THERMAL PHENOMENA OF MUSCULAR CONTRACTION 765 



not an effect taking place uniformly throughout the muscle substance 

 and related accordingly to the volume or mass of the muscular 

 substance (Blix). Much evidence has been accumulated in favour 

 of this hypothesis. For example, a muscle contracting isometrically 

 (p. 747) produces more heat the greater is the initial tension (the 

 more it is stretched at the begining of the excitation) 'that is, 

 the greater its length during contraction (Heidenhain). When a 

 muscle is allowed to shorten in a tetanus, the heat production as 

 compared with that of an isometric contraction of the same dura- 

 tion, and evoked by the same strength of stimulus, is diminished 

 by as much as 40 per cent. 



Relation between the Development of Mechanical Energy and 

 Heat Production in Active Muscle. There is no simple relation 

 between the external work done in a muscle twitch and the heat 

 set free. The efficiency of the muscular machine, as estimated 

 by the proportion of the work done to the total energy degraded, 

 varies with a number of factors e.g., the load, the number of fibres 

 excited, and the intensity of the excitation of each fibre, the two 

 lattjer factors depending upon the strength of the stimulus. 



The greater the resistance, so long as the muscle can overcome 

 it so as to do its utmost amount of external work,* the larger is 

 the proportion of energy which appears as work, the smaller the 

 proportion which appears as heat. For every muscle, under given 

 conditions, there is a certain load which can be raised more advan- 

 tageously than any other; but even in the most favourable case, 

 an excised frog's muscle never does work equal to more than of 

 the heat given off. Generally the ratio is much less, and may sink 

 as low as -g. In the intact mammalian body the muscles work 

 somewhat more economically than the excised frog's muscles at 

 their best; for both experiment and calculation show (p. 687) 

 that in a normal man under the most favourable conditions as 

 much as 1 of the energy is converted into work. According to 

 Zuntz and Katzenstein, 35 per cent, of the total energy appeared 

 as muscular work in climbing a mountain, and in bicycling only 

 25 per cent. Movements which have been much practised are 

 more economically performed than unaccustomed ones, and this 

 explains the superior efficiency of the muscles concerned in climbing, 

 for no movements can possibly be more familiar than those con- 

 cerned in locomotion. So far as this indication goes, it would seem 

 that in the treatment of obesity unfamiliar, and therefore physio- 

 logically expensive, forms of exercise should be recommended, in 

 so far, of course, as they do not injuriously react upon the general 

 condition, especially upon the circulation.' 



* This statement, based on experiments with excised frog's muscles, is not, 

 of course, inconsistent with the fact mentioned on p. 687, that in the intact 

 body the fraction of the energy transformed into heat is greater in hard than 

 in moderate work. 



