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BIOPHYSICAL STUDIES ON NERVE AND MUSCLE 



methods of kinetics in displaying the physical movements of molecules. 

 First, however, comes a discussion of the fact that activated muscle tissue 

 often behaves like a critically damped helical spring. The model is illus- 

 trated in Figure 10-8. 



The Helical-Spring Analogy 



The activated muscle has several physical properties in common with a 

 stretched spring. The latter obeys certain well-known physical laws, for ex- 

 ample that of Hooke: viz, the restoring force, F, is proportional to the dis- 

 placement, As, during stretching, or 



F 



k,As 



= k t (s - s f ) 



where s is length at any time, t, and Sj is the final (fully contracted) length 

 (see Figure 10-8). 



nerve endings 



muscle 



Figure 10-8. Stretched-Spring Model of Muscle. Defined are: s, the length 

 at any time during shortening; s , the initial (resting) length before contraction 

 starts; and s,, the length at complete contraction. 



