Popular Science Monthly/Volume 24/January 1884/The Source of Muscular Energy
|THE SOURCE OF MUSCULAR ENERGY.|
NEW and valuable scientific discoveries and inventions are not slow at the present time in making their way from the closets and laboratories of the investigators or discoverers to popular recognition. It is somewhat otherwise with the gradual development of knowledge on subjects once thought to have been tolerably clearly understood and of no immediate practical value. The gradual modifications which take place in generally accepted theories by the slowly accumulating results of the labor of many investigators are, to be sure, appreciated by the special student in the particular department of knowledge concerned, but are slower in meeting with public recognition. It thus happens that teachers and books, not dealing as a specialty with the subject involved, often adopt and repeat as authoritative views and theories which, by the specialists in those branches, have either been abandoned or brought seriously into question. Nor is it to be otherwise expected. Chroniclers are quick to seize upon and distribute the news of brilliant or startling discoveries or inventions, but those are fewer who will track patiently the slowly accumulating evidence of many workers, appreciate the bearing of their work, and produce it in a form in which it can be appreciated by those non-specialists most interested in the subject involved.
It is thus, to a certain extent, with the subject of the source of muscular power in the animal organism. It is needless to specify in this particular. Text-books and popular articles touching on the subject are continually asserting, as apparently unquestioned, theories which at the present time are either exploded or very much in doubt. It would seem, therefore, not without value to attempt, as far as practicable in a popular or semi-popular article, a general statement of the present condition of the theories on the source of muscular power, and of the main points of the evidence which tends to support these theories.
The general acceptance of the law of the conservation and correlation of physical forces had at once an important influence in directing attention to the source of muscular force. The idea was readily taken up that this form of force is at the expense of heat, which is produced by the oxidation of carbon and hydrogen in the body, the necessary oxygen being conveyed by the arterial blood to the muscular tissue. In other words, the somewhat trite comparison of the human body and the muscular system to an engine, which consumes just so much fuel to produce so much force, has pretty clearly formulated the idea as generally accepted. And so far as it goes the comparison is not bad.
When, however, we pass beyond this somewhat vague simile to an examination of the more intimate nature of these various processes, we find the questions raised are not so generally understood. Accepting that the muscular force is produced by the ultimate oxidation of carbon and hydrogen to carbonic-acid gas and water respectively, the next questions that suggest themselves are: "What is the immediate source of this carbon and hydrogen—the fuel material for muscular force?" and "What is the real nature of these processes which we call briefly oxidation?" The endeavors to answer these questions have given rise to many discussions and disputes, which are, even at the present day, by no means concluded.
Before taking up the discussion of the theories advanced to answer these questions, it will not be out of place to review very briefly the composition of the muscles and their general relations to the circulation—only in so far, however, as is necessary for a clear comprehension of the evidence and arguments involved in the discussion.
A muscle is essentially a collection of lengthened cells held together by a connective tissue. Each cell consists of a delicate cell-wall or membrane containing a fluid or semi-fluid mass of living (protoplasmic) matter. This gelatinous substance possesses the power of contraction under the stimulus of excitations of various kinds—nervous impulse, electricity, heat—and the cell becomes thereby shortened. This process, taking place simultaneously in all the cells of a given muscle under the influence of the same exciting cause, is what exerts the power of the contracting muscle. The intensity of this shortening or contracting power has been approximately measured—e. g., by ascertaining experimentally the weight necessary to prevent a muscle from contracting under excitation. The muscles are supplied with blood by the fine ramifications of the arteries, and the blood is conducted away again by the ramifications of the veins, the arterial blood losing oxygen and taking up carbonic acid during its passage, as is the case in the other tissues also.
