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 soft 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;
Yoga sutras 02:46
“sthira-sukham-āsanam”
“Asana must have the dual qualities of alertness and relaxarion.”
trans. TKV Desikachar