Measurements of the gaseous metabolism of man while engaged in muscular work have been made by different investigators and include such varied forms of work as riding a bicycle ergometer, lifting weights, turning a wheel, and walking on a horizontal or inclined path both out-of-doors and on a treadmill in a laboratory. In selecting the form of muscular exercise for experiments on muscular work, walking seemed most suitable on account of its being the natural and universal form of exercise. It brings into play a large number of muscles of the body and at the same time the element of training is practically negligible. Furthermore, the Nutrition Laboratory was equipped with apparatus for experiments in this line and considerable published and unpublished data were available upon which to draw for necessary comparison. Accordingly, in addition to the work done with the ergometer, a considerable number of observations were carried out with a treadmill. The first set of experiments was made with Squad B on January 6 before their diet restriction began. This was followed with the same squad on January 28 after a 20-day restriction of diet and on February 3 experiments were made with Squad A after four months of diet restriction. In these experiments the subject walked a definite distance at a definite speed and determinations were made of the carbon dioxide excreted, and the oxygen consumed.

Since the use of any form of mouth or nose appliance in metabolism measurements is open to criticism and requires some practice to avoid respiratory disturbances it was decided to build a closed chamber large enough to contain a treadmill and a subject walking on it. Thus all objections to the use of the mouthpiece and nosepieces would be avoided and the subjects would have much greater freedom of head and body movements while walking than when joined to a circulating air system by a mouthpiece. In fact, the subjects were as free as when walking in the open room. The temperature within the chamber could be controlled by cooling the air of the room by open windows and electric fans and the humidity kept at a low point by circulating the air of the chamber through a drying system. Under these conditions the gaseous exchange could be computed from the volume of the chamber, the percentage increase of the carbon dioxide, and the decrease in the oxygen in the air of the chamber during the period of walking.

1 Benedict and Cady. Carnegie Inst. Wash. Pub. No. 167, 1912, p. 5; see description of Er-gometer No. II.

It was also considered possible by this method to make one 20-minute walking period suffice for each subject. By taking the samples of air for analysis in duplicate the possibility of error from this source would be slight. As a further check, however, it was planned to draw air samples for carbon-dioxide determinations at 10-minute intervals, thus dividing the walking period into two 10-minute periods, which should show agreement in the carbon dioxide present.

The walking during the experiments was done on a treadmill which is described briefly on page 126, and more in detail in another publication.1 This- treadmill was placed inside the specially constructed chamber; both treadmill and chamber are shown in figure 14.