Many of the factors described above are found in the case of muscular exercise in man. For ex ample, muscle "viscosity" is very obvious in the case of move ments made at high speed and explains the very exhausting nature of such exercise; while, together with the fact of energy expendi ture during the maintenance of a contraction, it serves to deter mine an "optimum speed" for many types of movement. Recovery after exertion is well known, and its rate and extent can be meas ured by the amount of oxygen consumption which it occasions. Lactic acid is found in large quantities in the blood after exertion, and the "oxygen debt" allows its amount in the body as a whole to be calculated. Strong evidence has been found that in man glycogen is used preferentially as the immediate fuel of muscular exercise, as it is in laboratory experiments on isolated muscles. Recent applications indeed of academic muscle physiology have afforded an interesting and gratifying page in the history of applied science.
Various methods have been employed of measuring the energy liberated in the body, which of course all comes from the combustion of foodstuffs in the active tissues. A man has been placed in a calorimeter and his energy output determined directly; but, except for special pur poses, this method is laborious and inadaptable. The most con venient procedure is to measure the oxygen consumption
and the carbon dioxide production
which can be done by a collection, analysis and measurement of the air breathed out. Tables have been constructed from which, knowing the
and the
the energy value of i litre of oxygen can be read off : or, in exercise of short duration, it is usually simpler and just as accurate to assume that i litre of oxygen corresponds to the fol lowing amount of energy (calculated for the combustion of glyco gen). t litre of oxygen=5•i'4 calories=i5,86o foot-pounds.
The oxygen consumption can be measured during exercise of all kinds, even running, skating, ski-ing and swimming.
When exercise commences the oxy gen consumption begins to rise, and in two or three minutes (sooner in some, later in other forms of exercise) it attains its full value. This gradual rise is occasioned, or accompanied, by several events:— (i.) the acceleration of the heart and the quick ening of the circulation; (ii.) the deepening and quickening of respiration; (iii.) the gradual accumulation of lactic acid in tis sues and blood.
When exercise ends the oxygen consumption begins to decrease, and in 5 to 8o minutes, depending on the violence and duration of the preceding exercise, it falls to its original value again. During this fall (i.) the heart and circulation return to their previous rate; (ii.) the respiration reverts to its resting condition; (iii.) the excess of lactic acid in the blood and tissues disappears.
During the exercise, if moderate, a "steady state" may be at tained, the increasing rate of oxygen consumption leading to an increasing rate of lactic acid removal, until finally a balance is struck at levels of lactic acid concentration and of oxygen con sumption which are characteristic of the exercise. If, however,
the exercise be too intense no balance is possible, since there is a strict limit to the rate at which oxygen can be supplied to the muscle fibres, even with the most strenuous efforts of heart and lungs to cope with the situation. The rate of lactic acid forma tion is then never balanced by that of its removal, and fatigue and exhaustion gradually set in. Thus the limit of prolonged exertion is set by the "maximum oxygen income" of the body. This in athletic men of ordinary size is usually about 4 litres per minute, corresponding to a total energy liberation of about 63,400ft.lb. per minute. It is found that, under good conditions, the "mechanical efficiency" of a man at work, viz. (work done)--:-(total energy used), may rise as high as 25%, so that, during prolonged work the greatest possible mechanical output of a man is about 15,86oft.lb. per minute, or just under horse-power.
In less athletic individuals the maximum oxygen income is smaller and the upper limit of exertion lower; it is smaller also during exercise in which the body is held in a constrained position, or part only of it used. The role played by the heart in severe exercise should be noted. To distribute 4 litres of oxygen at least 30 litres of blood must be pumped round the body every minute; the whole of the blood must circulate six times.
In the isolated muscle suspended in oxygen recovery from considerable exertion takes many hours, owing to the slowness of the oxygen supply by diffusion alone. In the intact animal at rest after exercise the supply of oxygen, except in the earliest moments of recovery, is more than adequate and re covery goes on at its own intrinsic rate, unaffected by considera tions of oxygen supply. Af ter moderate exertion recovery is quick —there has been no considerable accumulation of lactic acid, and the body returns rapidly to its resting state. The "oxygen debt" at the end of the exercise may be of the order of r or 2 litres but not more; and this is soon paid off. After harder exercise, however, in which the oxygen requirement has been considerably in excess of the maximum oxygen supply, there may be large quantities (up to ioogms. or more) of lactic acid in the tissues, and this will require large quantities of oxygen for its removal. The highest recorded value of the "oxygen debt" at the end of exercise is nearly 20 litres, but one of 15 litres is a very fair performance. It will be seen at once that this possibility of "running into debt" for a large quantity of the oxygen required has a considerable influence on the degree of strenuousness of the exercise which can be undertaken for a limited period. Without this "accumula tor function" in muscle, no effort of any considerable severity could be made.