What is a low back strain or lumbar sprain? Muscle strains and lumbar sprains are the most common causes of low back pain. A low back muscle strain occurs when the muscle fibers are abnormally stretched or torn. A lumbar sprain occurs when the ligaments, the tough bands of tissue that hold bones together, are torn from their attachments. Differentiating a strain from a sprain can be difficult, as both injuries will show similar symptoms. Many doctors refer to both injuries as a category…