MUSCULAR CONTRACTION 909 



tracts at all it must not be supposed that under all circumstances the 

 maximal contraction is of the same magnitude. The ability of the fiber to 

 develop tension varies from time to time and may be shown to depend 

 on a variety of factors. The energy set free in the contractile process 

 is greater, the longer the muscle at the time when it begins to contract. 

 This is shown by the fact that a muscle which is stretched at the moment 

 when it begins to contract is able to develop a greater final tension 

 than an unstretched muscle. The heat liberated in the act of contraction, 

 which measures the total energy set free, increases in a similar manner 

 with the initial length of the muscle. This observation is of importance 



Fig. 225. The all or none nature of the contraction of a single fiber of skeletal muscle. The 

 lower line represents the strength of the stimulus applied to the muscle, which rises to a maximum, 

 and then is reduced to its initial level. The upper record is of the movement of a drop of mercury 

 resting on the contracting fiber. The amount of contraction does not change as the stimulus is in- 

 creased until the point A is reached, when a second fiber became excited causing a pronounced, 

 step-like increase in the record. Later a fiber responded and produced another step. The same steps 

 are observed as the stimulus strength is gradually decreased. (From Pratt and Kisenberger.) 



because it shows that the strength of the contraction is dependent upon 

 the surface area of the contractile elements in the muscle fiber, which 

 naturally becomes increased as the muscle is elongated, and not on the 

 volume of muscle substance, which is unchanged by stretching. We have 

 seen on page 217 the importance of this principle in enabling the heart 

 to compensate for an increased load by dilating. 



The previous history of the muscle fibers also has a great influence 

 upon the magnitude of the contractions of which each fiber is capable. 

 We have seen on page 178 that if a muscle is excited to a maximal 



