I could write a really long essay on this subject, but I'll settle for telling what I did to strengthen my back after an injury, and why.
I excersise with weights twice a week, and I try to get 10km on my excercise bicycle before breakfast three times a week as well. In total, I exercise about four days every week.
The key for me has been moderation (not training the same muscles too often, or too hard), and to mostly use compound exercises. By compound exercises, I mean for example the deadlift, bench press, and lateral row. Those particular ones train alot of muscle groups at the same time, which has the twofold advantage of saving time, and, as opposed to isolation exercies, they do not easily cause imbalances.
I fixed my back doing three sets of crunch situps once a week, three sets of back extension once a week, and three sets of deadlift once a week. I find this to be optimal for me, but it may well be different for you. I use other execises as well, in order to keep the rest of my body in check, but since this thread is about back exercises, I won't go there:yes:
