Magnetic resonance spectroscopy and optical spectroscopy (MRS/OS) provide novel, non-invasive and real-time insight into human skeletal muscle mitochondrial function. MRS/OS is a novel technique that measures maximal mitochondrial ATP production in-vivo following acute bouts of ischemia induced by exercise using phosphorous MRS (31P MRS). The rate of recovery of phosphocreatinine after cessation of exercise is used to characterize the rate of aerobic mitochondrial ATP resynthesis above that of basal anaerobic glycolytic ATP production. By combining this technique with optical spectroscopy post-exercise measuring the transport of oxygen from hemoglobin to myoglobin within the muscle the investigators are able to accurately determine the ratio of coupling efficiency between of ATP generation per unit of oxygen consumption. Skeletal muscle mitochondrial dysfunction measured by ATP generating capacity and coupling efficiency has been associated with processes of aging and insulin resistance. Furthermore studies in healthy subjects have shown that muscle performance and fatigue are linked to the ability to resynthesize ATP rather than lactate concentration.