In a recent article in SIB, Owen Anderson discussed research that showed how important it is to have good strength endurance of the spinal muscles. Specifically, the Erector Spinae and Multifidus muscles need to be able to hold the spine stable.
A common exercise for strengthening these muscles is the back raise (see picture below). The problem with this exercise is that the spine compression forces are actually over 6000 N, which is very high and completely unsafe for any patient recovering from a low-back injury, (McGill, Low Back Disorders, 2002, Human Kinetics).
A safer alternative is the 'Alternate Superman' performed in a kneeling position (see picture right). The compression forces are 2000N, which is tolerable, but the spinal muscles are still recruited. Because the opposite arm and leg are activated in turn, the left and right sides of the spine are worked alternatively. There are actually four sections of the spinal muscles: Thoracic left and right and Lumbar left and right. When the left arm and right leg are raised, the right Lumbar and left Thoracic muscle areas are working and vice versa. Hence to get an effective training benefit you need to either hold each position for significant time, or complete lots of repetitions.
I recommend starting with three sets of 30 seconds holding each position, building up to three sets of 60 seconds. Be sure to keep a nice straight back during the exercise as your leg extends back and arm raises forwards. The challenge is to stay aligned and balanced with only one hand and knee on the floor.