There's a really cool book called Yoga Anatomy by Leslie Kaminoff that shows crazy illustrations of what's happening to your muscules and your skeletal system in different yoga poses. (Kind of like that Bodies exhibit, if any of you have seen that... I saw it in Boston and thought it was wicked cool.)
I don't own the book, but I was thinking of it the other day because I was talking with Mei about the anatomy of the spine (shout-out!). I found in ebook form - yay!! - but it cuts off right before the part that I was trying to remember! Boo!! It is still an amazing read, though - the whole first chapter, which describes the anatomy of how we breathe, is available online right now: Yoga Anatomy Highly recommended FREE reading! It kinda makes my head spin, but it's amazing stuff, especially when you start thinking about pranayama breathing in an anatomical context and you realize what you are actually doing to your body during the exercise...
The part that I was looking for (which I did not find!) explained the different ways that your spine can move. There are five. Forward bending (extension), backward bending (compression), lateral bending, twisting, and axial extension (which I think was called by another name). If I'm remembering correctly, the vertebral discs (the little cushions between the vertebrae) get farther apart when you round forward (like rabbit) and closer together when you bend backwards. Axial extension, where the spine actually lengthens by reducing the curvature, is the only one of these motions that virtually never occurs in day-to-day movement, unless maybe you're reaching for a REALLY high shelf.
It's pretty easy to break down any yoga posture into the different actions of the spine that are involved. A few examples: we get pure forward bending in rabbit and all the "head to knee poses." Back bending is obvious - half moon, spine series, camel. Side bending is half moon, of course. Final spinal is pure twisting, though there are other twists; standing bow is sort of a composite twist and backbend. Axial extension - think of standing separate leg stretching, or the final stretching posture. I'm also CONVINCED that pranayama breathing is all about axial extension, which I think is brilliant. Think about all the length that you can create in your spine on the inhale, using the leverage of your hands on your chin. That's what REALLY prepares you for half moon, which hits you right away with three of the other actions. (Isn't it a great series?!)
Incidentally, this is why I think it's weird when people talk about avoiding compression in backbends, since they are compressions by definition. The vertebral discs move closer together. That's the anatomy. However, it IS true (and very important) that before you do any back (or side) bend, you need to create as much length in the spine as possible. Again, this starts in pranayama! This is also why, if you pay attention to the (pure) dialogue, you'll notice how many side and back bends start with an inhale breath and axial extension, either explicitly or implicitly. Like in half moon side bend, the last instruction before the bend begins is to "stretch up out of the waist one more time, touch the ceiling." In the half moon back bend, the instruction is to take a deep inhale ("inhale breathing, full lungs") immediately before beginning the backbend from the cervical spine ("drop your head back as far as it goes, look at the floor behind you"). Lengthening precedes compression, without exception!
Why was I talking about this? Oh yeah... because Mei posted measurements of her spine when it was compressed, extended, and in its natural state, and someone wrote her a comment saying "What the heck?! How can your spine change length?" Visual illustrations were requested, and I happen to have some handy, so here you go. I've got a bunch of random yoga pictures of myself, including a few in rabbit, camel, and pachimottanasana. Picture time!
My pachimo - close to max length, though standing separate leg stretching would really be an even better example
Rabbit: Maximum extension
Full camel: maximum compression.
These were all taken last year (some of you will be able to tell which one was taken over Christmas break by looking at the carpet...) and I can see TONS of room for improvement, but I guess it is kind of my job to look for those things since it is my own practice.
Cool though, right? Look at the spine! It can change so much! And in this post I have barely even scratched the surface of all the things that are interesting about the spine!! I would have to write a whole book to talk about how crazy important the spine is! Every sentence would probably end in an exclamation point!!
My sister is sitting in the living room with me and I kept shouting stuff out to her as I read the Yoga Anatomy ebook, and she did not agree that it was incredibly fascinating. This reminds me of a friend who was in grad school for geology, who one day, at the age of maybe 25, turned around with this expression of shocked realization and said "OH my gosh, I just realized something! Not everyone thinks that rocks are incredibly interesting!!!" To each her own, I guess...