Kelations between Factors of Metabolism. 225 



bicycle, either so adjusted as to rotate a small electric machine and develop a 

 current, 1 or in a more modified form with the electricity applied as a brake. 2 



With the close analogy between the human body and a machine, scientists 

 have for many years attempted to compute or determine the mechanical effi- 

 ciency of the human body, and these various types of ergometers have been used 

 extensively for this purpose. With the apparatus of Zuntz or Johansson the 

 mechanical work can be measured by noting the resistance of the brake on the 

 weights raised, but the total output of heat by the body of the subject must be 

 calculated indirectly from the respiratory gases. Both the carbon-dioxide ex- 

 halation and the oxygen absorption were measured in the Zuntz laboratory. 

 With the apparatus of Johansson, on the other hand, the heat was not deter- 

 mined directly and only the carbon-dioxide elimination was accurately meas- 

 ured, no attempt being made to determine the oxygen consumption. From the 

 factors obtained the total heat production was calculated by these investigators 

 and the mechanical efficiency of the body as a machine was computed from the 

 data. 



Using an early type of bicycle ergometer, in which the rear wheel was made 

 to rotate the armature of a small dynamo, a number of measurements were 

 secured as to the mechanical efficiency of man in the respiration apparatus at 

 Wesleyan University. Later, a more accurate machine was devised and utilized 

 for this purpose, and the results have been reported by us elsewhere. 3 The 

 measurement of the external mechanical work as performed on the earlier form 

 of bicycle ergometer with the respiration calorimeter at Wesleyan University 

 was subject to error, as the apparatus had not the accuracy or refinement that 

 could be desired. On the other hand, the determinations of the carbon-dioxide 

 elimination were made with the greatest accuracy, as were likewise the determi- 

 nations of the heat production of the man himself. It was possible to calibrate 

 the newer apparatus with great accuracy, and we believe that no measurements 

 exist in which the external muscular work performed and the total heat produc- 

 tion of man have been determined with as great accuracy. Consequently these 

 experiments offer especially advantageous data for computing the mechanical 

 efficiency of man. The computations for these investigations have been made 

 and discussed in considerable detail in the publication previously referred to. 



Carbon dioxide eliminated per Tcilogrammeter of work. As has been stated, 

 it is difficult to determine the amount of external muscular work performed in 

 the carrying on of certain simple operations. On the other hand, if a reason- 

 ably accurate relationship can be established between the increment in the 

 carbon-dioxide elimination above the resting value in experiments where the 

 external muscular work can be accurately measured, then the carbon-dioxide 

 increment can be used as a relatively accurate index of the external muscular 



1 Atwater and Benedict, U. S. Dept. Agr., Office Expt. Stas. Bui. 136, 1903. 



2 Benedict and Carpenter, ibid., 208, 1909. 



3 Benedict and Carpenter, U. S. Dept. Agr., Office Expt. Stas. Bui. 208, 1909. 



