The most common mistake made by beginners when they sit in a meditative asana is to hold the back too straight. That is, they sit with their back arched, the navel pushed forwards and the head thrown backwards. This may look very impressive, especially to people who do not practise yoga, but it is not very comfortable. Further, not many people will be able to maintain this position for more than a few minutes without slumping forwards. This is merely replacing one extreme with another.
Actually, when we say keep the spine straight, we are asking the impossible, for the normal shape or contour of the spine is certainly not straight, it is S-shaped. No matter how much you try, you will not be able to make your spine perfectly straight – the bone structure prevents this. When we instruct you to keep your spine straight, we mean that the back should be held upright without slumping forwards or being arched backwards. In other words, allow the spine to take the position in which it is most comfortable. You will find from personal experience which is the best position of your back.
We again emphasize that you should try to avoid either slumping forwards or straining backwards. Both of these positions prevent optimum relaxation and free breathing. It is impossible to relax if you are struggling to hold the spine in an abnormally arched position, just as it is impossible to relax with an aching back from a forward slumped position. And we assure you that backache is most likely to occur if you are slumped forwards. It is essential for optimum results to hold your head and back in a comfortable upright position.