Another thing you might want to absorb is that crossbows, just like compound or other bows DO NOT have knockdown power. They shoot an arrow with a razor sharp broadhead on the front. That broadhead is the only thing that kills. And it kills by cutting major blood vessels resulting in massive blood loss.
Not even guns possess knockdown power. Bullets do not kill by knocking down an animal. The result of a bullet mushrooming causes shock to an animals system rendering it unable to cope with that shock. Thus the legs collapse.
This stuff is just plain ole physics.