Many of the asanas have animal names, such as the fish posture and the cobra posture. This is because yogis devised their asanas partly by observing how animal instincts work in the wild. When animals are sick they would only eat certain herbs and grasses. Similarly, they would stretch and contract muscles in various postures instinctively.

Yogis also observed how animals relaxed. Cats, especially, are experts in relaxation. On awakening from sleep, they instinctively stretch, arch the spine in both directions and then relax.

Asanas are also based on a sound knowledge of human anatomy and physiology. Yogis knew that placing the body in certain positions would stimulate specific nerves, organs and glands. For example, the shoulder-stand posture causes the blood to be directed by gravity to the thyroid gland, and the tucking in of the chin causes a gently squeezing action on the gland. These two actions have a profound effect on the thyroid gland.

The asanas are based on five principles.

Organ massage. The position of the asana causes a squeezing action on a specific organ or gland, resulting in the stimulation of that part of the body.

Stretching muscles and ligaments. This causes an increase in blood supply to the muscles and ligaments as well as relaxing them. It also takes pressure off nerves in the area. This stretching is involved in all the asanas, since it has such a beneficial effect on the body.

Deep breathing. While holding the yoga posture we breathe slowly and deeply, moving the abdomen only (abdominal or low breathing). This increases the oxygen and prana supply to the target organ or gland, thereby enhancing the effect of the asana.

Concentration. As well as breathing slowly and deeply, we also focus our attention on the target organ or gland. This brings the mind into play, and greatly increases the circulation and prana supply to the organ or gland. This concentration has the second benefit of increasing your general powers of concentration through regular practice. This benefits every aspect of your life. Your mind is less distracted and swayed by external events and you are therefore calmer and worry less. You will be able to solve day-to-day problems better and have more success in whatever activity you undertake.