Regarding the composition of the muscular tissue, it may be simply noted that the tissue itself is composed mainly of albuminoid material (cell-contents) and of the substance of the connective tissue, which is, like the albuminoids, composed mainly of carbon, hydrogen, oxygen, and nitrogen, and in much the same proportions. Besides this, the blood and lymph permeate the muscular tissue throughout, and certain non-nitrogenous substances, mainly glycogen, a substance resembling starch or dextrine in composition and properties, are stored up in the muscular tissue, and always found to be present. Certain other simple compounds containing nitrogen are also present, and are considered to be decomposition products of the more complex albuminoids. When the muscular contraction takes place, mechanical force may be exerted which is produced at the expense of the force stored up as potential chemical energy in the materials which serve as the fuel material. This potential energy is set free or rendered active by the chemical processes which there take place, and appears as work, as sensible heat, or as electrical disturbances.
Before we inquire as to the nature of these chemical processes, it will be of advantage to glance briefly at the results of important investigations which have been made on this subject, as these form the only safe data by which we may judge of the tenability of any theory. It would be out of place here to attempt a full reference to the mass of investigations and experiments which have been published, and which bear on the topic under discussion. We shall therefore simply notice the principal facts which have been established as the results of those investigations, and which are most pertinent to the matter in hand.
The experimental researches on this subject may be classified under four heads: 1. The examination of the muscular tissue itself before and after muscular action. 2. The examination and comparison of the blood coming to the muscle, and that leaving it, during rest and exertion. 3. The examination of the gases given off or absorbed by the active muscle after excision from the animal, and under the influence of artificial irritation. 4. The influence of continuous muscular exertion on the respired gases and on the waste products of excretion.
1. With regard to the changes in the muscular tissue, it has been noticed that the proportion of water in the muscles is increased or the proportion of solids diminished by work, the amount of substances soluble in water is diminished and the amount soluble in alcohol increased; and particularly that glycogen disappears and sugar is increased (the latter probably as a product of fermentation at the expense of the glycogen).
2. Changes produced in the blood are for the most part difficult to trace with certainty; but, it has been observed that the blood coming from the active muscle contains more carbonic acid and less oxygen than that coming from the resting muscle; and, further, that the carbonic acid is increased in greater proportion than the oxygen is diminished. We shall recur to this later.
3. Investigations into the changes which occur in gaseous atmospheres surrounding an excised muscle made to contract under the influence of electricity are interesting and instructive. G. Liebig found that the excised muscle gave off carbonic acid and took up oxygen, but that muscular contraction took place also when the surrounding atmosphere contained no oxygen, carbonic acid being given off, however, in this latter case also. Later observers confirmed these observations, and Matteucci considered, from his experiments in the same direction, that the carbonic acid was not produced at the expense of the oxygen of the surrounding atmosphere, but from oxygen held in some form of combination in the muscular tissue itself. Herrmann found that a portion even of the oxygen absorbed from the air was absorbed in consequence of incipient putrefactions.
4. Investigations under the fourth head, as to the effect of muscular exertion on the general relations of respiration and excretion, have been very elaborate and very numerous. Pettenkofer and Voit, Ludwig and Sczelkow, and others, have investigated the relations of carbonic acid and water given off to food and oxygen consumed as influenced by muscular exertion. Their investigations have shown that the oxygen consumed and carbonic acid and water given off are largely increased by muscular exertion. This had been noticed as a general fact by Lavoisier a half-century or so earlier, but the experiments of the above-named investigators were carried on with a care and thoroughness which left little to be wished for in that direction.
