Anaerobic energy production during supramaximal exercise has been estimated from muscle metabolic changes. Based on such measurements the anaerobic energy production was determined to be 63 and 189 mmol ATP·kg-1d.w. for 60 and 142 s of exhaustive cycling exercise, respectively. These estimations do not, however, include release of lactate from the exercising muscles. Furthermore, the anaerobic production cannot be related to the work performed since the muscle biopsy sample may not be representative for the muscles involved in the exercise, and the total anaerobic energy production during whole body exercise cannot be determined because the mass of the muscles used is unknown. When a single muscle is exercised, the problems are minimized. With a one-legged knee-extensor exercise model, which uses a defined muscle mass, the anaerobic energy production has been estimated to be 370 mmol ATP·kg-1d.w. for a 192-s exhaustive exercise period. Estimated pulmonary oxygen deficit based on an energy demand extrapolated from a linear relationship between exercise intensity and oxygen uptake at submaximal exercise does not appear to represent the anaerobic energy production during whole body exercise.