Theoretically it is possible to determine the mechanical efficiency of a man by dividing the mechanical effect of his work by the increase of energy metabolism which the work involves. This gives the basis on which to ascertain how much extra food would be necessary to supply the energy required for the performance of any given task.

Zuntz and his associates in Berlin have carried out a long series of experiments of this kind which are described by Magnus-Levy in Von Noorden's Metabolism and Practical Medicine. The general bearing of these experiments may be summarized as follows:

The amount of oxygen consumed during work in excess of that during rest was regarded as a measure of that expended upon the work. As a rule the increased consumption of oxygen during work is relatively greater than the increased volume of air breathed, so that a greater proportion of the oxygen of the inspired air must be taken up by the lungs. As an example of the increase of oxygen consumption with muscular work the data obtained by Katzenstein in experiments upon the work of walking up an inclined plane may be given. The figures were as follows:

The constancy of the respiratory quotient indicates that there was no change in the nature of the material burned in the body on passing from rest to gentle, or from gentle to moderate exercise (though there is other evidence, as will be seen below, that vigorous exercise is apt to be accompanied by a rise in the respiratory quotient).

The weight moved (that of the subject and his clothing) was in this case 55.5 kilograms. From these data it was calculated that a consumption of energy equivalent to 0.223 kilogram-meter was required to move 1 kilogram of weight horizontally over a distance of 1 meter; and 2.924 kilogram-meters of energy to raise 1 kilogram through a vertical distance of 1 meter.

Experiments upon several other subjects gave similar results, indicating that these men who, while not trained in an athletic sense, were physically sound and thoroughly accustomed to this form of exercise, were able to perform 1 kilogram-meter of work in the ascent of the incline with an expenditure of only about 3 kilogram-meters of energy over that required at rest, so that the work was done with a mechanical efficiency of about 33 per cent. It is to be noted, however, that this applies only to walking done under the most favorable conditions, and not carried to the point of fatigue; also that robust men unaccustomed to this form of exercise showed efficiencies of only 20 to 25 per cent until after several days' practice, and for some subjects the maximum efficiencies found were 21 to 31 per cent.

On this basis it might be estimated that a man of average weight in walking one mile on level ground would do 8000 - 9000 kilogram-meters of work, or about the mechanical equivalent of 20 Calories. If this were accomplished with an efficiency of 33 per cent, it would involve an expenditure of only 60 Calories, but at an efficiency of 20 per cent 100 Calories per mile would be required.

The data of Benedict and Murschhauser's recent experiments lead to a similar conclusion. They found that the extra metabolism involved in walking at a speed of 4 miles per hour averaged 0.585 gram-calorie per kilogram-meter. For a man of 70 kilograms this would correspond to an increased energy metabolism of about 60 Calories per mile. Very fast walking (5.4 miles per hour) involved an expenditure of 0.932 gram-calorie per kilogram-meter, equivalent on the same basis to about 95 Calories per mile. To walk at a speed of nearly 5½ miles per hour required a greater expenditure of energy than to run at the same speed.

These figures may be helpful in estimating the food requirements of men who neither do active physical labor nor take vigorous exercise, yet move about more freely than in the so-called rest experiments already described. If, for example, it be assumed that a healthy man would require 2200 Calories per day when remaining in one room, and that the total additional muscular movements of a day at business and recreation were equivalent to walking five miles on level ground, his total food requirement for the day would become 2500 to 2700 Calories (36 to 39 Calories per kilogram), while activity equivalent to walking ten miles on level ground would bring the total daily requirements to 2800 to 3200 Calories (40 to 46 Calories per kilogram).

By means of the respiration calorimeter, Atwater and Benedict studied the question of mechanical efficiency with more accurate measurements of the energy involved than in the experiments of the Zuntz laboratory, but with a different form of muscular work. They placed in the calorimeter chamber an ergometer, which consisted of a fixed bicycle frame having in place of the rear wheel a metal disk which is revolved against a measured amount of electrical resistance, so that the mechanical effect of the muscular work is very accurately determined. The expenditure of energy involved in the performance of this work was estimated by comparing the total metabolism of a working day with that of the same man when living in the calorimeter chamber at rest. The average results obtained with three different men were as follows:

With an improved ergometer of the same type as that used in the experiments just cited, Benedict and Carpenter working with J. C. W. (one of the three men above mentioned) found efficiencies ranging from 20.7 to 22.1 per cent and averaging 21.6 per cent; with other men studied, the efficiencies ranged from 18.1 to 21.2 per cent.

Benedict and Cathcart, in similar bicycle ergometer experiments in which the basis of comparison was complete rest on a couch, found efficiencies varying from 10 to 25 per cent, depending on load, speed, and the familiarity of the subject with the work, the maxima for the six men studied being 23.1, 20.4, 21.6, 22.7, 20.8, and 25.2 per cent respectively.