Two studies published in 2011 should encourage you to try yoga if you have a bad back. In one study, British researchers compared a 12-week yoga program with usual care provided by Britain's National Health Service and, not surprisingly, yoga proved to be more effective than routine care. In the other study, researchers at the Group Health Cooperative in Seattle compared yoga with "self-care" and exercise classes that were specially designed for the study. The exercise classes included some warm-ups and strength exercises, but most of the time (about 50 minutes) was spent on 15 different stretching exercises targeting the trunk and legs. The study ended in a tie, with yoga and the exercise classes having a similar effect on people's back pain, as measured by self-reported "bothersomeness" and, as in the British study, by the effect on back-related problems with walking, standing, and so on. Self-care finished a distant third.
The Group Health researchers noted that at a purely physical level, the yoga and the stretching classes were pretty similar. In some sense, the study compared yoga with a yoga-like set of stretching exercises.
In the real world, yoga is the more practical choice. A stretching program tailored to the back is going to be hard to find, while virtually every health club and Y offers yoga classes and there are yoga studios everywhere.
Yoga is safe, but if you are getting into it as a treatment, talk it over with your doctor, especially if your problem is back pain. Certain yoga classes (and positions) are going to more suitable than others for people with back pain. In the British study, 5 (3%) of the 156 study volunteers who were randomized to yoga reported that their back pain increased and that the increase was related to yoga. In the Group Health study, 13 (15%) of the 87 yoga class participants reported mild to moderate problems, mainly related to back pain, and one person suffered a herniated disk.