Perfect Posture

The sanskrit word for posture is ‘āsana’

This comes from the root ‘ās’ .. to sit, reflecting the ancient seers' focus on seated practice
Modern yoga practice emphasises, for better or worse, movement and breathing

Whether in seated meditation/breathing, or during active bodywork/āsana practice, two elements of posture are involved; first, the work we do to achieve stability, supports, second, the blossoming into a spaciousness

Sthira: Stability, firmness, balance, steadiness, constancy, stillness
An active, doing-type quality, the application of effort

Sukha: Comfort, spaciousness, relaxation, ease, acceptance, surrender, openness
A passive, being-type quality, a letting-go, a surrender

In seated meditation our posture is upright, but not rigid, relaxed, but not slack
In our āsana practice the two are engaged in a constant, more dynamic, play

To survive and thrive a tree must have firm roots and a strong trunk
But it must also flex and move with the wind

References;

“sthira-sukham-āsanam”
Yoga sutras II:46