Yoga is among India’s most popular cultural exports. People across the globe have adopted the ancient practice for its physical, mental and spiritual benefits. But along the way, yoga has morphed and been adapted to suit the modern gym-going public, leading some yogis to voice concern that it has become little more than a series of stretching exercises, divorced from its roots as a meditative discipline. In an attempt to find out the true essence and importance of yoga, TIME spoke to traditional Indian teachers for the International Day of Yoga, which falls on June 21.