She leaps up from her semi-prone position to a panther's crouch in the space of a breath.
She prefers a prone or semi-prone position because if she tries to sit up, she has extreme symptomatology - i.e. she becomes pale and her pulse rate increases among other things.
She twisted and fell out of the car onto her backside, and there she sat looking up at him, stunned from a slumped semi-prone position, in the middle of the road.