ADVANCED EXPERIMENTAL PHYSIOLOGY 99 



(Weber's paradox). If this were not so, we should, when trying 

 to lift a load greater than the muscle could move, run a great 

 risk of rupturing our muscles. 



CHAPTER V 



LOAD AND AFTER-LOAD. WORK DONE WITH INCREASING 



LOADS x 



Muscles may be loaded in two ways : the load may be applied 

 before the muscle has begun to contract, or only after it has already 

 begun to contract ; this latter method, in order to distinguish it 

 from the former, is called " after-loading." Most of the muscles 

 in the body are both loaded and after-loaded ; that is, they are 

 constantly loaded by the pull of their antagonists, and it is only 

 after they have already begun to shorten that the main load the 

 weight of the limb, etc. is applied to them. The deltoid, however, 

 is an instance of a muscle constantly loaded by the weight of the 

 arm ; the ventricle of the heart, on the other hand, is a muscle 

 which is only after-loaded. 



The effect of load, and of its method of application on a single 

 muscular contraction, will be studied in the following ways : 

 (a) the contraction given by a muscle loaded and after-loaded 

 with the same weight will be compared ; (6) the muscle being 

 just completely after-loaded, the height of contraction, with in- 

 creasing loads, will be measured and the work done with each 

 calculated. 



Comparison of the Contractions of a Loaded and After-loaded Muscle. 

 Arrange the apparatus for stimulating a muscle with single maxi- 

 mal induction shocks, using the drum as a key in the primary circuit. 

 Fix a gastrocnemius preparation to a myograph lever, provided 

 with an after-loading screw ; by raising the screw the metal part 

 of the lever can be supported at any level. Hang a weight of 

 50 grms. near the axis and raise the screw until the whole of the 

 weight is just after-loaded ; this point can be ascertained by 

 supporting the weight with the finger, and when the muscle no 



1 The magnification of the movement of the muscle recorded by the lever 

 is calculated by dividing the distance of the writing point from the axis by 

 the distance from the axis of the point of attachment of the thread from the 

 tendon. 



The amount of actual shortening a muscle undergoes during contraction 

 can be calculated by measuring the vertical height of the top of the curve 

 above the base line and dividing it by the magnification. 



The actual load which the muscle raises is not the whole of the weight 

 hung near the axis of the lever, but a proportion of it, calculated by multi- 

 plying by the distance from the axis of the point of suspension of the weight 

 and dividing by the distance from the axis of the point of attachment of the 

 muscle. 



