The third limb of Ashtanga yoga in Raja Yoga is Asana. Asana is important in so far as one must have practice to sit for long periods in a firm posture that is conducive to meditation (Asana Siddhi). The essence of it is that the spine should be held relaxed and erect, with the three upper parts of the body – the chest, the neck and the head, held in a straight line, and the weight of the upper part of the body evenly distributed on the ribs. In the practice of concentration, nerve currents will have to go through new channels, and the main part of this activity of a very subtle nature will lie along the spinal column. Hence the importance of holding the spine erect.
- Swami Vivekananda
(Excerpts from The Four Yogas of Swami Vivekananda, condensed and retold by Swami Tapasyananda)