2D Fighting Games use rectangles and circles that are pinned to bones or in this case, imagepoints to simulate a moving hitbox,
as for the physics behind the impact, You'd be better off using pre-made animations as most ragdolley games look very unrealistic and getting it right might take a lot of time
I could knock you an example of the latter, hopefully tommorow.
One more thing, physics in real life are in 3D space and you can't replicate a humanoid character perfectly if you only have x and y to work with, let me gove you an example, imagine you hit someone on the top of the left leg, as a result, the crotch zone will pivot but the right leg won't move the torso will have to be at a 30 degrees angle from normal, which would vary depending on the force of impact and can't be reproduced with a single torso sprite, if you see what I mean, you'd need 3D physics