344 MUSCLE IN ACTION.- [BOOK I. 



shortening of the muscle \ The force varies with the stimulus : as 

 this gradually increases, that enlarges, quickly at first but after- 

 wards more slowly, until a maximum is gained, which is known as 

 the ' absolute force '. The absolute force is usually stated to be 2800 

 3000 grms. per sq. centimetre of tetanized frog-muscle ; and between 

 6000 and 8000 grms. per sq. centimetre in the muscles of man 

 voluntarily contracted 2 . 



These numbers cannot be taken as the direct or exact equivalent 

 of that portion of the chemical changes which is devoted to me- 

 chanical effect. For the result so obtained is less than the true 

 absolute force of the muscle experimented on by an amount which 

 depends on its extensibility. If muscle were more elastic than it is, 

 although the process of contraction with all its chemical changes re- 

 mained the same, the absolute force would seem to be less. This may 

 readily be demonstrated by interposing an elastic band between the 

 muscle and the weight about to be raised. The absolute force of such 

 a system is less than that of the muscle alone; whence we may conclude 

 that the proper extensibility of muscle has a like diminishing effect 3 . 



If a loaded muscle be made to contract by the application of 

 a stimulus, the height through which the load is raised is called the 

 lift; and this multiplied into the load gives the value of the mechani- 

 cal work actually done. As the stimulus is increased, the lift grows 

 proportionally to the stimulus up to a maximum, beyond which 

 it remains constant 4 . Inasmuch as muscle is extensible, and 

 its extensibility is increased during the state of contraction, it is 

 clear that the lift is the expression of the actual shortening of 

 the muscle minus the difference between the extension of the 

 uncontracted and the extension of the contracted muscle. For the 

 full illustration of this the reader is referred to Ed. Weber's article 

 'Muskelbewegung' in Wagner's Handwarterbuch, and to the Text- 

 books of Physiology. 



The lift varies also with the load, becoming smaller as the load 

 increases; and the variation is such that the product of lift into load 

 first of all increases and afterwards decreases as the load varies from 

 nothing onwards. In other words, within certain limits, the more a 

 muscle is weighted the more mechanical work will a given stimulus 

 produce. The increased tension to which the muscle is for the time 

 subjected converts it into a body capable of yielding a larger amount 

 of mechanical work than the same muscle less tense. Not only does 

 the state of tension in the very act of contraction influence the work 

 done, but the state of tension immediately prior to contraction has 

 the same effect : the greater the tension the greater within certain 

 limits 6 the yield of mechanical energy. 



1 E. Weber, Wagner's Handivorterbuch, in. 2, p. 84. Helmholtz, Arch. f. Anat. 

 Physiol. u. wiss. Med. (Miiller), 1850, p. 276 ; 1852, p. 199. 



2 See Hermann's Handbuch, Bd. i. Abth. i. p. 64. 



3 See Hermann's Handbuch, Bd. i. Abth. i. p. 65. 



4 Fick. See Hermann's Handbuch, Bd. i. Abth. i. p. 108. 



5 Heidenhain, Mechanische Leistung, Wamieentwicklung und Sto/umsatz lei 

 Muxkelthatigkeit. Leipzig, 1864, p. 84. 



