570 MUSCULAR WORK. 



cause they are distinguished from all machines of human construction by 

 the fact that, as a result of repeated exercise, they become stronger, 

 better developed, and capable of increased activity. 



According to the usual method of estimation, the amount of work 

 performed by a muscle is equal to the product of the weight lifted (P) 

 and the height to which it is lifted (s); hence, A = s P. From this it 

 follows, first, that if the muscle is not at all weighted, therefore, if P equals 

 o, then A must equal o ; that is, no work is performed if there is no weight- 

 ing. Further, if the muscle is burdened with an excessively heavy 

 weight so that it is no longer able to contract, therefore, s equals o, then, 

 likewise, no work is performed. Between these two extremes the active 

 muscle is able to execute work. 



Strictly speaking, the contracting muscle lifts, in addition to the suspended 

 weight P, half of its own weight p, which should be added to P as % p; hence, 

 A=(P + ip) S. 



With the strongest possible stimulation, or maximal stimulus, that 

 is, a strength of stimulus that causes the maximum degree of contraction 

 in the unweighted muscle, the work performed increases progressively 

 with each contraction as the weight increases to a certain maximum. 

 If, as the weight is increased, the muscle can raise it to a gradually dimin- 

 ishing height, the amount of work diminishes progressively; and, finally, 

 as already noted, it becomes o, when no elevation is effected. 



The following table will illustrate the work performed by a frog's muscle, 

 according to Edward Weber: 



Weight Lifted, Height in Milli- Amount of Work Performed 



in Grams. meters. in Gram-millimeters. 



5 27.6 138 



i5 25.1 376 



25 11.45 286 



30 6.3 189 



If the weight be increased at any given moment during the contraction of 

 the muscle, more work can be performed, but only if the stimulus applied does 

 not fall below a certain minimum. The duration of the contraction is longer. 



If a muscle has contracted as much as possible for the purpose of lifting a 

 heavy weight, it can be made to perform still more work by gradually diminishing 

 the weight. It contracts still further and performs additional new work by 

 raising the diminished weight. 



If the amount of work performed by the muscle be diminished by raising the 

 weight before the contraction to a part of the height to which it would have 

 been lifted by the muscle stimulated to the maximum, then the muscle will raise 

 the weight to a still higher level. 



The investigations concerning muscular work yield the following 

 results : 



1. The muscle is capable of lifting a greater weight the larger its 

 transverse section, that is the more fibers it contains arranged side by 

 side. 



2. The muscle is capable of lifting a weight the higher the longer it is, 

 that is the more muscle-fibers it contains arranged in succession. 



3. The muscle is capable of lifting the greatest weight at the com- 

 mencement of contraction; as the contraction progresses, it is capable of 

 lifting only a progressively smaller weight, and near the maximum con- 

 traction only a relatively light weight. 



4. By the term absolute muscular energy is meant, according to Ed. 

 Weber, the weight that the muscle stimulated to the maximum is no 



