Mirror, Mirror . . . How Can I Get a Stronger Back?
What is known about back muscle strength and back pain? Patients with low back pain seem to have weak back muscles that get tired easily. Poor muscle endurance has a part in causing low back pain.

But there is a complicated interaction between the muscles that cover the back, buttocks, and hips. It's hard to separate the actions of these muscles. They usually work together. When a load is placed on the back, the buttock muscles are quick to respond. Researchers find that to work the smaller muscles along the sides of the spine (paraspinal muscles), actions in the larger buttock muscles have to be minimized. The question is "How?"

Various styles of equipment and exercise are used. One type of machine used is the Roman chair, which can be adjusted to help work the back muscles.

But which equipment is best, and how much exercise is needed? Are three sets of 10 repetitions of each exercise needed? Is one set of eight to 12 repetitions enough? Do the number of sets and amount weight have to be increased in order to strengthen just the paraspinal muscles?

Researchers at an exercise science lab think that these paraspinal muscles are mainly used to hold the spine steady. They aren't designed for moving higher loads. The buttock and hamstring muscles begin to kick in with higher loads. Adding more load to the spine while using the Roman chair may only increase the forces against the discs between the spine bones. This could put the person at increased risk for back injury.

Multiple sets of exercise to strengthen back extensor muscles are under question. According to these researchers, only one set of exercise is really needed. This is true for young, healthy people who participated in the study. Patients with low back pain may need a different amount of exercise.
References
Brian C. Clark, MS, et al. Electromyographic Activity of the Lumbar and Hip Extensors During Dynamic Trunk Extension Exercise. In Archives of Physical Medicine and Rehabilitation. November 2002. Vol. 83. No. 11. Pp. 1547-1552.