Age and Gender also influence the work output of people. The curve opposite shows data on the maximum oxygen consumption rate (and hence steady state power output) of different subjects. The power output capability is about 15 to 25% larger for adult men than adult women, and for both genders the peak of capability occurs between 20 and 35 years of age. Part of the gender difference seems to be due to the lean muscle weight, which is less in women, and to the hemoglobin content of the blood (and hence oxygen transport capability)

From: Abbott & Wilson, "Human-Powered Vehicles,"
Human Kinetics (1995)

which is also less in women. The decline in oxygen consumption rate with age is dependent on the general activity level of the person, being greater in sedentary people (by a factor of three) as compared to active people. In athletes the period of oxygen consumption rate decline may be delayed until 40 years of age. Estimates of the overall efficiency of participants in a cycling marathon yield overall mechanical efficiencies between 17 and 20%. Automobile numbers are on the order of 15 to 20%.