From The Science of Pranayama, Swami Sivananda Saraswati

Prana may be defined as the finest vital force in everything which becomes visible on the physical plane as motion and action and on the mental plane as thought. The word pranayama, therefore, means the restraint of vital energies.

It is the control of vital energy which tingles through the nerves of persons. It moves his muscles and causes him to sense the external world and think his internal thought. This energy is of such a nature that it may be called the vis viva of the animal organism. The control of this force is what is aimed at by the yogis by means of pranayama. He who conquers this is not only the conqueror of his own existence on the physical and mental plane, but the conqueror of the whole world.

For prana is the very essence of cosmic life, that subtle principle which evolved the whole universe into its present form and which is pushing it towards its ultimate goal. To the yogi the whole universe is his body. The matter which composes his body is the same that evolved the universe. The force which pulsates through his nerves is not different from the force which vibrates through the universe.

Pranayama while walking

Walk with head up, shoulders back and with chest expanded. Inhale slowly through both nostrils counting Om mentally 3 times, one count for each step. Then retain the breath till you count 12 Om. Then exhale slowly through both nostrils till you count 6 Om. Take the respiratory pause or rest after one pranayama counting 12 Om. If you find it difficult to count Om with each step, count Om without having any concern with the steps.

Kapalbhati can also be done during walking. Those who are very busy can practise the above pranayama during their morning and evening walks. It is like killing two birds with one stone. You will find it very pleasant to practise pranayama while walking in an open place, when a delightful gentle breeze is blowing. You will be invigorated and innervated quickly to a considerable degree. Practise, feel and realize the marked, beneficial influence of this kind of pranayama. Those who walk briskly, repeating Om mentally or verbally do practise natural pranayama without any effort.

Pranayama in shavasana

Lie down on the back, at ease, over a blanket. Keep the hands on the ground by the side and legs straight. The heels should be kept together, but the toes can remain a little apart. Relax all the muscles and the nerves. Those who are very weak can practise pranayama in this pose while lying on the ground or on a bed. Draw the breath slowly without making any noise, through both nostrils. Retain the breath as long as you can do it with comfort. Then exhale slowly through both nostrils. Repeat the process 12 times in the morning and 12 times in the evening. Chant Om mentally during the practice. If you like you can practise the ‘easy comfortable posture’ also. This is a combined exercise of asana, pranayama, meditation and rest. It gives rest not only to the body but also for the mind. It gives relief, comfort and ease. This is very suitable for aged people.