A mass' moment of inertia depends on how that mass is distributed about an axis of rotation.For example, the leg's distal mass, which moves about the knee's lateral axis (designated by the x in the diagram to the left), possesses moment of inertia with respect to that axis. 

We can calculate the lower leg's moment of inertia by measuring


Then:
The more parts into which we divide the mass, the more accurate our estimate of its moment of inertia.
Anthropometric tables often include estimates of body segment moment of inertia. Researchers use these, rather than direct measurements, to perform biomechanical analyses.
Reference