Muscular Activity

Fic. 1. Tap Respiratory Quotient DuriInG AND AFTER MuscuLAR _ These figures show the initial phase of recovery only. The final phase is shown in figure 2. (Hill, Long and Lupton, 1924.) tude of that process—that is, of the amount of lactic acid | removed. Assuming an “efficiency of recovery” of 5.221, 1 litre of oxygen consumed after exercise is equivalent to 7 grams of lactic acid removed. The greatest oxygen debt — hitherto recorded is one of nearly 19 liters, which corresponds to a concentration of not far from 0.35 per cent in all the muscles of the subject; he was well-nigh exhausted. The changes in the lactic acid concentration in the

blood are very instructive. During moderate continued exercise it rises, in a few minutes to a steady constant value, remaining there so long as the exercise lasts. After the exercise it returns slowly to its original value. During severe exercise it rises continually, till the exercise is stopped by distress (fig. 3). Afterwards it may continue to rise for a few minutes, by diffusion from the muscles: Fig. 2. Tur ResprraTory Quotient Arrer SpverE Muscunsr Exercise

Note that in the later phases, while carbon dioxide is being retained to compensate for that initially driven off, the respiratory quotient falls to a very low level, returning to its final value at about 80 minutes. (Hill, Long and Lupton, 1924.) then it begins to return to its normal value, attaining that some time in the second hour after the exercise ended. Its changes give the same information, in another form, as is given by the changes of the respiratory quotient.

The greater part, however, of our knowledge of the events occurring in human muscular exercise has been arrived at by studies of the oxygen consumption. The measurement of oxygen intake is possible in many ways. One involves — the continuous circulation of oxygen in a closed system, — which includes the lungs of the subject, the CO. being — absorbed and the diminution in volume owing to consumption of oxygen being recorded. The method is accurate

Fia, 8. Lacric Acip In Human Buoop Arter Srverp Muscurar Exercisn; Two EXPERIMENTS IN Arr, OnE IN 49 Per Cunt Oxyaern, One 1n 100 Par Cunt OxyGEn Note that the recovery process is not quite complete at the end of the time shown in the diagram. (Hill, Long and Lupton, 1924.) but inadaptable; it is not capable of analysing the timecourse of the oxygen consumption when the latter is changing rapidly, as at the beginning and end of exercise. An- other method, used by Krogh and Lindhard in their recent and admirable researches, is to place the subject in a closed room of known volume, and to determine the small changes in the gas concentrations inside it by a highly

sensitive gas-analysis technique. The method is very accurate and convenient to the subject; it is elaborate, however, and—without very violent mixing of the air in the chamber—could not be used over very short intervals. For such purposes the technique of the Douglas bag, in which the subject expires into a large bag carried on his back or in front of him, and the expired gases are analysed and their volume measured, is sufficiently accurate and is capable of following very rapid changes. It is possible to use this method when walking, running, riding a bicycle, even swimming, and to plot the time course of the oxygen consumption in intervals as short as half a minute. Under certain circumstances, even quarter minute intervals may be employed.

An isolated muscle contracting in an atmosphere of oxygen rapidly attains a steady state; in such a muscle there is a balance between breakdown and recovery. In an atmosphere of nitrogen such a steady state is never attained: fatigue progressively occurs, and the muscle finally becomes inexcitable. ‘The recovery process is more rapid when the total amount of breakdown causing it is greater; lactic acid is a sensitive “governor” of oxygen usage. Hence, if we start with a given rate of breakdown,

the rate of recovery, starting from zero, has to work up gradually until it balances the rate of breakdown (fig. 4). It is necessary, moreover, in the intact animal, for the circulation and respiration to work up also. In a subject starting to run, having been previously at rest, the oxygen intake rises to its full steady value in a period of about two to two and a half minutes. If the running be continued at a constant speed the oxygen intake also remains constant thereafter till the end of the exercise; the subject is

in what we may call a “steady state.” This state corresponds to a constant concentration of lactic acid in the muscles, constant owing to a balance between formation and removal. As the exercise, however, is continued, the Fia. 4. Tam ArrarnMenT or a “Steapy Stats,” iv Runnine at Various Constant SPEEDS ; Horizontally, time from commencing to run; vertically, rate of oxygen intake In excess of standing. Speeds of 181, 203, 203, and 267 metres per minute. The lower three curves represent a genuine steady state, the uppermost curve only an apparent steady state in which the oxygen

