Swami Kriyananda, a direct disciple of Paramhansa Yogananda, founder of Ananda Sangha, and a known authority on Kriya Yoga, elucidates the difference between yoga and ordinary exercise.
Yoga postures are very different from ordinary exercise. It is a mistake even to call the postures exercises, in the usual sense of the word. Their purpose is not to strengthen the muscles. They emphasize relaxation quite as much as they do tension. Unlike most physical exercises, they do not excite; rather, they eliminate excitement from the system.
An important difference between these postures and other systems of exercise is that in yoga practice one must never strain. Relax, never force yourself, into the prescribed positions. Stretch only slightly, if at all, beyond the point of comfort. You will be astonished to see how many poses you can accomplish by progressively deeper relaxation.
The yoga postures are not only a series of physical positions but exercises in mental awareness. The yogi must try to become conscious of the energy as it directs the muscular movements.
Between poses, he should calmly withdraw his energy from the periphery of his body; he should rest within himself. Savasana, the Corpse Pose, is particularly recommended for these peaceful interludes.
The yogi is enjoined to practice moderation in everything. He should avoid eating too much, or too little. He should not sleep too much, nor too little. (More than seven hours' sleep at night only drugs the nervous system.) He should be especially moderate in his sex life. Sexual over-indulgence causes tremendous drain on natural vitality. Continence, if it has the full consent of the mind, can be a tremendous factor in helping one to achieve full vigor, mentally and physically, and to attain deep spiritual insight.
Yoga practices help one to live in harmony with the forces of nature. The yoga postures should always, if possible, be practiced out-of-doors, or by an open window.
They should be practiced on an empty stomach or at least three hours after eating. It is preferable that the body be warm when performing them. But don't practice immediately after strenuous activity; or so long that the postures themselves result in over-exertion and fatigue.
Women should use caution if they wish to do yoga postures during the first day or two of the menstrual period.
The postures should not be practiced, save with the greatest of caution, when the body is unwell. Any posture that gives rise to a feeling of pain (other than muscular) in the chest, abdomen or brain should be abandoned until the cause has been ascertained. People with high blood pressure should avoid all but the most gentle poses.
The duration of each posture must be increased gradually. People beginning these postures after middle age should be particularly careful to start slowly, with the easier poses, bit by bit working up to the more difficult ones.