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A MANUAL OF PHYSIOLOGY 



The work done by a muscle in raising a weight is equal to the 

 product of the weight by the height to which it is raised. Beginning 

 with no load at all, it is found that the weight can be increased up to 

 a certain limit without diminishing the height of the contraction ; 

 perhaps the height may even increase. Up to this limit, then, the 

 work evidently increases with the load. If the weight is made still 

 greater, the contraction becomes less and less, but up to another 

 limit the increase of weight more than compensates for the diminu- 

 tion of 'lift,' and the work still increases. Beyond this, further 

 increase of weight can no longer make up for the lessening of the 

 lift, and the work falls off till ultimately the muscle is unable to raise 

 the weight at all. 



FJG. 171. INFLUENCE OF TEMPERATURE ON THE MUSCLE CURVE. 



2, air temperature ; i, 25 30* C. ; 3, 7 10" C. ; 4, ice in contact with muscle. 

 The 5th curve was taken at a little above air temperature. 



The manner of application of the weight has an influence on the 

 work done by the muscle. If it is applied before the contraction 

 begins, so that the muscle is already stretched at the moment of 

 stimulation, a cause of error and uncertainty is introduced ; for it is 

 known that mere stretching of muscle affects its metabolism, and 

 therefore its functional power. So that it is usual in experiments of 

 this kind to after-load the muscle that is, to support the lever and 

 its load in such a way that the weight does not come upon the 

 muscle till contraction has just begun. The ' absolute contractile 

 force ' of an active muscle may be measured on this principle by 

 determining the weight which, brought to bear upon the muscle at 

 the instant of contraction, is just able to prevent shortening without 



