Patanjali describes: Sukha Sthira Asana.  This translates to: Ease and Steadiness in Postures.  Padmasana is the quintessential pose representing this approach.  The vertebra must be naturally straight allowing free movement of cerebrospinal fluid without muscular engagement of torso; legs must be comfortable to support the structure of the body as well as enable Mulabandha (root lock) and Udiyanabandha (diaphragmatic lock); palms can face forward or down based on what keeps the shoulder-blades naturally down.  In this position, lungs must have the freedom to breathe fully, as well as engage in pranayamas and kumbak (holding of breath). Finally, wrists will create further steadiness by creating a pyramid-like support using straight elbows. The body and mind must be able to support this pose with ease and steadiness.