There is just two ways to work this out.

There is no general answer to this question because every resin has different properties. 

with something relating to prosthetics, I would be looking at making the sockets with a sandwich layup so using a foam(rohacell or honeycomb core) and having carbon skins. This will give you a lighter component and still have the desired strength, one other material you could add to the mix would a be a resin compatible rubber like Kraibon or Smac which will add durability and impact protection to the component.