As a Physical Therapist, let me begin by breaking a myth. Doing exercises to increase the strength of your spinal extensors aggressively as if you were lifting to grow your pecs is not going to improve your back pain. Get examines and evaluated by a Physical Therapist to find out why your back hurts, and use their help to address that.
Now, there are plenty of ways to strengthen your lower back. You may want to begin by learning how to control your CORE (see the previous blog). If you have no lower back pain or issues; then, here is what you need to take into account any time you are strengthening your lower back:
Your spine is a chain of multiple joints interlocked together to provide you with immense mobility. However, when you increase your mobility you also increase your risks for instability. Learning to control and move properly during each exercise you do to strengthen your lower back becomes critical. If you force your spine to move loads or at speeds for which is not able to control the movement properly, you will eventually disrupt a part of the amazing chain the spine is.
Make sure you have the flexibility around and along the spine as well as in the extremities. If during any movement your legs or arms are limited by muscle or joint tightness, your spine will compensate to achieve the movement desired. Its enormous flexibility and degrees of freedom (planes of movement) allow it to take on the movement other joints cannot do.
Strengthen in the available ROM you have. To gain muscle hypertrophy (growth), you need to challenge the muscle repetitively with a high load. However, you can only properly challenge a muscle when its joints allow it to move. If you go beyond its reach, you engage other structures that you are not thinking of and end up injuring yourself. Only attempt to increase your strength during larger ROM once you have increased your joints ROM. Then, start the strengthening process for the newly attained ROM.
Not only trained the spine linearly to isolates its many muscles, train it during dynamic activities that require coordination to improve its movement. It will also decrease the chance of injury during other normal tasks.
I hope this gives you some guidance to those of you who have healthy backs. For those of you who have lower back issues, please seek the help of a Physical Therapist.