A Little Anatomy

From Wikipedia::Back_Pain :

Muscle strains (pulled muscles) are commonly identified as the cause of back pain but in fact muscle imbalances are by far the most common cause of low back pain, and pain from such an injury will often hang around for as long as the muscle imbalances persist. The muscle imbalances cause a mechanical problem with the skeleton which leads to pressure building up at certain points along the spine.

Next : Upper Back Stretches