An embodiment of the invention includes (a) modeling a first internal force applied to a model of a user's joint based on a first external force externally applied to the joint at a first position； (b) modeling a second internal force applied to the model based on a second external force externally applied to the joint at a second position unequal to the first position； (c) comparing the first and second modeled internal forces； and (d) simulating the user based on the comparison. Other embodiments are described herein.
