The third limb of yoga is asana or posture. Asana brings steadiness, health and lightness of limb. A steady and pleasant posture produces mental equilibrium and prevents fickleness of mind. Asanas are not merely gymnastic exercises, they are postures.
Asanas have evolved over the centuries so as to exercise every muscle, nerve and gland in the body. They create a physique which is strong and elastic without being bound by muscle. Asanas also help keep the body free from disease, reduce fatigue and soothe the nerves. Their real importance, however, is in the way they train and discipline the mind.
Yogasanas are merely one aspect of yoga. The yogi seeks refinement and balance not only in the body, but in the senses, mind, intellect and soul. Thus, the yogi conquers the body by the practice of asanas and makes it a fit vehicle for the spirit.
By performing asanas, the sadhaka (practitioner; one who is yet to reach the goal) first gains health - this is not a commodity which can be purchased with money, but an asset to be gained through hard work alone. It is a state of complete equilibrium of body, mind and spirit. The yogi is freed from physical disabilities and mental distractions by practicing asanas.
The yogi does not neglect or mortify the body or mind, but cherishes them both - the body is not an impediment to spiritual liberation, nor the cause of its fall, but rather an instrument of attainment. The yogi seeks a body as strong as a thunderbolt, healthy and free from suffering so as to dedicate it to a higher purpose. The Self cannot be attained by one without strength, nor through heedlessness, nor without aim. Just as an unbaked earthen pot dissolves in water, so the body soon decays. So bake it hard in the fire of yogic discipline in order to strengthen and purify it!
- B.K.S. Iyengar