You may have heard by now of a book causing a stir in the yoga world called The Science of Yoga by William Broad. I thought it might be a good idea to give it read and decide for myself whether it had merit or not. Far from arousing my objections, the book actually made me feel justified in several of my practices as a yoga teacher. (See me patting myself on the back here.)
For instance, Broad writes about the hidden stroke danger of some poses, especially shoulderstand and headstand. While I admit I wasn't aware of this danger before, I had given up practicing shoulderstand a few years ago because it kept giving me headaches afterwords. Besides it creates extreme neck flexion and the backs of our necks are already overstretched from things like driving and computer work. It never seemed to me like a good idea to add to it through yoga.
I do believe, however, that shoulderstand does have many benefits for the body such as lowering blood pressure and heart rate. A few years ago I was shown a simple alternative that just about everybody can do safely. It involves placing a block up under the hips and then elevating the legs. It's much easier to get into and it comes with the added benefit of helping to balance the sacrum.
As for headstand, I still practice it because I enjoy it. But I don't hold it quiet as long as is usually recommended in Iyengar classes where 10 minutes is expected at the advanced levels. In fact, I'm a little worried about being able to keep up in some of the classes I wish to take at an Iyengar studio in New York during my trip there in May. I guess I'll have to practice what I preach and let go of expectations. (And maybe not mention that I'm a yoga teacher.)
The book also pointed out (to many people's dismay) that there's isn't much evidence that yoga is cardio-exercise. I've never thought that it should be. I think today we are too focused on multitasking and getting as much done in as little time as possible. We want to make something that is great for strengthening the body, especially the core, and for creating healthy flexibility, and make it into aerobic exercise so we can have a fitness trifecta. I think this is part of the reason fast paced styles of yoga have emerged promising it all.
For me, the faster the class moves the less relaxing it is. It's also challenging to make sure that you are properly aligned in the poses which can lead to injury. If I want cardio benefits I'll go run. I fell in love with yoga because it slowed me down and helped me learn to unwind.
Some may argue that Billows-breath and Shining Skull breath can create cardio benefits such as increase lung capacity. It's possible but I find the dizziness and nervousness that accompany these rapid styles of breathing to do more harm than good. Broad sites a few cases of students who actually collapsed a lung as a result. He also mentions that instead of increasing your oxygen levels what you are actually doing is decreasing your carbon dioxide levels which can result in dizziness and even blackouts. The bottom line here is you are still going to need to do some form of aerobic exercise such as walking, swimming, cycling etc. in addition to yoga for complete physical fitness.
I'm writing all this not to show what a great yoga teacher I am and how my teaching aligns with science, rather the importance of following your instincts and listening to your body. The way that I practice and teach, the things I do or avoid, are mostly because they either benefited or bothered me in some way.
I couldn't have told you until I read the book that strokes were associated with shoulderstand, or that rapid breathing depletes your carbon dioxide levels. All I knew was they didn't feel good to me and so I stopped practicing them.
No matter how many benefits are associated with a particular exercise, even yoga, it's a mute point if your body objects. Chances are there's a good reason something doesn't feel good. Don't ignore that.