Playing guitar involves contracting muscles of the upper body for up to 8 hours at a time. Guitars weigh 1-10 pounds. Your body must bear this weight while making light, but vigorous, and often fast, motions with the hand, arm and shoulder.
If your body is already messed up, playing guitar has the potential to make it worse.
Here are a few range of motion tests you to see if you are in the minimum physical condition to play guitar: