I've had so many questions regarding back pain and injuries in yoga (usually after class- rather than before when I can monitor the student and give more specific options.) While Yoga can be very healing, many classes you can go to, are meant for the general population (without injuries) or are often too fast to address the needs of someone with this type of injury.
Low back pain is often the result of minor muscle, ligament or joint issues in the lumbar spine area as a result of strain from one activity (how you use your body AND how you sit, stand and sleep) or years of misalignment (poor posture), muscle tightness or weakness, stress (life) or other injuries.
Lack of body awareness is a huge contributor as well: rounded shoulders, forward head, and kyphosis (concave spine), swayback, or lordosis (arched spine). Sciatica is much more severe (with pain shooting down the leg from the back) and happens when the spinal vertebrae bulges out and compresses the nerve from the spinal cord. If you have a "radiating, constant pain which could infer an impingement of the nerves from a constricted piriformis muscle pressing on the sciatic nerve or herniated disk, and are not able to find a pain free posture (even lying down)", Judith Lasater, author and PT recommends a medical evaluation. In addition, there are several life-threatening causes of back pain (infection, cancer, spinal fracture and neurological problems) which seeking a physician is in order. However, most injuries are resulting from postural or poor body awareness.
Are you slumped in a computer chair while you're reading this? Move to the edge of your chair, pull your sitz bones together (and higher than your knees), place your feet flat on the floor in front of you, and draw your belly and chin back. There, now don't you feel better already? (Alexander technique is great and was founded 100 years ago.); )
"Movement heals." (J. Pilates)
Resting (lying in bed) will not allow the muscles to rebuild to support the spine. The more you lay around, the more muscle loss you'll have (studies show up to 3% a day). Studies also show that most pain will subside within 6-8 weeks, regardless of what you do and that surgery is generally not the most effective option. A recent study of yoga practitioners found that 98% of 1000 people with back pain found yoga helpful. Now that's powerful.
What can you do in your Yoga practice?
Create internal warmth first
Focus on breath
Placement or stacking and proper rotation of joints (ask your teacher for adjustments)
Avoid overstretching or forcing a pose
Develop core strength & stability (ask your teacher to evaluate what muscles need strengthening and to recommend specific exercise for that)
Use of Bandhas, which are muscular and energetic locks to protect the spine and generate energy
Choose the appropriate choice of movement by knowing when to give yourself permission to stay in a pose or modification rather than progress with the teacher
Avoid seated forward folds (supine is a better option)
Avoid jumping into poses
Aim to feel balanced through the pelvis and lower spine, not overarching the lower back and not over tucking the pelvis. It should feel like you're standing right down through the center of your body with the feet firm and the crown of the head lifted. "Engage your mind, bring attention to what you're doing, and precisely how your doing it to build the ability to feel your body's signals." (Yoga as Medicine, by T. McCall, MD)
Enjoy a healthy practice and back...