CHAPTER 



6 



Physical Aspects 

 of Muscles 



INTRODUCTION 



The study of muscle action provides a good illustration of the different 

 points of view of the biophysicist and physicist. Consider a trapeze 

 performer hanging from a trapeze, supporting a (usually beautiful) lady 

 trapeze artist. The physicist's definition of work is the product of a 

 force on a body and the distance d that the body is displaced in the 

 direction of the force: 



W = Fd. 



From this point of view, the artist holding the lady is surely exerting 

 a force, but since there is no motion in the (upward) direction of the 

 force, the physicist claims that no work is being done. This is patent 

 nonsense, even to the physicist, as he discovers by trying to do this job 

 himself. Of course, the reason the physicist insists on talking this non- 

 sense is that he is talking about mechanical work being done against the 

 gravitational pull downward. The work we have in mind — the work that 

 results in sweat and fatigue of the performers — is biophysical and bio- 

 chemical work. It takes energy to keep the muscles in their tensed state, 

 and the biophysicist (and biochemist) is primarily concerned with this 

 aspect of the performance. 



Today, the physics and chemistry of muscle action constitute one of the 

 more interesting research areas of biology. In describing the action of 

 muscles, we will first set forth the over-all aspects of the situation in 

 terms of forces exerted and the contraction and relaxation of the muscles. 

 Then we will inquire into the substructure of the muscles which make 

 possible the observed actions. Finally, we will indicate the molecular 

 aspects of the substructure, and will sketch the possible connection with 

 known biochemical facts of muscle action. 



1 . The over-all action 



Contraction of a muscle, or the attempt to contract it, results in a 

 force. Contraction is readily studied since the length of the muscle 

 before and after the contraction can actually be measured and (essen- 



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