

MECHANICS OF MUSCLE 



365 



cross-section than are the fme-fibered muscles. Fick estimates the average strength 

 as about 10 kg. per square cm. This is known as the absolute muscle strength. 

 The total strength of a muscle would be equal to the number of square centimeters 

 in its physiological cross-section X 10 kg. 



PCS. 



FIG. 365. A, fusiform; B, unipinnate; C, bipinnate; P.C.S., physiological cross-section. 



The Work Accomplished by Muscles. For practical uses this should be expressed 

 in kilogrammeters. In order to reckon the amount of work which a muscle can 

 perform under the most favorable conditions it is necessary to know (1) its physio- 

 logical cross-section (2) the maximum shortening, and (3) the position of the joint 

 when the latter is obtained. 



Work = lifted weight X height through which the weight is lifted; or 



Work = tension X distance; tension = physiological cross-section X absolute 

 muscle strength. 



If a muscle has a physiological cross-section of 5 sq. cm. its tension strength = 

 5 X 10 or 50 kg. If it shortens 5 cm. the work = 50 X .05 = 2.5 kilogrammeters. 

 If one determines then the physiological cross-section and multiplies the absolute 

 muscle strength, 10 kg. by this, the amount of tension is easily obtained. Then 

 one must determine only the amount of shortening of the muscle for any particular 

 position of the joint in order to determine the amount of work the muscle can do, 

 since work = tension X distance. 



The tension of a muscle is, howevsr, not constant during the course of contraction 

 but is continually decreasing during contraction. It is at a maximum at the begin- 

 ning and gradually decreases. 



This can be illustrated by the work diagram Fig. 366. 

 AMD (ordinate) = tension. 

 A V X (abscissa) = shortening. 



A D = tension of muscle in extended or antagonistic position. 

 AV= amount of actual shortening. 

 A M = tension in midposition = absolute muscle strength. 



D V = shows how the tension sinks from maximum (in the extended position of 

 the muscle) where it is about double that in the midposition (M) to 

 nothing on complete contraction. 



