Posture, stance, or a state of readiness/preparation; the physical position or the prepared condition for an action or situation.
試合に向けて体勢を整える。
Prepare one's condition (or posture) for the match.
バランスを崩して体勢が崩れた。
I lost my balance and my posture/stance gave way.
緊急事態に備えて防衛体勢を強化した。
They strengthened their defensive posture in preparation for an emergency.