84 



Suppievient to '^ Nature T July 14, 1923 



prowess depends not only upon a large oxygen supply, 

 but upon a low oxygen requirement. 



Mechanical Efficiency. — Finally, let us consider the 

 " mechanical efficiency " of muscular movement in its 

 more technical sense, of work done divided by energy 

 utilised in doing it. The mechanical efficiency of a 

 steam engine may be from 5 per cent, to 20 per cent. : 

 of a gas engine it may be higher, say up to 30 per cent. 

 In man, the mechanical efficiency of muscular move- 

 ment may be as high as 25 per cent. ; the remaining 

 75 per cent, loss of energy is a serious thing : to what 

 is it due ? It seemed, from the purely physico-chemical 

 point of view, that an efficiency of 100 per cent, was 

 conceivable : the free energy of the oxidation of food- 

 stuffs is very large. We know, however, that the body 

 has been organised so that it can go on for a while 

 without sufficient oxygen ; it is like an accumulator : 

 it can be discharged and then recharged : it can run 

 into debt for oxygen, and pay off its debt afterwards. 



If an animal like man were forced to live within 

 his " oxygen-income," and were able only to make 

 efforts which were possible on his contemporary 

 oxygen supply, he would be a very feeble creature : 

 only about ^th as energetic (for short-lived effort) as 

 he actually is. Moreover, oxidation in the body is 

 a very slow thing ; it takes minutes to complete, and 

 it would be a disadvantage to take three minutes over 

 every muscular movement. Hence the mechanism of 

 the muscle has been evolved and differentiated on a 

 different plan : oxidation is not the chemical reaction 

 which directly and immediately provides the mechanical 

 energy of the muscle : the actual process which produces 

 the mechanical energy appears to be some kind of 

 explosive transformation of a glucose di-phosphoric 

 ester into lactic acid, and the subsequent physical or 

 physico-chemical reaction of this lactic acid with the 

 protein structures of the muscle. In recovery the 

 lactic acid is restored, about |^th of it, to the precursor 

 from which it came, the remaining ^th (or its equivalent 

 amount of glycogen) being oxidised to provide the 

 energy for the reversal. Mechanical energy is liberated 

 only in the first stage, which appears to have a very 

 high "efficiency " — probably about 100 per cent. In the 

 recovery stage, however, 150 units of heat are liberated 

 by oxidation for every 100 units in the initial stage, 

 and this reduces the efficiency of the whole cycle to 

 about 100/250, i.e. to about 40 per cent. Apparently, 

 therefore, a big reduction in efficiency is effected simply 

 by taking proper account of the recovery process, and 

 is due to the need the animal often experiences of 

 taking violent exercise, so to speak, " on credit." 



Even so, however, 40 per cent is far higher than 

 the efficiency actually found in man : the remaining 



reduction of efficiency is due to two other factors : (a) to 

 the rapidity of the usual type of muscular movement, 

 and to consequent frictional loss inside the muscle ; 

 and (ft) to the physiological effort associated with 

 maintaining a contraction. 



With regard to (a), muscle is made up of a viscous 

 material, not unlike egg-white or treacle, with a fine 

 network of membranes, fibres, and tubes throughout 

 it : the joints, the tendons, the connective tissue, the 

 blood-vessels and the blood within them, are similarh 

 of a viscous nature. Now, when a viscous fluid i 

 forced to flow, mechanical energy is wasted and turned 

 into heat : the faster it is made to flow, the more energy 

 is degraded. But when a muscle changes its form, and 

 produces a movement in a limb, the tissues have all to 

 fall into a new form, viscous fluid has to flow into a 

 new disposition, energy is degraded into heat : and in 

 the more rapid movement we should expect more 

 energy to be wasted. Experiment amply confirms this 

 expectation : the frictional loss is greater, the greater 

 be the speed of movement. This explains why it is so 

 laborious to pedal a bicycle on too low a gear, and why 

 very rapid running requires such an enormous amount 

 of energy. In both cases the external resistance may 

 be small or negligible. The internal resistance, however, 

 is large, and increases directly as the speed of movement, 

 until finally a limit is reached at which no further 

 increase in speed is possible ; every muscle fibre is then 

 working to its physiological limit of speed and power, 

 merely in overcoming its own internal resistance. 



With regard to (ft), just as it is inefficient and tiring 

 to move our limbs too rapidly, or on too low a gear, so 

 also it is inefficient and tiring to move them too slowly, 

 or on too high a gear. This simple observation gives 

 us the clue to the third and final reason why the 

 efficiency of muscular contraction is relatively so low ; 

 a contraction which continues too long requires energy 

 to maintain it, as well as energy to set it up, and from 

 the point of view of doing external work the maintenance 

 of contraction is ineffective. Experiments were made 

 in which the heat produced by a muscle was determined 

 as a function of the duration of the stimulus exciting 

 its contraction. After an initial outburst of energy 

 associated with setting up the contraction, the heat- 

 production increases uniformly as the duration of the 

 stimulus is increased. Hence we see why slow and 

 prolonged movements are inefficient : a large and 

 unnecessary part of the energy is used in maintaining 

 the contraction. This is the phenomenon we all know 

 in our own bodies : to attempt to lift a thing which is 

 too heavy for us to move is more tiring than actually 

 to lift a thing we can move, even though no work at all 

 — in the mechanical sense — be done in the former case. 



