It is possible to strain the spinal erectors or other lower-back muscles by overstressing the area, especially when you do a movement that hyperextends the lower back-like Deadlifts, or bench exercises such as Bench Presses or Leg Raises, in which the lower back is lifted clear of the bench and hyperextended. A certain amount of curvature of the lower back is normal, but bending it too far under stress can cause problems.
When you strain the lower back, you may feel pain radiating down into the hips or upward toward the middle back. Sometimes these muscles will go into spasm to prevent further injury.
You can also have a sprain in the lower back when there is an injury to the ligaments in the area. It may often be difficult for you to tell whether you have incurred a strain or a sprain, but in any event, the treatment is virtually identical.
Another lower-back injury you can incur is a ruptured disc. The discs are situated in between the vertebrae, and when they rupture, the pulpy material inside the disc can extrude and press upon adjacent nerves. You may feel pain anywhere along the back or even down into the legs, but it is this specific pressure which causes the pain, and treatment involves alleviating that pressure.
One specific type of nerve problem is "sciatica." The sciatic nerve is the largest nerve of the body, extending from the back all the way down the leg, and when pressure is put on this nerve the pain is severe and disabling.
Another cause of lower-back problems can be abdominal work such as Straight-Leg Sit-Ups and Straight-Leg Raises, which both put a lot of stress on the lower back. Bodybuilders who have been able to do heavy Deadlifts or Good Mornings with no difficulty have sometimes been surprised to find themselves incurring back injuries while doing abdominal training.