What are”ASANAs? “Asana” means “a position in which one feels comfortable”-’Sthirasukhamasanam.’ Asanas are a kind of exercise by regular practice of which the body stays healthy and hardy and many
diseases are cured. But asanas are not prescribed for the general cure of diseases: only those diseases which create trouble in the path of meditation may be cured with the help of specific asanas, so that sadhana may more easily be done.

The relation between the physical body and the mind is very close. Mental expression is brought about through the vrttis, and the predominance of the vrttis depends on different glands of the body. There are many glands in the body and from each there is a secretion of a particular hormone. If there is any defect in the secretion of hormones or any defect in a gland, certain vrttis become excited. For this reason, we find that in spite of having a sincere desire to follow the moral code, many persons cannot do so; they understand that they should do meditation, but they cannot concentrate their minds because their minds become extroverted due to the external excitement of this or that propensity. If a person wants to control the excitement of these propensities, he or she must rectify the defects of the glands. Asannos help the seidhakn to a large extent in this task, so asanas are an important part of sadhana.

The same asanas are not necessary for everyone. There are more than 50,000 asanas: among them a few are named here which are most necessary for the path of meditation. The acarya will teach particular asanas to a person according to his or her need.

There are many asanas whose forms are similar to those of different animals, and therefore they are called by the names of those animals. And there are many characteristics found in animals and birds which are not commonly found in human beings: the physical structures of those animals and birds are such as to further certain glandular secretions, and as a result certain special qualities are developed.

The tortoise can easily retract its extremities. If human beings can also sit in that way for some time, they can withdraw their minds from the external world. The name of that posture is Kurmakasana (tortoise posture). -source:Ananda Marga-