The most common cause of back pain is an injury to a muscle(strain) or a ligament(sprain) of the back. These are caused by improper lifting of heavy objects, excess body weight, awkward sleep angles and poor posture. Other causes of back pain include disk injury(disks are the cartilage-like pads that act as cushions between the segments of the spine), abnormal spine curves, accidents, arthritis, kidney problems, pregnancy some cancers and infections involving the spine.
Attribution: Straight Talk Foundation