Most of the time the exact cause will not be known, but recovery will occur. A wide variety of back problems are loosely termed back strain or sprain. The term sprain is generally used when a ligament is partially torn, and strain when a muscle is over-stretched. Neither X-rays nor a physical exam will help tell the difference. In fact, the difference is not important.
Treatment is the same and the expectations of recovery are about the same.
Two other conditions, muscle spasms and ruptured discs, are more clearly defined. They can be caused or aggravated by poor posture, long periods of sitting in the same position, tense back muscles, weak abdominal muscles, and emotional stress. Discs act as shock absorbers and are subject to wear and tear that thins their coverings, permitting the material inside to rupture to the outside. These are not common causes and usually there will be symptoms other than back pain to help identify the cause. Additionally, back pain will sometimes accompany abnormal curvature of the spine.