Supplement to '' Nature,'' July 14, 1923 



79 



however, by a delicate electrical device that 40 to 50 

 obvious vibrations per second occur in them, and that 

 they are really reacting discontinuously to a rapid 

 stream of stimuli : even the shortest voluntary con- 

 traction of which the human muscles are capable is 

 due to a volley of impulses shot at it, along the nerves, 

 by the brain. Each separate unit of effort, however, 

 which goes to make up the complete contraction is ex- 

 pensive^each requires energy just as each stroke with a 

 pump requires energy. It is obvious, therefore, why the 

 maintenance of contraction is expensive and fatiguing. 

 Fatigue. — Nearly all the recent and important 

 advances in muscle physiology have resulted from a 

 study of the phenomena of fatigue. We all know that 

 there is a limit to muscular exertion, a limit which is 

 set by what we call fatigue. If an able-bodied man 

 take exercise at a very small rate, e.g. by walking, he 

 remains comparatively untired for long periods : if he 

 takes exercise more violently he becomes tired more 

 quickly : if he exerts himself with the extreme effort 

 of which he is capable, he is completely exhausted in 

 less than a minute. There are many different kinds 

 of fatigue, but the one we are discussing now, from the 

 study of which so much light has been shed on the 

 nature of muscles, is the extreme athletic fatigue which 

 results rapidly from very violent effort. By it the 

 finest athlete in the world may be overcome within a 

 minute. It is a simple and comparatively intelligible 

 thing. We can reproduce it readily in isolated muscle, 

 deprived of its circulation. Let us subject an isolated 

 frog's muscle, every second or two, to an electric shock, 

 and record its contraction • we find that the response 

 changes in a regular and progressive way, the force 

 exerted becoming less, the contraction being developed 

 rather more slowly and continuing much longer, 

 relaxation being much drawn out. Finally, the muscle 

 becomes inexcitable. Now in the intact animal, in 

 man, we know that even extreme fatigue is rapidly 

 recovered from, and this recovery is attributed to the 

 circulation. If the circulation be hindered by a 

 cramped position recovery is slower. If the fatigued 

 isolated muscle be left in a chamber free of oxygen, no 

 sign of recovery occurs : if, however, it be left in 

 oxygen, in a few hours complete recovery will take 

 place, and the muscle will now be capable of repeating 

 its previous effort. 



The realisation, especially by Fletcher about twenty- 

 five years ago, of the extreme importance of this 

 observation led directly to the most striking advances 

 in our knowledge of the working of muscle. Recovery 

 , from fatigue is possible only in the presence of oxygen, 

 \ and it was natural to suppose that the oxygen was 

 / used to oxidise some waste product, the presence of 

 which acted unfavourably on the muscle. The next 



great step was due again in part to Fletcher, this time 

 in co-operation with Hopkins. Lactic acid was known 

 to occur in muscle, and Fletcher and Hopkins found the 

 lactic acid to be increased by exercise, and diminished 

 or abolished by recovery in the presence of oxygen. 

 Furthermore, there appeared to be a certain definite 

 maximum, beyond which the lactic acid content of 

 the muscle could not be driven, even by the most 

 vigorous stimulation : clearly this corresponded to the 

 maximum effort a muscle could make. What was the 

 function of this lactic acid, was it indeed to be the 

 keystone of the bridge which physiologists were 

 building from physics and chemistry on one hand to 

 muscular activity on the other ? 



Heat-production. — Muscles, in activity, give out 

 heat. External mechanical work is produced by the 

 muscle with an efficiency of only about 25 per cent. 

 Hence for every 25 ft. lb. of energy turned into external 

 mechanical work at least 75 ft. lb. are degraded into 

 heat inside the body. In a maintained contraction, 

 in which no actual work is done, all the energy used 

 is turned into heat : while in such movements as 

 running, the energy is indeed turned in part into 

 kinetic energy, which, however, is chiefly reabsorbed 

 into the body as heat, owing to the jolts and jerks 

 and rapid movements of the limbs, just as the energy 

 of a motor car on a bumpy road is absorbed largely 

 as heat in the tyres. In a single muscle twitch the 

 rise of temperature is only about 0-003° C., and if one 

 wishes to measure to i per cent. — and for some purposes 

 one must measure to o*i per cent.— it is necessary to 

 read to the nearest 0-00003° C. This can, however, be 

 done, and with the wonderful electrical measuring 

 instruments now available it has become comparatively 

 easy. It is worth doing, because the heat accompanies, 

 and is a measure of, the chemical processes occurring 

 in muscular activity, and its production can be followed 

 continuously, and so made to give us the time course 

 of those chemical processes. 



If the electrical record of the thermal response of 

 the muscle to stimulation be carefully analysed, it is 

 found that the heat-production is by no means simple 

 in its time relations. In the first place, if the muscle 

 be in oxygen, there is an evolution of heat lasting for 

 many minutes after the contraction is over : and this 

 evolution of heat is not small, but actually larger in 

 total extent than the heat which occurs early in 

 the contraction. In the absence of oxygen this 

 delayed heat almost disappears. Clearly it is some- 

 how connected with the recovery process Fletcher had 

 noticed in an exhausted muscle, which we all know 

 in our own bodies ; it is accompanied, as Fletcher 

 and Hopkins had shown, by a disappearance of lactic 

 acid. The recovery heat - production occurs more 



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