These experiments, however, were the first to demonstrate exactly that mechanical work was done at the expense of a dynamic equivalent of metabolism - a splendid confirmation of the law of the conservation of energy.

In one other experiment Atwater and Benedict calculated for J. C. W. a metabolism amounting to 9981 calories, divided as follows: Protein, 478 calories; fat, 7744 calories; carbohydrates, 1759. The man worked for sixteen hours on the bicycle. The work done measured an equivalent of 1482 calories; the body heat production was 7382 calories, both of which were measured in the Atwater calorimeter, and the total energy loss reached 9314 calories,1 a height of metabolism attained also by Maine lumbermen2 actively employed (see p. 348).

Later work by Benedict and Cathcart3 includes an experiment on a professional bicycle rider who rode a stationary bicycle for four hours and twenty-two minutes, accomplishing 208,000 kilogrammeters of work during this period, or nearly 13 • kilogrammeters per second. The work was the equivalent of more than a "century run," or over 100 miles (161 kilometers). The subject rode to exhaustion. When lying on a couch before the experiment the basal metabolism of this man was 1.14 calories per minute, the R. Q. was 0.85, pulse 63, and respiration 20 per minute. The basal value for the work experiment was ascertained by determining the heat production of the man sitting on the bicycle and revolving the wheel when it offered no resistance.

1 The calories calculated from the metabolism and those directly measured by the calorimeter did not exactly agree in this particular instance - an exception in a brilliant series.

2 Woods and Mansfield: U. S. Dept. of Agriculture, 1904, Bulletin 149.

3 Benedict, F. G., and Cathcart: "Muscular Work," Carnegie Institution, 1914, Publication 187.

The following table presents the results:

Metabolism During A "Century Run" On A Bicycle

Subject, M. A. M.; Weight=65.o Kilograms.

* No load experiments, without motor.

It is of great interest that the respiratory quotient should have remained at about 0.90 throughout the experiment, which indicated that the body's glycogen was being used in goodly measure throughout the whole period. A calculation shows that 368 grams of glycogen must have been consumed during the time of the ride. The average respiratory quotients of thirty-four days of experimentation with this individual presents the following results:

The lower respiratory quotient after mechanical work indicates the exhaustion of body glycogen.

The production of 600 calories per hour is probably in the neighborhood of the highest possible maximum of human physical capacity for sustained effort (see p. 431). The mechanical efficiency of 33 per cent, is the same as that previously described by the Zuntz school for raising the body of an individual in mountain ascents. The leg muscles are, therefore, remarkably efficient machines.

This work confirms that of Johansson,1 that the subjective sense of strain or fatigue has no influence upon metabolism.

Benedict and Cathcart further report a considerable increase in the basal metabolism obtained lying down after severe muscular work, the stimulating influence persisting for five or six hours. For example, a man whose basal metabolism was determined as 1.15 calories per minute rode a bicycle seventy-four minutes, doing work which was the equivalent of 2.06 calories per minute. During four and a half hours of subsequent rest the basal metabolism was determined eight times, and gave values between 1.35 and 1.33 calories per minute in each instance. The rate of the pulse fell from 93 in the first observation to 75 in the last, that of the respiration from 24 to 22.

Mettenleiter2 states that after hard exercise there is a fall in carbon dioxid tension in arterial blood lasting several days, due to a long continuing slight acidosis (see p. 421).

The stimulus to the increased metabolism is undoubtedly due to lactic acid. The rise in metabolism after giving alanin, which is convertible into lactic acid, is sufficient evidence that lactic acid stimulates metabolism (see p. 240).

Barcroft3 climbed a straight path to a height of 1000 feet (303 meters) in thirty minutes, a performance which involved only moderate effort. Observations of the carbon dioxid content of the alveolar air and the hydrogen ion concentration of the blood gave the following results:

The difference in acidosis corresponds to an addition of 0.023 per cent, of lactic acid to the blood. In another subject (Roberts) who made the same ascent the amount of lactic acid necessary to reduce the alkalinity of his blood to the level actually found was estimated at 0.029 per cent., and the increase, as determined by analysis of the blood, amounted to 0.032 per cent. Barcroft gives the following analysis of this state of affairs: During the ascent lactic and carbonic acids, and these only, were added to the blood. On account of the increased hydrogen ion concentration, the hemoglobin at a given pressure takes up oxygen less readily than usual and the respiratory center is stimulated. The increased respirations cause the excessive carbon dioxid produced to be expired, and not only the excess but somewhat more than this; the carbonic acid pressure in the alveolar air therefore falls. Lactic acid, however, is not got rid of so quickly as the carbon dioxid, and is retained. The increase in the hydrogen ion concentration of the blood causes a readier dissociation of the oxyhemoglobin contained in the large and quickly flowing volume of blood which passes through the capillaries of the muscle. At the same time the increased ventilation of the lungs increases the oxygen tension in the alveoli, and, since the absorption of oxygen by the plasma is proportionate to the oxygen pressure, the decreased avidity of hemoglobin for oxygen caused by the increased hydrogen ion concentration is compensated for.

1Johansson: "Skan. Archiv fur Physiologie," 1901, xi, 273; Frumerie: Ibid., 1913, xxx, 409.

2 Mettenleiter: "Deutsches Archiv fur klinische Medizin," 1915, cxvii, 517. 3 Barcroft: "The Respiratory Function of the Blood," 1914, p. 236.

The formation of lactic acid may be attributed to a local anemia during mechanical work. (See Chapter XV (Metabolism In Anemia, At High Altitudes, In Myxedema, And In Exophthalmic Goiter)).