202 PHYSIOLOGY 



length of the latter by a certain weight, when it will be in a con- 

 dition of tension. In their natural position in the body muscles may 

 possess any length between extreme shortening and extreme elongation 

 whether they are in a resting or in an excited condition. Since the relaxed 

 muscle requires only a minimal force to extend it to the maximal length 

 possible in its natural relationships in the body, it is usual to speak of the 

 different lengths of an excited and unexcited muscle, the lengths being in 

 this case those which are impressed on the muscle by a minimal load. When 

 we measure by means of the isometric method the maximum energy set free 

 in a muscle as the result of excitation, we find, as Blix first pointed out, that 

 this energy depends on the length of the muscle fibres during the period of 

 contractile stress set up by the excitation. With increase in the length of 

 the muscle the tension developed on excitation increases until the length of 

 the muscle is somewhat greater than that which it possesses in its normal 

 relationships in the body. To lengthen the muscle beyond this point a 

 certain stretching force must be applied to it which rapidly increases. The 

 tension developed on excitation however soon begins to diminish. These 

 relationships are shown by the diagram (Fig. 61), where the ordinates repre- 

 sent the length of the muscle and the abscissae the tension on the muscle. 

 The left-hand thick line represents the muscle in a state of rest, the right- 

 hand curved line the muscle in a state of excitation. The horizontal 

 distance between the two lines gives the increase of tension (as measured 

 by the isometric method) produced when the muscle passes from the 

 resting into the excited state as the result of stimulation by a single induction 

 shock. 



j Since the tension set free on excitation depends on the length of the 

 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. As A. V. Hill has pointed out, 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 1 TZ, where T is the maximum tension developed 

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



