From the 1975 programme:
The body can be regarded as a rather special internal combustion engine, insofar as its ability to perform mechanical work is concerned. Some of this work is expended to keep it alive, for instance by pumping the blood round and breathing, whilst the rest appears as external activity. Walking, running and lifting sacks of coal all involve the expenditure of energy which ultimately has to be derived from the fuel, i.e. the food supplied to the body. Just as in a car engine, not all the energy contained in food can be converted into mechanical work; at best only about 20% appears as work, the rest being converted into heat.
There are many occasions when one wants to measure how hard an individual is working or more correctly how much energy he is consuming. This is important in fields as diverse as the organisation of some industrial production processes in order to minimise the load on the worker, and the investigation of the factors causing obesity.
In a petrol engine we could measure the rate at which fuel is being consumed quite easily but in man it is impossible to measure fuel consumption directly, because our bodies contain large and variable energy stores on which we draw to supply our immediate needs.
However, we cannot store any substantial amounts of the oxygen which is required to 'burn' the fuel, and if therefore we were to measure the amount of oxygen removed from the inspired air this would provide an indirect but convenient method of measuring energy expenditure. This procedure is known as 'indirect calorimetry' and is performed these days by using interesting and complex physical methods of air flow measurement and gas analysis. We will endeavour to measure the efficiency of the human engine and compare it with that of a small petrol driven generator.