42 



EXPERIMENTAL PHYSIOLOGY 



The rise of tension is seen to be, on the whole, fairly uniform in rate, 

 being slow at the commencement, and near the apex. Undula- 

 tions on the curve are instrumental in character, and are due to the 

 extremely disadvantageous position from which the lever is moved. 



The measurements of such curves should be arranged in tabular 

 form, as in the following table : — 



From this table it is seen that the maximum tension attained 

 during a twitch becomes greater when the initial tension is raised. 

 With rise in initial tension the latent period, apex time, and total 

 time all increase, but the greatest increase is in the period of falling 

 tension. 



By comparing these three measurements with those given for an 

 isotonic twitch (pp. 33-36) it is seen that the apex time for an isotonic 

 twitch is longer than for an isometric, but that the period of relaxa- 

 tion is practically the same as the period of decreasing tension. 



The aim of an isometric twitch is to be able to record the tensions 

 a muscle is able to exert at each instant of a twitch carried out when 

 it is prevented from shortening. It is important to recognise that 

 the methods employed for this purpose can only give us an approxi- 

 mate result. This is due to the fact that we cannot prevent the 

 muscle fibres from shortening, at any rate in part. When a muscle 

 is stimulated the whole length of each fibre does not commence con- 

 tracting at the same instant, but one part is first affected, and from 

 this, as a centre, a wave of contraction travels along the whole fibre. 

 As a result the part which first contracts exerts an increased tension 

 upon the rest of the muscle fibre, which it stretches, and at the same 

 time this extension allows it to contract. We could get a better 

 solution to the problem if we could simultaneously stimulate the 

 whole length of each muscle fibre, so that all its parts commenced 

 contracting at the same instant. 



THE METHOD OF AFTER-LOAD 



In our study of the muscle twitch up to this point we have mainly 

 been dealing with contractions carried out whilst the load on the 

 muscle was as nearly as possible constant. There is, however, another 



