Yoga and back pain. The article below goes on to mention that during a study of over 300 patients, those who attended between 6 and 12 yoga classes over the course of 12 weeks (many were Iyengar classes) found their back pain to have eased significantly, compared to a group who received normal care through their GP.
http://www.york.ac.uk/news-and-events/features/yoga-back-pain/