Specific Aim 2. The second specific aim is to rigorously test mathematical models to extract quantitative physiological information from dynamic PET imaging. We will test a novel model that specifically addresses the kinetics of substrate delivery within the interstitial space of skeletal muscle (i.e. a 5 rate constant, 4 compartment model with respect to FDG. Model testing will entail use of non-compartmental models e.g. spectral analysis).