Whether the subject of the experiment be kept on a constant diet during both work and repose, or whether it be allowed to eat and drink according to desire, or even if no food be permitted during the experiment, the general fact remains the same, that the quantities of carbonic acid and water eliminated during work are much greater than during rest, in many cases the ratio being as high as two to one. It is also found that the oxygen taken up, though increased during muscular exercise, is not increased in proportion to the carbonic acid eliminated. The result is, that the ratio of the volume of oxygen consumed to the volume of carbonic acid eliminated, which is normally somewhat less than unity, tends to approach unity during muscular work. It should be here remarked that investigations dealing with total respired gases, although doubtless in the main reliable, are not without certain defects. If we could be certain that muscular exercise left all other organic functions unaffected, we could safely attribute the observed changes to the muscular contraction alone. But such is probably not the case. The functions of organs are influenced by the activity of others, and hence the changes noticed in products of elimination or in the consumption of oxygen can not with safety be attributed solely to the muscular work performed, as these substances are consumed or produced by the combined activity of all the living tissues of the organism. Hence the value of the corroborative testimony of the other methods of investigation noticed above.
The influence of muscular exertion on the elimination of nitrogen has also received much attention, inasmuch as the nitrogen eliminated (mainly in the form of urea by the kidneys) may be taken as a measure of the amount of nitrogenous food or tissue decomposed in the organism. The influence, then, of muscular exertion on the excretion of nitrogen is of importance as showing also its influence on the decomposition of albuminoids (foods or tissues). The results of the numerous investigations on this subject have been somewhat at variance. Many have found no material increase in the elimination of nitrogen during muscular exertion; others find a slight increase, but not sufficient to indicate any immediate relation of the nitrogen eliminated to the work performed. Passing over the work of earlier investigators, we will consider briefly the results of some of the later investigators. Voit was one of the first to make careful and exact experiments extending over a considerable period of time, and he determined that the increase in elimination of nitrogen during muscular exertion is very slight; that it bears no constant relation to the work done, and is more influenced by diet than by work. Fick and Wislicenus made an ascent of the Faulhorn in the Alps, with the purpose of determining the possibility or impossibility of albuminoids being the fuel-material for muscular power. They estimated the mechanical work necessary to raise their own bodies through the vertical distance to which they ascended. They then calculated the amount of albuminoids necessary to produce so much force by its combustion. They determined experimentally the amount of nitrogen in their excreta during the period of the ascent, and, having taken no nitrogenous food during that period, they were enabled to estimate what relation the albuminoid decomposition bore to the amount necessary to supply the power for the ascent. By this method they demonstrated that the whole amount of albuminoid material decomposed during the ascent, even if completely oxidized to carbonic acid, water, and nitrogen (instead of yielding its nitrogen in the form of urea, as is actually the case), would produce less than half the force necessary to raise their bodies through the vertical height to which they ascended. Thus it is shown that the amount of force represented by the actual decomposition of albuminoids during work is by no means adequate to account for the work done, even supposing that all the nitrogenous material decomposed in the body went for that purpose, and that no other muscular work were performed during the ascent than the mere lifting of such a weight to the given height. Both these suppositions are evidently incorrect, as the nitrogen is eliminated in almost equal quantities when no voluntary muscular action is exerted, and the muscular work, voluntary and involuntary (lungs, heart, etc.), on such a trip, would evidently far exceed that necessary for the simple elevation of a dead weight to a specified height.
Experiments conducted by Dr. Parkes on two soldiers proved that a small increase of nitrogen elimination was produced, and also, that this increased elimination of nitrogen may extend for many days after the exercise has ceased.
Dr. Austin Flint, Jr., in an elaborate and thorough investigation on the pedestrian Weston, found a decided increase in the nitrogen eliminated during work; also, a decided increase in the ratio of nitrogen eliminated to that taken in with the food. The value of his results is somewhat impaired for our present purpose, in so far as they relate to the influence of muscular exertion simply, because the condition of the subject during the working period was not such as was favorable for a fair test. His appetite fell off; he slept poorly; was extremely nervous and irritable much of the time; became at times much exhausted and prostrated even to nausea. When the influence of the nervous state and of an exhausted condition on the functions is taken into account, it will be evident that deductions as to the effect of muscular exertion alone would in this instance be open to doubt. Dr. Pavy's experiments on the same pedestrian indicated also an increase in the nitrogen elimination, but only a slight increase as compared with Dr. Flint's results.What, then, seems tolerably certain is, that muscular exertion increases the nitrogen elimination but slightly, and perhaps only very slightly, so long as the muscular system is moderately exercised and not overtaxed. And, indeed, the pertinent question here would seem to be, "Is the normal muscular action accompanied with any elimination of nitrogen showing a decided relation of the work done to the nitrogen eliminated?" and not "Is the excessive and exhaustive exertion of the muscles accompanied with any increase of nitrogen elimination?"
