Your back starts seizing up, so you bend over to stretch it. Feels good, but it may be the worse thing you can do for your back. In fact, it may be the reason you can't get out of pain, and your back flairs up an hour or two later. The reason for this is do to the nature of a herniated or bulging disc.

When people say they "threw their back out" they are usually feeling this way because they have a disc bulge or herniation. The conditions happen when the outer layer of the vertebral disc (kind of like a water balloon between the bones of your spine) expands out unevenly, like a bubble, or ruptures completely, putting pressure on the spinal cord or nerves exiting the spinal cord. This can cause excruciating pain, numbness and tingling, sciatica, drop foot, and a whole other host of problems. Most herniations or bulges are posterior, meaning they bubble out the back, towards the nerves of the spinal column. This is why bending forward to stretch is a bad idea when you have back pain, because it actually pushes the bubble out further, which in time, cause more pain and negative conditions.

Part of the reason the lower back feels so tight when you, "Throw it out" is because the lower back muscles tighten, attempting to prevent you from bending forward. See, your body knows what's right for you, unfortunately, we just don't always know why it's doing what it's doing.

With other injuries, such as knee, shoulder or hip injuries, the muscles can tighten up around the joint to protect it. When we keep stretching all the muscles we just perpetuate the instability of the joint, which is probably why we injured it in the first place. In these cases, it is usually better to strengthen the muscles around the joint instead of stretching them in order to relieve the pain and prevent further injury. Craig Zuckerman breaks it all down for you in this video.