An increase in exercise capacity depends on the magnitude of increase in maximum aerobic capacity. Central and peripheral factors may limit oxygen uptake. Central oxygen delivery depends on cardiac output and maximal arterial oxygen content. Peripheral extraction of the delivered oxygen is expressed as a-v O2. With increasing intensities of exercise, the respiratory system may become limiting in some trained individuals. Most studies have shown a higher stroke volume in maximal as well as submaximal exercise in the trained vs untrained individuals. A variety of peripheral factors determine vascular tone. Maximal oxygen uptake depends on all components of the oxygen transporting system, but stroke volume appears to be the prime determinant in the trained subject. At maximum exercise the capacity of the muscle capillary network is never reached.