Asana is a Sanskrit word which means posture.

According to the Patanjali Yoga Sutras, Asana or physical posture must be steady and comfortable. Steadiness of physical posture will eventually lead to the mastery of the 4 Postures of the Mind which are Friendliness, Compassion, Indifference to the ignorant people and Joy at meeting a spiritually advanced person.

 

Range of asana

The tradition also maintains that the benefit achieved through the practice of asana is knowledge of the three dimensions. Any layperson trying to study traditional literature will begin to wonder how one can achieve knowledge of the three dimensions by the practice of asana. The three dimensions can be understood in any sense, such as physical, mental and spiritual; gross, subtle and causal; waking, dreaming and sleeping states, and so forth. However, looking at the different descriptions of the three dimensions we can see that these dimensions are best understood in terms of the koshas: annamaya, manomaya and pranamaya, because the extent of experience of the physical body is limited to these three dimensions, and asanas are physical in nature.

1. Annamaya kosha is the physical aspect, the bones, muscles, nerves, blood circulation and the internal organs, such as the lungs, heart, kidneys, liver and pancreas. Everything that is physical in nature is included within annamaya kosha. The practice of asana is never unconscious or mechanical. It should always be done with awareness and concentration. For the practice of each asana, the yogis have described where we have to concentrate, which chakras and what kind of sensation and feeling we have to be aware of, and also what kind of pain we may experience. There is a wide range of experiences of which we have to become aware and observant.  This faculty of awareness or observation will increase our sensitivity to our own body. Therefore, through the practice of asanas we must first learn to observe our own body. What the body needs, what type of stretch, what type of release the muscular system needs, what kind of tension underlies which system, whether there is any disturbance being experienced anywhere in our physical body. What is the physical experience in the state of comfort, in the state of stillness and quietness?  This in-depth awareness of the body is the first benefit of asana practice at the level of annamaya kosha.
2. Manomaya kosha is the mental aspect. At the level of manomaya kosha the effort made to still the mind is very important. While making a physical movement, we are adding awareness and trying to concentrate. We are trying to practice asana pratyahara by withdrawing the mind from the distractions that pull it outside and merging it with the physical posture. So, asana pratyahara is the experience of manomaya kosha, where one begins to know the movement, nature, quality, strengths and weaknesses of the mind through the practice of asanas.
3. Pranamaya kosha is the energy aspect. When the body and mind are attuned with each other, pranamaya kosha is definitely affected. This is because prana relates very much to physiological experiences such as muscular stiffness and tiredness, and different pranic blocks are experienced in the physical body in the form of discomfort, energy depletion or disease. If there is dissipation of mind, one cannot be aware of this pranic energy or direct it into the right channels, where it can become the tool for the awakening of the subtle sensitivity of mind. If the mind is not sensitive enough, there cannot be an experience of prana. Without concentration and awareness there will be no experience of prana. This knowledge of pranamaya kosha, which is awakened through the practice of asana, prepares the practitioner for the techniques of pranayama, which deal specifically with the field of prana. So, asana becomes the stepping-stone to the practice of pranayama. The purpose of pranayama is to awaken the pranas, and this is where the range of asana ends. This is the knowledge of the intuitive mind which dawns through perfection of asana. To continue beyond this physical range, techniques of pranayama, pratyahara and dharana are required.

-Yoga Darshan-Swami Niranjanananda Saraswati