82 TEXT-BOOK OF PHYSIOLOGY. 



very slight upward movement at its axis, close to which the muscle 

 is attached, will be considerably magnified. That the ordinate value 

 of an isometric curve may be known, the apparatus must be graduated 

 by subjecting the spring to a series of weights playing over a pulley 

 supported by the muscle clamp. 



The curve of the variation in tension obtained by the isometric 

 method is shown in Fig. 30, b, in which the two curves are con- 

 trasted. The form of the curve indicates that the muscle attains its 

 maximum of tension more rapidly than its maximum of shortening ; 

 that the tension endures for a certain period of time unchanged ; that 

 the fall in tension takes place more rapidly than the muscle relaxes. 



The Work Accomplished by a Muscle during the Time of a 

 Single Contraction. By work is meant the overcoming of opposing 

 forces. In the physiologic activities of the body the muscles at each 

 contraction not only overcome the resistances of antagonistic muscles, 

 the weight of the limbs, the friction of joints, etc., but in addition 

 overcome various external resistances connected with the environ- 

 ment e. g., gravity, cohesion, friction, elasticity, etc. The muscles 

 may therefore be regarded as machines for the accomplishment of 

 work. Experimentally the work done by an isolated muscle may be 

 calculated by multiplying the weight by the height through which it 

 is lifted. In the following table 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 ; but as weights differing by a com- 

 mon increment are added, the height of the contraction diminishes 

 until with a given weight it is nil. The work done is shown in the 

 following table: 



WEIGHT. HEIGHT. WORK DONE. 



o grams 14 mm. o gram -millimeters 



50 

 100 



150 

 200 

 250 



45 

 700 



750 



400 



o 



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 

 maximum 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. 



The maximum amount of force which a muscle puts forth during a 

 contraction is naturally measured by the amount of work done; but 

 as this varies with the degree to which the muscle is weighted, another 

 measure has been adopted, to which the term absolute muscle force 

 or static force has been given. The absolute force is measured by 



