I've said before, I'm not a doctor. But I hope you can see what I'm getting at.)
The other problem with extended bed rest is that your muscles can completely sieze up.
I experienced this first-hand a few years ago when I had the misfortune to break my elbow falling over a wall. (I know, not very glamourous at all, but that's how I did it.) I completely dislocated my elbow, and broke a piece off it. "Smashed" was how the consultant described it, but I'm relieved to say he decided not to operate.
Anyway, to help the bones to mend I had to wear a cast for a few weeks - and when it came off I was completely surprised to find that my elbow was frozen solid. It wouldn't move at all - in fact, it took weeks and weeks of physio and careful exercise to get it back to normal.
So it's no surprise to me that instead of bed rest, moderate exercise is now prescribed for back pain. (I do wonder if there's a better way for joints to heal than to completely immobilise them, but that's a topic for another day.)