Yogic exercises are all based on the formula of stretching, relaxation, deep breathing, and increasing circulation and concentration. Modern physical exercises require more energy than yogic exercises.
Every violent movement burns up lots of energy and more lactic acids are formed in the muscle fibers by such quick movements which is more tiresome to the muscles.
The slow gentle movements of the joints during yoga practice waste no energy. Deep breathing with mild retention during the practice allows for more oxygen absorption. Less lactic acid is produced and, as this is easily neutralized by the alkali, it avoids muscle fatigue.
When twisting and other movements of various joints occur the blood vessels are pulled and stretched and blood is equally distributed to every part of the body. The stretched muscles and ligaments during the yogic practice are immediately relaxed, carrying more energy to the muscle fibers. Energy flows into the relaxed muscles, just as water flows through an open vessel.