Exactly, I first thought of the idea Jok suggested but that only applies to cases where the joints normal range of motion is from 0° to 180°. I'm really not sure what the exact joint limits are for the glutes or ankles or wrist (or chest), but if different joints have different limits then an extension value has to be in relation to a specific range of motion.
I guess this isn't something you can easily do with the available lua functions, since nothing in the SDK mentions anything about what range of motions are available to which joints, although it seems obvious that most of the joints are functionally "hinge" joints, because they work in one plane, and have the dimension of forward and backward.
Whats unusual is the chest joint since it actually flexes by rotating rather than hinging back or forth; still, its motion is one dimensional and limited to a specific range like the other joints, so theres nothing stopping Hampa from making the current extension of the chest available to a script either.
Somewhere in the physics/movement code, Hampa HAS to have some code that checks if the joint will move outside of its limit and prevents it from freely moving that way; thats how fractures work in Toribash, by turning that limit off.
Ideally, Hampa will provide some LUA function that returns the current extension/contraction of a joint, or at the very least, he might be considerate enough to explain how one might implement their own.