From the scriptures
1) Maintaining integrity of the spine in meditation
A previous article I wrote on Vajrasana and its variants, and how they are thought to strengthen the spine, will not be complete without information on the proper meditation posture because correct alignment of the spine is essential for being able to sit long in meditation without strain or torpor. In any meditation retreat conducted by a knowledgeable meditation teacher, instructions on the proper meditation posture will be given. Correct position of the spine so that it does not slouch is important to maintaining mental alertness and prevent drowsiness and lassitude during meditation. At the same time there should be no strain.
2) The Bhagavad Gita on the meditation seat and posture
The Bhagavad Gita, considered to be the holiest of all yogic texts uses 2 shlokas (given below) to describe the seat and body alignment for meditation in chapter 6 (Dhyana Yoga). The first of these two shlokas talk about the seat preparation. Even though the Bhagavad Gita implies a variant of padmasana, as we saw in the previous article on Vajrasana, the Vajrasana is a legitimate posture used by Rishis throughout history and the Buddha himself is thought to have attained enlightenment on that fateful night while sitting in Vajrasana. The key item to glean from Shloka 6.11 (the first of the two below) is that the bottom has to be raised above the floor with some padding material. Since Kusha grass was used in those days, an additional covering of deer skin and a cloth over it was recommended though unanimously all the scholars and saints of modern times say that this is one area where one can do away with the deer skin and use the conveniences of modern materials.
Sitting cross legged
Sitting in modified Vajrasana on a Seiza bench
Sitting on a flat floor is not recommended because as shown above, the spine slouches after a while
3) Variants of Padmasana
The figure above shows three meditation postures. Two of them are recommended in the scriptures and the last one, where one sits on level floor, is not advisable due to the tendency to slouch. The first position can use any variant of the padmasana (full lotus). It is difficult for most of us to hold padmasana with the soles of the foot facing the sky for a long time. The Hatha Yoga pradipika as well as other texts on Hatha Yoga describe other variants of padmasana some of which can be used for long periods of time without strain. The most popular of this is Virasana. Shiva is often depicted meditating in Virasana (below).
Shiva in Virasana
The Hatha Yoga Pradipika on Virasana
4) Variants of Vajrasana
As seen above, Vajrasana is a recommended posture for meditation because it naturally keeps the spine upright, though in practice sitting in it for long durations makes the calf muscles ache. A modification using a seiza bench (Second picture in section 2 above) is often used.
Two postures for meditation, Padmasana and Vajrasana and their variants, commonly described in the ancient yogic texts are described. In both Padmasana, Vajrasana and their variants, the important aspect is that an obtuse angle be maintained between the spine and the legs (figure below). Sitting on a level floor results in a right angle (90 degrees) which puts strain on the spine and causes it to slouch.
Maintaining a obtuse angle between the spine and the legs is essential for maintaining an erect spine