Lost in Doubt
Read David Swenson's reply:
The fact that you are questioning your approach to teaching and practicing yoga is a good thing. Life exists in the questions, which spur us forward into new places of knowledge. Introspection is the springboard for growth.
Your queries tell me that you care about your students and want to make sure that they are safe. That is a healthy attitude for a teacher to have. Keep in mind that just because we teach yoga doesn't mean we won't be confronted with difficulties and obstacles in life. The true test of one's character comes in times of crisis or struggle.
When we are confronted with injury, illness, or other obstacles, we must take the full breadth of our knowledge and apply it to understand how best to deal with the situation. Our practice may not diminish the problems, but it will change our perceptions and reactions to the challenges. We have no control over what difficulties may confront us. Our only control in life is how we react to them. The practice of yoga provides a valuable tool that allows us to react with greater clarity and insight. Learn from your back condition, and pass along the knowledge to your students.
As far as how to approach your back pain, I recommend that you seek out instructors who specialize in working with yoga therapeutics and back issues. I'm certain that you will be able to find a teacher who'll have specific insights about how best to approach your practice. Without knowing more details, I can only offer some very general advice. You didn't mention whether your back condition was due to an injury, a degenerative condition, or another cause. The answer will greatly affect the appropriate approach for treatment and healing. It is important to create as much length in the spine as possible when you're practicing. By creating length in the spine, you'll create space between the vertebrae rather than compressing them. This is true for both forward bending and backbending. You must avoid the compression of the disks between the vertebrae involved. Avoid rounding your lower back when you fold forward, and keep the tailbone moving away from the head in backbends. These are very general observations, but they may be useful in avoiding further complications in your lower spine.
There's much more to yoga than bending forward. So even if you have limited mobility, it doesn't diminish the depth of yoga that you may experience—and it certainly does not diminish your abilities as a teacher. If yoga were only about physical prowess, then the greatest yogis in the world would be gymnasts and circus contortionists. Teach from your heart. Be honest with your students. They will respect you for your integrity far more than for your ability to do a forward bend!
David Swenson made his first trip to Mysore in 1977, learning the full Ashtanga system as originally taught by Sri K. Pattabhi Jois. He is one of the world's foremost instructors of Ashtanga Yoga and has produced numerous videos and DVDs. He is the author of the book Ashtanga Yoga: The Practice Manual.