TEXT-BOOK OF PHYSIOLOGY 



tracted, after which it gradually relaxed and returned to its original con- 

 dition. The record shows also that during the actual passage of the cur- 

 rent the muscle substance was being stimulated by it. 



The Work Accomplished by a Muscle during the Time of a Single 



Contraction.-^By work is meant the overcoming of opposing . forces/ 



I In the physiologic activities of the body the muscles at each contraction not 



Wly overcome the resistances of antagonistic muscles, the weight of the 



limbs, the friction of joints, etc., but in addition overcome varjous external 



resistances connected with the environment e.g., gravity, cohesion, friction, 



lelasticity, etc. The muscles may therefore be regarded as machines for 



the accomplishment of work. Experimentally the work done by an iso- 



lated muscle may be calculated if the height of the contraction is first obtained 



and then multiplied by the weight 

 raised. The influence of the 

 weight on the height of the con- 

 traction is shown in Fig. 31, in 

 which only the height of the con- 

 traction or the degree of shorten- 

 ing and hence the lift of the weight 

 is represented. From this tracing 

 it will be observed that the extent 

 to which a muscle will shorten in 

 response to a maximal stimulus 

 is greatest when it is unweighted; 

 FIG. 31 .-TRACING SHOWING THE GRADUAL but as weights differing by a com- 

 DIMINUTION IN THE HEIGHT OF THE CONTRAC- mon increment are added, the 

 TION AS THE WEIGHT WAS INCREASED BY A COM- height of the contraction dimin- 

 MON INCREMENT OF 10 GRAMS FROM o TO 180 . , .. . , . t^ f* 



untl1 Wlth a g lven weight it 



GRAMS. MAGNIFICATION OF THE LEVER, 4. 



is nil. 



A careful study of the results of this experiment will show that the 

 work done gradually increased as the load was increased from o to 70 

 grams, when it amounted to 210 gram-millimeters; but that after this, 

 even though the weight lifted was' greater, the height to which it was lifted 

 was less, and hence the work done gradually decreased, until it amounted 

 to nothing. 



The following table will also show the work done by a frog's muscle 

 according to Rosenthal. 



Weight, 

 o grams 



50 grams 

 100 grams 

 150 grams 

 200 grams 

 250 grams 



Height. 

 14 mm. 



9 mm. 



7 mm. 



5 mm. 



2 mm. 



o mm. 



Work 



o gram- 



450 gram 



700 gram- 



750 gram 



400 gram- 



o gram 



Done, 

 millimeters, 

 millimeters, 

 millimeters, 

 millimeters, 

 millimeters, 

 millimeters. 



From the preceding figures it is evident that the mechanical work of a 

 muscle increases with increasing weights up to a certain maximum, and 

 then declines to zero. Equally when the muscle contracts to its maxi- 

 mum without being weighted, and when it does not contract at all from 

 being overweighted, no work is done. Between these two extremes the 

 muscle performs varying amounts of work. 