Having thus glanced at some of the more important experimental results bearing on this subject, let us return to the consideration of the two questions previously enunciated. First, then, "What is the fuel-material for muscular force? is it albuminoid and nitrogenous, or is it non-nitrogenous?" That it is not essentially nitrogenous will appear from the experiments last described, for if such were the case we should find nitrogen eliminated in much greater quantities during muscular work than during rest, which is not the case. The material which supplies the force by its decomposition must, then, be mainly non-nitrogenous. Here, again, are various possibilities. Fats, sugars, glycogen, are all non-nitrogenous, and we have next to inquire whether the fuel-material be fats, sugars, or glycogen. The facts above stated of the constant occurrence of glycogen in the muscular tissues, and its disappearance in part during muscular exercise, suggest at once the possibility of this substance being a fuel-material. We shall obtain light on this question from the facts regarding the influence of muscular exertion on the ratio of the volume of carbonic acid expired to that of the oxygen taken up. The three principal classes of foods consumed in the animal body are the fats, carbohydrates (starch, sugars, glycogen, etc.), and nitrogenous substances. For the present purpose it may be considered that the fats and carbohydrates are ultimately converted into carbonic acid and water, and that the nitrogenous substances are ultimately converted into carbonic acid, water, and urea. The nitrogenous foods are usually subdivided into albuminoids proper, and substances not albuminoids. All these nitrogenous substances are composed mainly of carbon, hydrogen, oxygen, and nitrogen, and usually also sulphur, in proportions which vary with different substances, but within very narrow limits. For the sake of simplicity, therefore, it will be permissible to take a certain average composition to represent the entire class, and the deductions will apply with sufficient accuracy to the nitrogenous foods as a body. For the sake of easy comparison we may also represent this average composition by a formula which may be considered as representative of the class; e. g., C143H326N38O46S. If we now consider this to be oxidized to carbonic acid, water, and urea (and the sulphur to be oxidized to SO3, as would be the case in the formation of a sulphate), we might represent the process by the following equation:
|C143H326N38O46S 124CO2 75H2O 19CON3H4 SO3299O|
This would give 248 volumes CO2 produced for 299 volumes of oxygen taken up, or a ratio of 248 0·83.
If we consider the fats, and take stearine as a fair example of this class, we should have for such an equation—
|C57H110O6 163O 57CO2 55H2O.|
or the ratio of volumes of carbonic acid and oxygen would be 114 =0·70. Other natural fats would give results differing little from this ratio.
The carbohydrates, on the other hand, contain relatively more oxygen than the other classes of foods, and contain hydrogen and oxygen in just such proportions as exist in water. Hence by their oxidation just enough oxygen must be consumed to convert the carbon to carbonic-acid gas, e. g.:
C6H10O6 12O 6CO2 5H2O.
C6H12O6 12O 6CO2 5H2O.
The ratio is hence 1 for all this class, since the carbonic acid formed is equal to the volume of the additional oxygen consumed. It follows, then, that the oxidation in the organism of carbohydrates would tend to cause the ratio CO2 to approach unity. The extensive investigations of Regnault and Reiset on small animals have shown that with carbohydrate food the ratio does approach unity, sometimes almost attaining it, though of course it is impossible to eliminate entirely the decomposition of fats and albuminoids in the organism, and hence the ratio is kept below that figure.
