Endurance power of muscles is determined largely by the capacities to oxidize substrates in mitochondria in the process of making ATP by oxidative phosphorylation. This review explores physiological and morphological factors that may cause limitation of carbohydrate and fat utilization by muscle cells. The pathways for oxygen and substrates converge in muscle mitochondria. In mammals, a structural limitation of carbohydrate and lipid transfer from the microvascular system to muscle cells is reached at a moderate work intensity (that is, at 40-50% of VO2max). At higher work rates intracellular substrate stores must be used for oxidation. Because of the importance of these intracellular stores for aerobic work we find larger intramyocellular substrate stores in endurance trained athletes. The transfer limitations for carbohydrates and lipids on the level of the sarcolemma implies that the design of the respiratory cascade from lungs to muscle mitochondria reflects primarily oxygen demand.