intake is at its maximum and the ‘‘oxygen debt” is rapidly increasing. — lactic acid present in the active muscles gradually passes, — by diffusion, into the blood and thence to other parts of the body, This is probably one cause of the fatigue resulting from long-continued moderate exercise. The oxidative removal of the lactic acid formed in one muscle may, in this manner, occur in other distant muscles by its passage, via the blood, from the one to the other. As Barr and Himwich have found, the venous blood from a resting muscle, during the vigorous activity of others, may contain less lactic acid than the arterial blood coming to it. In- deed, in this sense it is possible to recover in one’s arms from exercise taken in one’s legs! This may be an important factor in recovery from vigorous exercise taken by

Fig. 5. Diagram TO REPRESENT OxYGEN INTAKE AND OxYGEN REQUIRE- MENT FOR THREE Duarezs or Exercise: I, Mitp; II, Mopmratery Severe; III, Very SEVERE small portions of the muscular system; it cannot have much effect in severe general exertion. It is to be noted that an apparent “steady state’ may occur, during muscular exercise, in which the oxygen intake is constant, which is not, however, really “steady” at all; there is a continual onset of fatigue. The oxygen intake may attain its maximum and remain constant merely because it cannot go any higher owing to the limitations of the circulatory and respiratory system (curve III, fig. 5). _ Such a condition causes an accumulation of lactic acid in _ the muscles and can end only in complete exhaustion.

When the exercise terminates the lactic acid in the muscles must be removed and oxygen must be taken in to provide the necessary energy. The lag in the oxygen intake at the beginning of exercise, during which the lactic acid was concentrating, must be compensated by an extra intake afterwards when it is diminishing again. After a short bout of moderate exercise the oxygen intake falls Horizontally, time from cessation of exercise; vertically, rate of oxygen intake in excess of standing. The lower curve, falling rapidly to the base line, represents recovery from a short bout of moderate exercise. The — upper curve, falling less rapidly, represents recovery from a rather longer bout of much more strenuous exertion. (Hill and Lupton, 1923.)

in a period of five to ten minutes to its resting value (fig. 6). After severe or prolonged exercise it may be 80 minutes before the oxygen intake is at its resting value again (fig. 7). The total amount of oxygen used in the recovery process may be measured by collecting the whole of the expired gases over any desired interval, by an estimation of the total oxygen used in that interval, and subtraction of the amount of oxygen which the subject would have used had he remained throughout at rest.

There are apparently two phases in the recovery process. The first and rapid phase (fig. 6) involves the oxidative removal of lactic acid in the muscles where it was formed. The second and prolonged phase (fig. 7) deals with the removal of lactic acid which has had time (after severe exercise) to escape, by diffusion, from the muscles where it was liberated, into the blood and thence into all the Five ExPERIMENTs PLorTEep (FoR CLEARNESS) TO Dirrerent Base Lines

other tissues of the body. The slowness of the diffusion process makes the second phase a very prolonged one. It is desirable here to introduce a new term—namely, the “oxygen requirement” of exercise (fig. 8). If the exercise be mild, then the oxygen requirement may be met . by the actual oxygen intake once the steady state has exercise be persisted in, the body necessarily incurs what we may call an “oxygen debt.” Were it not for the fact that the body is able to obtain its energy in this way, by —

would be impossible in man, We should never be able to run upstairs, or to run on the flat at more than eight or Fia. 8. Diagram To ItLustratTH FurtuHer (cr. Fie. 5) tar OxyG@En INTAKE AND THE OxyGEN REQUIREMENT nine miles an hour. We can do so only because Nature has provided us with an arrangement, like an accumulator or of a given isolated movement or series of movements, can always be measured. The oxygen requirement may be many times higher than any possible oxygen intake. If, however, the movement be carried out only for a short

time, and the total oxygen used during it and in complete recovery from it, be measured, then the total oxygen so found is the requirement of that piece of effort. In this way it is possible to study the energy requirement of even the most violent and discontinuous movements. To take an example, Lupton has measured the maximum efficiency of the process of climbing a staircase, in an endeavour to Show that there exists, for many muscular movements,