So, also, as we have seen above, the tendency of muscular exertion is to increase this ratio and cause it to approach unity. The evidence, then, seems to point with tolerable conclusiveness to the fact that the immediate fuel-material is mainly non-nitrogenous and carbohydrate in its character. To what extent this supply of carbohydrates is derived from the glycogen of the muscles, to what extent from sugars absorbed from digestion, or produced from the glycogen of the liver, is not yet established with sufficient accuracy, though the participation of the muscle-glycogen is hard to doubt.
We have said the immediate fuel-material is apparently carbohydrates, for the possibility still remains that this carbohydrate material may itself be in part derived from albuminoids. It is certain that the liver-glycogen is in great part, possibly entirely, derived from albuminoids. Parke's experiments, above mentioned, showing a continuous elimination of increased quantities of nitrogen in the form of urea for days after continued muscular exertion, would be in harmony with such an origin, as they might indicate a gradual replacement of glycogen consumed, at the expense of albuminoid material with elimination of urea as a a waste product. Sugars (grape-sugar and maltose) absorbed from digestion or formed from liver-glycogen, are doubtless consumed in the tissues and organs and assist in producing animal heat. Whether muscular tissue consumes these sugars in greater quantity than other tissues it is difficult to say with certainty.
We come now to the second question as to the nature of this decomposition to which we have alluded as oxidation. This question is still contested. The older theory is that the oxygen, taken up by the blood, is given up in the form of active oxygen, or ozone, and by its energetic oxidizing power burns up or oxidizes the carbon and hydrogen of the fuel-material, with formation of carbonic acid and water.
The newer theory is that the decomposition processes are essentially fermentative in their character; that under the influence of appropriate ferments the substances combine with water, splitting up into simpler and simpler products with evolution of heat or force, as is the case with all fermentative changes. The oxygen present in the arterial blood gives these processes the character of fermentative changes in the presence of oxygen; secondary oxidation takes place, as in putrefaction in presence of air, the final products being mainly carbonic acid and water, as also is the case in putrefactive processes.
Some of the objections raised to the older theory are that we know of no similar changes produced by ozone in watery solutions, such as exist in the animal organism; that the oxygen obtained from the arterial blood under the air-pump contains no ozone. Also certain compounds are found in the blood and tissues which are essentially deoxidized products, which could not be supposed to exist in the presence of ozone, but the presence of which accords with the supposed fermentative character of the processes (Hoppe-Seyler). The fact that the evolution of carbonic acid from the contracting muscle is in great part independent of the presence of oxygen at the time would harmonize also with such a fermentative character of the changes, as carbonic acid is the product of many fermentative changes out of the presence of oxygen, as, for example, of the alcoholic fermentation of sugar. Matteucci's supposed storing up of oxygen in some form of combination in the tissues would then be interpreted rather as the storing up of fermentable substances (like glycogen) rich in oxygen. The combustion theory, on the other hand, would seem to demand that the evolution of carbonic acid and consumption of oxygen should be simultaneous, which is apparently contradicted by the experiments of G. Liebig, Matteucci, and others above mentioned. It would exceed our limits to enter more fully into a discussion of these two opposing theories. The conflict between them is still in progress, and new evidence is constantly accumulating. Both theories agree in this, that the material which by its decomposition produces the force for muscular work is finally decomposed, with evolution mainly of carbonic acid and water. They differ in their views of the nature of the process and the steps by which these ultimate products are obtained.
