1. Lumbar Strain
WHAT IT IS: An unfortunately vague diagnosis that accounts for about 90 percent of all lower-back injuries. It's caused by a pull or tweak of any of the muscles, ligaments, connective tissue, joint capsules, or cartilage in your lower back. HOW IT HAPPENS: Lift something heavy without bending your knees, land awkwardly, wake up with a random soreness—all are probably strains or sprains. WHAT IT FEELS LIKE: A hard-to-pinpoint ache or stiffness that can radiate into your rear end. PROGNOSIS: With proper rest and low-impact exercise, most cases remedy themselves within three months. The bad news: It often recurs.

2. Herniated Disc
WHAT IT IS: A tear in the outer fibers of a disc, the shock-absorbing pad between the vertebrae, which may cause the disc's jellylike nucleus to push through the fibers. Also called a slipped or ruptured disc. HOW IT HAPPENS: Causes can be similar to those of a strain or sprain. Discs also lose fluidity with age. WHAT IT FEELS LIKE: If minor, you'll feel the type of ache associated with a lumbar strain. The discomfort becomes worse if the slipped disc presses against a nerve; even more acute if the disc rubs against the sciatic nerve, causing shooting pain (called sciatica) and possible numbness in one leg. PROGNOSIS: More than 95 percent of people recover through rest and therapy. Steroid injections, which control pain and swelling for several months, are a common treatment for the hardest to cure.

3. Compression Fracture
WHAT IT IS: A break in the bone structure of the vertebra. It's most common in the vertebrae about two-thirds of the way down your back. HOW IT HAPPENS: A sudden impact coupled with flexing your back—think of lurching forward when smacking hard on the landing of a big jump on a bike or skis. WHAT IT FEELS LIKE: Intense pain directly on the spine where the fracture has occurred. PROGNOSIS: You'll typically wear a back brace for upwards of six weeks while the fracture heals.

4. Spondylolysis
WHAT IT IS: Something as bad as it sounds, unfortunately: a crack in the solid part of the vertebra. Like a herniation, it occurs at the base of the spine. Worse still is isthmic spondylolisthesis, a crack that forces a vertebra to slip over the one below it. HOW IT HAPPENS: Some 4 to 8 percent of people are born with the defect. It can develop into isthmic spondylolisthesis, which can occur during growth spurts or as a result of repetitive exercise, like a stress fracture. WHAT IT FEELS LIKE: Either one can cause pain that's no different from the typical strain or sprain; amazingly, in some cases there are no symptoms at all. PROGNOSIS: Taking a break from pain-causing activities and doing core-strengthening exercises is usually sufficient, though surgery may be required.