36 



PRACTICAL PHYSIOLOGY 



necessary to bring back the writing point accurately on to the base-line 

 before each contraction is recorded. 



The general effects to be noticed are that, as the load is increased, 

 the latent period becomes slightly longer, the height of the contraction 

 generally becomes less, the rise of the lever during the period of active 

 contraction becomes more gradual, and the period of relaxation, which 

 may be at first much decreased, gradually lengthens out again. 



If the muscle be fresh and in really good condition, the early effect 

 of increasing the load may be to increase the height of the first few 

 contractions (Fig. 43). This stimulatory effect of initial tension on the 

 power of a muscle to liberate energy during a subsequent contraction, 

 is seen, within certain limits, in all kinds of muscular tissue (see 

 p. 293); and it is of importance. For, in the body, as has been 

 already pointed out, the skeletal muscles are, even when relaxed, 

 unaer a certain tension produced by the pull of their antagonists and 

 their being really shorter than the distance between their points of 

 origin and insertion. 



But when we study the work done by the muscle during a series of 

 contractions with increasing loads, and not merely the height of the 

 individual contractions, the stimulating effect of increased load is much 

 more obvious. After the tracing has been varnished and dried, 

 measure off the vertical heights of the curves corresponding to the 

 different loads, and calculate the work done during each contraction 

 (see p. 29). In the following table are given the details of the work 

 done during the contractions recorded in Figs. 40, 41. 



It will be seen that, although the height of the contraction decreases 

 as the load increases, the work performed increases throughout. This 

 process of course has limits, which will be dealt with on p. 288. The 

 important deduction to be made from these results is that muscle as 



