Information about mechanical efficiency during intense exercise is limited, which is partly due to the difficulties that exist in quantifying anaerobic energy production. Based on human studies examining the isolated quadriceps muscle, the mechanical efficiency during intense dynamic exercise has been estimated to be around 22 to 26%. For cycling, the mechanical efficiency shows a wider range (14 to 34%) which, to a large extent, can be attributed to the uncertainties regarding the mass of the muscles used and whether muscle biopsy material is representative for the muscles involved in the exercise. Data from studies of the isolated quadriceps muscle suggest that mechanical efficiency increases as intense exercise is continued and that mechanical efficiency is greater when high intensity exercise is repeated, even with a rest period of 1 hour between exercise bouts. It is unclear whether a period of training can affect mechanical efficiency during intense exercise.