Back injury

This is because the human spine and associated muscles and other soft tissues are vulnerable to some types of injury. The lower back in particular is prone to injury, as it is both highly flexible, allowing us to bend and twist in all directions, and subject to a great deal of stress as the main load-bearer of the torso. Though lifting, placing, carrying, holding and lowering are involved in manual materials handling (the principal cause of compensable work injuries) the BLS survey shows that four out of five (or 80%) of these injuries were to the lower back, and that three out of four (or 75%) occurred while the employee was lifting. No approach has been found for totally eliminating back injuries caused by lifting, though a substantial portion can be prevented by an effective control program and ergonomic design of work tasks.
While many instances of back pain can be traced to a specific injury (e.g. a muscle strain), often the acute onset of back pain is actually a result of a long term process of general degeneration, such as degenerative disc disease or a spinal disc herniation.

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from http://en.wikipedia.org/wiki/Back_injury