In many cases of low-back pain, no cause can be determined. Contributory factors are general lack of fitness and occupational and psychological factors. 
There are two main causes in cases where a mechanical cause can be identified: 
€¢ soft-tissue injury: strain of spinal muscles and ligaments (lumbago, fibromyalgia) 
€¢ trapping of a nerve root, usually the sciatic nerve, due to a slipped intervertebral disc (sciatica).