We have here endeavored to show briefly what has been gained in comparatively recent times by the growth of knowledge in regard to the source of muscular power. Let us attempt a brief summary of the main points brought forward in the preceding discussion: 1. The source of muscular energy is in the chemical decomposition of certain substances, which is accompanied with a release of energy. 2. The muscular contraction produces a greatly increased production of carbonic acid and water, and an increased consumption of oxygen, in the general respiration. To what extent this is due to the mere muscular contraction, to what extent to the influence of muscular exercise on other functions, is difficult to estimate with certainty. 3. The excised muscle, when caused to contract, gives off carbonic acid, and this action is in great part independent of a simultaneous absorption of oxygen. 4. The blood coming from the contracting muscle contains more carbonic acid and less oxygen than that coming from the resting muscle, and less oxygen than that coming to the contracting muscle. 5. The ratio of carbonic acid given off to oxygen taken up is increased by muscular exertion. 6. The nitrogen elimination is but slightly increased during muscular exertion. No considerable amount of nitrogenous muscular tissue is consumed. 7. The immediate fuel-material is mainly non-nitrogenous and carbohydrate in its character, probably in part at least derived from the muscle-glycogen, and perhaps from some other substances stored in some manner in the muscular tissue, possibly also to some extent from sugars conveyed to the tissues by the blood. 8. It is not certain to what extent this glycogen or other non-nitrogenous fuel-material is derived from nitrogenous or albuminoid material during rest or repose of the muscles, but such an origin, for a portion at least of the fuel-material, has some evidence in its favor. 9. The nature of the decomposition of this fuel-material is as yet an unsettled matter. The older theory of direct oxidation has been to a great extent replaced by the more modern theory of fermentative decomposition, i. e., splitting up by combination with water into simpler products with an accompanying release of energy, and this process followed by secondary oxidations exerted by the oxygen of the blood. Satisfactory experimental evidence for deciding with respect to these theories as yet fails us.
In conclusion, it is well, however, to recollect that at best the questions touched upon are but secondary to the more fundamental question upon which no investigation has as yet thrown even the most dim and feeble light, viz., "What is muscular force?" It seems impossible to conceive how a collection of cells with thin, elastic walls, and filled with a fluid or semi-fluid mass, can contract in such a way as to manifest the power familiar to us as muscular force. We are here brought face to face with the same difficulties that meet us whenever we attempt to explore the mysterious physics and chemistry of living matter. The attempts which have been made to account for the peculiar selective power of the living cells of the rootlets of plants, to explain the selective action of the gland-cells of the kidneys which act partly according to laws of transudation and diffusion, and partly in opposition to those laws, have given us no satisfaction on those points. And it is the same with regard to the essential functions of other living tissues—all are carried on under the influence of the peculiar and uncomprehended properties of living matter.
We have gained, and are constantly gaining, valuable knowledge as to very many of the processes taking place in the living body, but as to the processes which take place in the truly living cells of gland, muscle, brain, or nerve, we are in almost complete darkness. At the doors of these most refined and mysterious of Nature's laboratories, we must lay down our rude tools and methods, and confess to ourselves that "thus far and no farther" may we hope to press our eager search for truth.
- This value has been found in man at about 6,000 to 8,000 grammes per square centimetre of cross-section of muscle (85 to 114 pounds per square inch) for the maximum for voluntary contraction. It is of course evident that the intensity of the force exerted varies with the kind and degree of excitation, so that too much dependence must not be placed on any particular values thus obtained. They simply give an approximate value for ordinary muscular activity.
- Quite full references may be found in the excellent and quite recent text-books of F. Hoppe-Seyler, "Physiologische Chemie," and of A. Gamgee, "Physiological Chemistry of the Animal Body."
- It will, I think, be evident that the widely entertained theory of Herrmann, regarding the chemical processes taking place during muscular action, is not contradicted by the considerations here advanced. According to this theory, a complex nitrogenous substance of the muscular tissue is decomposed during muscular activity with evolution of carbonic acid, and other non-nitrogenous residues, together with a simpler nitrogenous substance which is supposed again to unite with other (non-nitrogenous) matter to form the original compound, which may be again decomposed during contraction. This still leaves the non-nitrogenous matter the fuel-material, but assumes it to be stored up in the form of a combination with a complex nitrogenous substance which then yields it again in the form of carbonic acid and water. This theory lies too far in the field of speculation for its discussion to come within the scope of the present article.