B.K.S Iyengar is considered the world's greatest living yoga master. He refined and perfected the technique of doing yoga poses that is most widely taught in the United States.
Iyengar began life as a frail and sickly child — but now, at the ripe age of 87, the yoga master can still stand on his head and hold a conversation at the same time.
In his latest book, Light on Life, he recounts how physical weaknesses led him to the ancient practice of yoga:
"My poor health was matched, as it often is when one is sick, by my poor mood," he writes. "A deep melancholy often overtook me, and at times I asked myself whether life was worth the trouble of living.
"Seeing that the general state of my health was so poor, my brother-in-law recommended a stiff regime of yoga practice to knock me into shape and strengthen me up to face life's trials and challenges as I approached adulthood."
Over the next seven decades, Iyengar used his own body as a living laboratory to explore how different yoga postures — called asanas — can alleviate health problems. He used props like ropes, belts and bricks to help even the elderly, weak, and inflexible experience yoga's therapeutic effects.
But Iyengar says yoga goes beyond the physical motions: "The practice of yogasana for the sake of health, to keep fit, or to maintain flexibility is the external practice of yoga," he continues in Light on Life.
"While this is a legitimate place to begin, it is not the end... Even in simple asanas, one is experiencing the three levels of quest: the external quest, which brings firmness of the body; the internal quest, which brings steadiness of intelligence; and the innermost quest, which brings benevolence of spirit.
"Often, we hear people saying they remain active and light when they do just a little bit of asana practice. When a raw beginner experiences this state of well-being, it is not merely the external or anatomical effects of yoga. It is also about the internal physiological and psychological effects of the practice."
NPR Kroc Fellow Diane Geng contributed to this report.