Preventing default pose from deforming

I have a model created from scratch just to test building everything from the get-go. It has a body, a head, and four legs.

I've slowly got it to the point where I can translate it around, and jump. However if there's too much jumping too fast, it slowly deforms the pose. The head and legs twist and eventually it falls apart.

I have an active keyframe of its default pose on all the time, and I also have a bump detector for the ground triggering a copy of the same keyframe, but none of this seems to have helped.

Any ideas what I'm missing?