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.
Patanjali, 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.