On the Cavalry Commander (1.13-1.14)

The haunches must be broad and fleshy, that they may be in right proportion to the flanks and chest, and if they are firm all over, they will be lighter for running and will make the horse speedier.

If the gap that separates the hams under the tail is broad,[*] he will also extend his hind legs well apart under his belly; and by doing that he will be more fiery and stronger when he throws himself on his haunches and when he is ridden, and will make the best of himself in all ways. One can infer this from the action of a man: for when he wants to lift anything from the ground, a man invariably tries to lift it with his legs apart rather than close together.