Q: "I have Yoga student who has a bad back. Poses like the Fish and Bridge bother his back. I haven't found anything in the books about not doing them because of a bad back. However, I also don't want him to hurt himself. Should he do a different pose, relax and breathe, during that pose, or work only as far as he can to help build up strength?"
A: This could be a very serious pre-existing neck or spinal injury. This might not be a case where your student can strengthen it, because his pain may originate directly from within the spine or neck. It is important to know if the pain is in the neck, spine, or if it is a sore muscle.
Either way, you should strongly advise him to see a physician as soon as possible. As you know, muscle frequently repairs itself, over time, but the spine is entirely another matter.
In a case such as this, some Yoga instructors will refuse to have a student in the class, until he has his doctor's approval. That may seem harsh, but student safety is a primary concern.
If it is a sore or pulled muscle, it will pass in due time. In the case of mild muscular discomfort, Cobra pose (Bhujangasana), or a version of modified Cobra, might be a good substitute for Fish pose (Matsyasana).
Do not speculate as to the cause of pain with your student. This could possibly be interpreted as giving medical advice, or practicing medicine without a license. There are so many variables when considering the source of pain in the neck and spine, that there is no place for speculation from a Yoga teacher.
Very few Yoga teachers are medical doctors. Therefore, this is a problem for medical professionals to solve. My guess is your student should also avoid plow and shoulder stand. Either way, tell him to avoid anything, which causes pain by substituting a painless posture.
In some cases, particular forward bends and backbends can cause pain. You could try modifications to decrease angles, but for the sake of your student, you may want to avoid some of these postures all together. If that is the case, substitute relaxing postures, which keep his neck and back flat.
