The greater the proportion of one of the three substances of which the food taken consists, the more is the use made of this substance increased, which is shown partly by respiration and partly (with regard to protein) by the quantity of nitrogen in the urine.

All muscle work, static and motor, eccentric and concentric, increases the consumption of oxygen and the production of C02 (Johansson), and both these substances are increased, irrespective of the work, by the supply of all three kinds of food substances, protein, carbohydrates, and fat (Speck, Tisot).

Johansson in Stockholm has shown that during rest the production of carbonic acid is increased both by taking in protein and carbohydrate (laevulose), which may both be supposed to form fat by the liberation of carbonic acid, but that the production of carbonic acid, on the other hand, is unaltered by the ingestion of fat, which during rest is entirely stored.

It is well known that the change to muscular energy and heat of the chemically stored energy in protein, fat, and carbohydrate takes place by the oxidation of these substances, and their breaking up into simpler molecules in the tissues wherever blood and cells are present, therefore, even in resting muscles (Meade-Smith), but that these changes take place much more quickly and strongly in working muscle than elsewhere. Chauveau and Kauffmann showed experimentally that a muscle doing moderate work consumes many times as much sugar as glandular tissues or resting muscle. Under normal conditions the consumption of oxygen and the production of carbonic acid in the lungs correspond to the complete oxidation of the assimilated food substances, although the respiratory quotient, or the proportion of oxygen to carbonic acid (Co2 / O), varies somewhat, for purely chemical reasons, with different foods, and is greatest with pure carbohydrates. It amounts to about 1, and may even rise above this. With pure unmixed proteid food it is about 0.8. With unmixed fatty food, 0.71. In starvation it is at first at a fairly high figure while the glycogen of the liver is being consumed; afterwards it sinks quickly and approaches the same figure as that of fat (Koraen). Speck's experimental rest cases gave off 0.461 to 0.601 grm. oxygen (= 0.322 to 0.420 litre) and 0.535 to 0.717 grm. carbonic acid (= 0.271 to 0.364 litre) per kilo of body weight every hour. The respiratory quotient was in this case on the whole greater than 1 when the oxygen took part in other oxidation processes than that of forming carbonic acid, forming in particular water. The muscles are able to form carbonic acid by destructive processes without taking up oxygen, but oxygen must take part in the formation of the substances capable of being broken up. During work therefore the quotient rises; during rest it sinks.

By as complete rest as possible a person weighing 80 kilos can limit his production of carbonic acid to 20 grm. per hour. In changing from rest to a stiff standing position the gaseous changes show an increase of about 20 per cent. In walking the consumption of oxygen and production of carbonic acid rises two to four times its amount during rest (Zuntz); hill-climbing and other hard mechanical work raise it extremely (to over 130 grm. per hour), and heat production rises at the same time, as I have already pointed out (Atwater and Benedict). A person who during rest expended 2,357 calories, of which 429 were supplied by protein and 1,928 by fat and carbohydrate, expended during work 5,119 calories, of which 462 were supplied by protein and 4,657 by fat and carbohydrate. In hill-climbing Zuntz found that 1 kilogrammetre of work used 0.0072 calorie. Since a kilogrammetre corresponds to 0.0026 calorie, this agrees with the view that about one-third of the force used is transformed into work, two-thirds into heat. According to Johansson and Koraen every kilogrammetre of external work caused a production of 0004 grm. carbonic acid. Here we must call to mind that the Co2 in expired air corresponds to different heat values, according to whether protein, fat, or carbohydrate has been oxidised in the body.*

In static muscle work, according to Johansson and Koraen, the production of carbonic acid is proportional to the muscles" time of contraction, and rises more than in proportion to their shortening, so that when the muscles are shortened to the maximum the production of carbonic acid is about five times greater per second than when they are contracted bei naturlicher Lange. Both in static muscle work and in eccentric resistance movements ( = "negative muscle work ") the production of carbonic acid increases in proportion with the time of contraction within certain limits; and the increase during eccentric muscle work is not greater than can be ascribed to static muscle work. On the other hand, as soon as fatigue arises the production of carbonic acid increases more quickly. (The explanation of this is given later.)

Johansson's and Koraern's experiments show that in concentric resistance movements the production of carbonic acid increases proportionately with the number of movements after some practice, but that its production increases more quickly than in this proportion before the individual has had some practice. The carbonic acid increases in proportion to the time of contraction and to the resistance ( = the load); with a heavy load and slow contractions, however, more quickly than according to this rule. It is, especially during slow contractions, greater during the later than during the earlier stages.

Since muscle work, consumption of oxygen, and production of carbonic acid increase with the time of contraction, and since the same movements may require very different work, according as the contracting muscles alone or their antagonists also are innervated (which according to Kohnstamm is especially the case in fatigue), it may easily be that slow movements, even slow walking, expend more than quick movements and quick walking, although quick walking, for the following reasons, costs more in consumption of food substances than slow walking (see further on Self-resisted Movements).