Sthira sukham asanam
Asana is a steady and comfortable body posture.
When the mind concentrates on a particular object, the body becomes fixed and somewhat relaxed. But one is unable to hold that position for long. Apparently the body was not in a relaxed natural posture, though one may have felt so for a little while. The idea behind the practice of asanas is to discipline the body to assume a posture in which greater concentration can be achieved.
In this stage of yoga
, the sadhaka is taught to discipline his body by making it remain fixed and relaxed. This ability is essential for keeping the mind still.
, the great Indian sage who composed Yoga Sutra
, has this to say on the effect of asanas:
Tatah dvanda anabhighatah
– From then on, the Sadhaka is undisturbed by the dualities.