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PHYSIOLOGY 



muscle fibres during the production of the condition of tension, the tension 

 developed will be diminished if the muscle be allowed to shorten before its 

 maximum tension has been reached. This is the case with all isotonic 

 records of muscular contraction, so that it becomes difficult to give any 

 exact expression for the total energy changes in a muscle which is allowed 

 to shorten. On the other hand, in the body the bony levers are so arranged 

 that the muscles at their greatest length work at a maximum mechanical 

 disadvantage which lessens continuously as the muscles shorten and approxi- 

 mate their points of attachment. The load on a muscle is thus lessened 



Tension 



FIG. 61. Diagram to show the relation between the initial length of a 

 muscle and the tension developed in it during excitation (as measured 

 by the isometric method). The tension developed at each initial 

 length is measured by the horizontal distance between the two thick 

 lines, the left line representing the resting muscle, and the curved thick 

 line on the right the contracted muscle. (From BLIX.) 



continuously as the muscle contracts. A muscle is a machine primarily 

 for developing tension, and the potential energy thus set up may be used 

 for the production of work to any degree the conditions of loading allow. 

 The work done by a muscle when it contracts is measured by multiplying 

 the weight lifted by the height through which it is lifted, w X h. Since 

 however the result will vary according to the conditions of loading of the 

 muscle, a much more useful quantity is obtained by measuring the tension 

 produced in a muscle which is stimulated but not allowed to shorten. The 

 potential energy available due to the new elastic conditions of the fibres is 

 found to be approximately ^ 17, where T is the maximum tension developed 

 in the twitch and I is the length of the muscle (A. V. Hill). 