Fig. 9. ‘“Errictmncy’’ eee or OxyYGEN eran or Srair- CLIMBING AT DirreRENT SPEEDS (Lupton, 1923) a certain optimum rate (fig. 9). The subject ascends the staircase once, at any required speed varying from very slow to extremely rapid. The oxygen used during the climb and in the succeeding 20 minutes, after subtraction of the resting value for the same time, gives the oxygen consumption produced by the exercise. This decreases at This existence of an optimum speed does not occur in all | types of exercise. In running (fig. 10) the oxygen require- —

ment increases continuously as the speed increases, attaining enormous values at the highest speeds; the actual no effort can drive it. Once the oxygen requirement is | larger than the oxygen intake, a steady state is no longer — Fig. 10. Oxyaun InTAKB (A) anp OxyGrn REQUIREMENT (B) or RUNNING AT Various SPEEDS Some of the most consistent physiological data available are contained, not in books on physiology, not even in books on medicine, but in the world’s records for running different horizontal distances. If one plots the average speed at which the record was made, against the length of the race (compressed in some manner—as by taking its logarithm—

to bring all the points on to the same diagram), then a curve of almost perfect smoothness is obtained, in which a few points only lie just below the curve (fig. 11). These latter are for the races in which athletes have not been so concerned to break the record as in the rest. The relation shown in the curve may be accepted practically as a natural constant for the human race; it would require almost a superhuman effort to change one of the points by 2 per cent; and it is interesting to consider what determines in

Fig. 11. Wor~tp Recorps Runnina: SPEED As A Function or DISTANCE (Hill, 1924) general the shape of this relation. At very short distances the speed is constant; it is the maximum which a man can attain, though it falls off a little as the accumulation of lactic acid in the muscles affects—first—their speed of relaxation. At very long distances the speed again tends to become constant, practically a steady state being attained. the athlete can take in oxygen to provide his muscles with their necessary energy, and by the economy with which he uses it. Speed at intermediate distances is determined,

partly by the maximum oxygen debt which the body can incur, partly by its maximum oxygen intake. We assume, of course, that we are dealing with a man in perfect condition, highly trained to carry out his movements in the most economical manner possible, ready and willing in a race to exhaust himself completely. In the magnitude of the oxygen debt, that is in the concentration of lactic acid which his muscles can tolerate, we have what we may regard as a man’s “capital.’’ In his oxygen intake, determined by the capacity of his heart and lungs, we have what we may regard as his “income.” In a race an athlete will finish—if he can, if the race be not too short—with the whole of his reserves gone, having spent both “capital” and ‘income’ completely. In a short race, therefore, he can spread his “capital” over a shorter time, he can expend energy more rapidly than he can in a long one; the form of the curve is determined mainly by these factors.

Recently a number of estimations have been made of the “oxygen debt.” These are small after moderate exercise—even when prolonged—large after violent exercise, even of comparatively short duration. The most striking* of the higher ones are perhaps those (a) in which 23 seconds of exercise led to a debt of 8.7 litres, (6) running for 33 minutes at a medium speed only of 7.9 litres, and (c) “standing running” with extreme violence for 4 minutes of 18.7 litres. The larger values mean enormous amounts of lactic acid accumulated in the body; how can this acid be tolerated? Inside the muscle are alkalies capable of. neutralising the acid, and the hydrogen-ion concentration in the active muscle does not rise very far so long as the amount of these alkalies is adequate. It would seem probable that the limit to which it is possible to press a muscle

*A still more striking one (in a man weighing abant 140 Ibs.) has recently been attained, of 7.1 litres in 13.1 secs. heard, of recent years, a great deal about the buffers which maintain a constancy in the hydrogen ion concentration of the blood. The buffers of muscle, fulfilling the same function, must be just as effective as, and their total amount far greater than, those of the blood. Training may consist, to some degree, in an increase in the amount and disposition of the available alkali.

off during the exercise and immediately after it (fig. 1). When the lactic acid is removed in recovery, the same quantity of CO, must be retained as was originally eliminated (fig. 2); sodium lactate must become sodium bicarbonate again. Consequently, we find very low respiratory quotients in the later stages of recovery. The CO, retained is a measure of the lactic acid got rid of. The oxygen used in recovery is a measure of the carbohydrate (or lactic acid) oxidised. The comparison of the two enables us to determine in man the “efficiency” of recovery. Again, we find that only a small fraction of the lactic acid has been oxidised, the value determined for the “efficiency” of recovery being almost identical with that found in the case of isolated frog’s muscle; a sufficient reward for the faith that Nature is consistent!

