 Two groups of subjects will participate in this study. Group I n25 subjects with PAD will undergo two sets of assessments. Each assessment will consist of completing three questionnaires, two exercise tests, and one PET scan session. Additionally, they will receive one MRI of the legs. Then they will participate in a three-month exercise training intervention consisting of treadmill walking and calf muscle exercise. Finally, they will repeat one set of assessments listed above with MRI. Group II n25 healthy control subjects will undergo two sets of assessments. Each assessment will consist of three questionnaires, two submaximal exercise tests, one MRI, and one PET scan session no exercise tests. Additionally they will receive only one MRI. They will not participate in an exercise training intervention. 