That's one of the primary problems that makes it difficult to make a specific diagnosis, especially for physicians who have less experience with the subtleties in determining what the real cause of back pain is. They're going to tell you what the cause of the pain is based on what the initial problem was. It's no different than any other kind of muscle pull, whether it's shoulder, bicep or the muscles in the neck. That's probably the most common form of low back pain and fortunately the simplest to resolve. It gets better on its own.
More serious problems present identically, with localized or regional low back pain.
The answers to that question in regard to chronic low back pain is that it's generally a group that we have difficulty making a definitive diagnosis. If I had to choose one primary cause of long term chronic back pain, over a year, it would be damage to the intervertebral disc. That might be herniation, tearing of the peripheral disc or anulus, or damage to the inside of the disc. Unfortunately, these particular problems are often most difficult to treat, surgically and non-surgically. Most often they're due to repetitive movements or repetitive trauma related to somebody's previous physical activities or job related activities.