q during the steady state of continued exercise this lactic acid accumulates to a constant concentration characteristic _of the violence of the exercise. In an earlier lecture I gave curves which showed that the speed of the recovery process depends upon the magnitude of the breakdown from which recovery is necessary. The rate of recovery, bis an adequate supply of oxygen, is proportional roughly to the square of the concentration of lactic acid in the active muscle. The lactic acid is a sensitive “governor” of oxidation. If we regard the resting oxygen consumption of man as being due to recovery from very small amounts of exercise, slight bodily movements, movements of heart and lungs, muscular tone, then for four times the oxygen consumption, such as occurs in a steady state of ordinary walking, we should expect not four times the lactic acid concentration in the blood, but only twice; for 16 times the oxygen consumption —the limit approached during hard exercise—we should expect not 16 times, but 4 times the lactic acid concentration in the blood. Recent experiments by Long, still only preliminary, tend to confirm this expectation. It will be a very fortunate thing for the body if, when we take 16 times as much exercise, the lactic acid concentration in our blood only rises four times; it would have been very awkward had it been the other way round! The fact is an interesting example of the application to man of one of the by-products of experiments first performed on frogs. The lactic acid, however, is more interesting when we are not dealing with the steady state. After very severe exercise the lactic acid in the muscles cannot fall very rapidly owing to the inadequacy of the oxygen supply. It diffuses out, therefore, into the blood and all round the body. It reaches a maximum in the blood a few minutes after it has attained its maximum in the muscles (fig. 3). Hence the maximum in the blood is lower than in the muscles. At the particular moment, however, when it is at its maximum in the blood, we may be sure that it is the same in the muscles; equally it is certain to be the same in both towards the end of recovery, when both are becoming constant.

It is possible, therefore, by a direct analysis of lactic acid in the blood to ascertain the concentration in the muscles and other soft tissues of the body at two moments: (1) about ten minutes after exercise, and (2) towards the end of recovery. By such direct analyses, and assuming that about 50 per cent of the body weight is in intimate contact with the blood, it is possible to determine directly —if somewhat roughly—the amount of lactic acid which has disappeared in the recovery process. This may be compared, as in the case of the lactic acid calculated from the CO, retention, with the oxygen used in that removal, and it is found, as in the frog’s isolated muscle, that the acid is not oxidised; only about one-fifth of the oxygen is used which would be required were all the lactic acid broken down to CO, and water. The same ratio for the “efficiency of recovery” is attained in man as in the isolated muscle.

It has long been discussed whether the proper fuel of muscle is carbohydrate or whether any foodstuff may be employed to provide the energy for muscular movement. The most conclusive evidence available, up till recently, was that of the experiments of Krogh and Lindhard, who showed in a beautiful and consistent series of observations that the “mechanical efficiency” is appreciably higher with a high respiratory quotient and on a carbohydrate diet, than it is with a low respiratory quotient when the fuel is mainly fat. They suggested that carbohydrate is the substance actually used by muscle for its activity, and that when fat has to be oxidised to provide the necessary energy, it must first be transformed into carbohydrate, and in that process an appreciable fraction (about 10 per

has found, in the isolated muscle, a respiratory quotient of about unity during the recovery phase. It was possible, however, that the recovery process might indeed involve the oxidation of carbohydrate only in the isolated muscle, and yet, in the intact animal with its circulation and the interaction of its various organs still at work, fat might equally well be oxidised directly to provide the muscle with energy. The ordinary method of examining the respiratory quotient in a man carrying out prolonged exercise is not

adequate to decide this problem, since a respiratory quotient — of (e.g.) 0.85 might be given by the oxidation primarily — of the carbohydrate at the expense of fat. It has proved possible, however, by an extension of the methods employed in measuring the oxygen requirement of a short bout of exercise, to isolate the muscular processes from the meta- — bolic changes of the body as a whole. These experiments, made mainly by my colleague Furusawa, have shown quite clearly that the primary fuel of muscle is carbohydrate.