THE MOLECULAR BASIS OF MUSCLE CONTRACTION 



281 



r max 

 (or rn max ) 



Velocity of Shortening , v "" 



Figure 10-10. Force-Velocity Relationship (see text). 



In review of the discussion in Chapter 7, we recall that the physical proc- 

 ess derives its energy from chemical reactions, and that the heat of reaction, 

 A3C, can appear in several forms: 



A3C = AS' + q' + TAS 



= a^' + a^ + ^ + taS 



where A3 1 ' is the external work; AO 1 ^, is the internal work, degraded into 

 heat and given off by the system as heat; TA S is the reversible entropic 

 heat, unavailable for work; and q' m is the extra heat produced because of the 

 nonreversibility (inefficiency) of the process. It was also shown that q' can 

 be expressed as q' bm + <7ex> so tnat 



A3C = AJF' + ^ + ^ + TAS 



where q' hm is the basal metabolic heat given off, and q' ( . x is the excess heat 

 given off during exertion. (These r/'s are irreversible heats, can be factored 

 into T A S "s, and are sometimes called entropic heats.) 



Now although A JC and TA S depend only upon the amount of material 

 reacting, and q' bm is substantially constant since it refers to a particular 

 physiological state, values of A^ ' and q[. x generally depend markedly upon 



