The human body can deliver lots of work. Consider, for instance, the athlete running a marathon, or the cyclist racing in the Tour de France. We also know that human body temperature is normally 37°C and that usually the environment is cooler, say 20°C. From this we could suggest that there is some resemblance between a heat engine, in which the body is the heat source, and the cooler environment could act as a heat sink. So let’s make a few simple calculations to see how closely the body resembles a heat engine. We know that the efficiency of a heat engine is determined by the temperatures of the heat source (the body temperature, Tbody = 310K) and the heat sink (the environmental temperature, Tsink = 293K):
Efficiency = [Tbody – Tsink]/Tbody = [310-293]/310 = 5.5%
Thus, based on this temperature difference, the body would be able to achieve only 5.5% efficiency. Fortunately, scientific studies already have estimated the human body’s efficiency  in other ways. One study reasons that for an average man to produce 75 Watts of power, he will need to breathe about one liter of oxygen per minute. That liter of O2 is combusted in body cells to form carbon dioxide (CO2). It has also been determined that one liter of oxygen generates in this way about 300 Watts of power. Thus, we can conclude that the efficiency of the human “engine” is 75/300 = 25%. What causes the difference between the 5.5% efficiency as calculated above, and the 25% from the combustion determination? The explanation is that the human body cannot be considered a heat engine. The work is not generated in the same way as a steam engine, which directly transforms heat into work and lower-temperature waste heat. Instead, the human body is more like a fuel cell, where chemical energy is transformed into work (see also Whitt et. al.). For this kind of transformation, one obviously cannot use the efficiency formula of a heat engine.
 Whitt, F.R. and Wilson, D.G., Bicycling Science, MIT Press, Cambridge (1976)