THE MECHANICAL RESPONSE. 



357 



700 800 900 1000 HOD 1200 



the principle is formulated by Fick, " If a muscle is completely 

 tetanised, and thereby overcomes an external force which at each 

 moment nearly balances its tension, the mechanical effect, i.e. the 

 negative work overcome, is exactly equal to the positive work as 

 calculated from the extension curve." x 



The utmost amount of external work which can theoretically be 

 done by a tetanised muscle during the period of effort is, in accordance 

 with the principle which has just been enunciated, obtained by multiply- 

 ing the load lifted by half the lift. It may be represented by a 

 triangular area (Fig. 191), hereafter to be referred to as the work-diagram. 

 If a c be the equilibrium-length of the muscle when tetanised, a I its 

 equilibrium-length when relaxed, and bg represent the amount of the 

 load required to stretch the excited 

 muscle from the length a c to the length 

 ab, it is obvious that the triangle c~bg 

 represents the product in question, i.e., 

 on the one hand the smallest amount 

 of negative work required to extend 

 the tetanised muscle to its unexcited 

 length, and, on the other, the maximum 

 amount of positive external work which 

 it is capable of accomplishing under the 

 most favourable circumstances. 



The normal length of a muscle in 

 the body, when said to be at rest, is not 

 that of equilibrium, but of extension, i.e., 

 its length at the moment of beginning 

 any effort exceeds that which it would 

 possess if it were perfectly free to 

 assume its natural form. This fact is 

 advantageous to the organism in a way 

 that the diagram helps to explain. In 

 constructing it, it was assumed that, in 

 order to extend the excited muscle to 

 the length ab, a weight of 700 grms. 

 was required. If, before excitation, an 

 extending force of say 250 grms. had 

 been applied to it, so that it would 

 have assumed the length ad, it would 

 require, as the diagram shows, in 

 order to stretch it to the same length in the excited state, the weight 

 of 1200 grms. It would then be able, under perfectly favourable 

 conditions, to do, when released from extension, an amount of work 

 represented by the much larger area cdf. As the diagram also 

 shows, by far the greater part of this work would be accomplished 

 by the proper energy of the muscle at the expense of material 

 contained in itself. But the remainder, indicated by the vertically 

 shaded area b d e, would be assignable to the negative work which must 

 have been done on the muscle before excitation, in order to bring it to 

 the length a d. The curved line b e is, according to Fick, the " extension 

 curve " of unexcited muscle, a curve of which the ordinates represent the 

 successive lengths of such a muscle when extended by a load increasing 



1 Fick, op. cit., S. 61. 



FIG. 191. Work-diagram, a b, length 

 of unexcited muscle ; and b e, its 

 extension curve, ac, length of 

 muscle when excited ; and c/, its 

 extension curve, cdf, the whole 

 work capable of being done by an 

 excited muscle in shortening from 

 a length a d, to its proper length 

 ac. The vertically shaded area 

 represents the work done on the 

 muscle in bringing it to the length 

 ad before excitation ; the hori- 

 zontally shaded area the "useful 

 effect." After Fick. 



