“Sthira Sukham Asanam”, Sadhana Pada Sutra 46
Patanjali says, Asana should be steady and easy. In Sutra 47 and 48 he says to master asana restlessness should be reduced and one should feel timelessness while maintaining the pose. Then the mind becomes free from the dualities of the body.
Asana shows us the tensions held in the body. The experience is a mental one and the holding pattern of muscle tension is revealed as an action of the mind. This tension held in the body has its root in the mind. If we remain still while maintaining a pose we are releasing tension from both body and mind. The unconscious program in the mind which organises the posture of the body is altered if we can find a new comfortable balance. Over time the body finds a more appropriate symmetry. Over time left and right, upper and lower, inner and outer become balanced.
The other ancient authority we look to is Swatmarama who composed the Hatha Yoga Pradipika over 1000 years ago. This classical text describes Asana in detail and gives instruction on some 15 different poses. The Pradipika gives a graphic explanation of the relationship between mind and body and the interplay of subtle energy known as prana. In the Pradipika Asana is used to purify the Nadis, meaning the network of channels for Prana. This is an important preparation for harnessing the dormant potential or Kundalini which comes with other practices of pranayama and dhyana (meditation).
If a posture is not maintained for a certain period of time, with the right mental attitude it is not considered asana.