This review presents data on human muscle oxygen consumption in the initial phase of exercise as well as on muscle maximal oxygen uptake. It also discusses mechanistic limiting factors related to oxygen utilization at the onset of exercise and of maximal aerobic power of skeletal muscle. Direct measurements of oxygen utilization of a well-defined muscle show that contracting muscles utilize oxygen within a few seconds of exercise onset and that it takes some 45 s before oxygen extraction is maximal. The delayed oxygen utilization in the initial phase of intense exercise does not appear to be caused by insufficient oxygen availability. But it may rather be the result of a non-optimal distribution of blood flow in the exercising muscles and a limitation in the rate of oxygen extraction by the contracting muscle cells. The latter limitation does not appear to be caused by an insufficient activation of the enzyme pyruvate dehydrogenase. The maximal oxygen uptake of skeletal muscle is around 300-400 mL min-1 kg-1. This uptake rate corresponds to a TCA cycle rate of 4-5 mmol min-1 kg-1, which is of the same magnitude as the activity of oxyglutarate dehydrogenase and pyruvate dehydrogenase, suggesting that these enzymes may be rate limiting for oxygen uptake when an isolated muscle is exercising.