The cross-sectional study will test the hypotheses that a) chronic lower extremity arterial ischemia is associated with specific pathophysiologic findings in lower extremity muscle and nerve and that b) these ischemia-related pathophysiologic findings are associated with lower extremity functional limitation.The longitudinal study will test the hypotheses that a) greater baseline lower extremity arterial ischemia as measured by ABI is associated with greater progression of pathophysiologic findings over two year follow-up and that b) greater ischemia-related pathophysiologic findings in the legs at baseline is associated with greater functional decline over two year follow up. Results will be used to develop interventions designed to improve lower extremity functioning and prevent functional decline in persons with PAD.