THE MOLECULAR BASIS OF MUSCLE CONTRACTION 



283 



Power of Contraction 



The power — the rate of energy release, or "energy flux," as some people 

 call it — is given by 



P = d(A$')/dt + dSll/dt 



= P W + tin 



the first term being the rate at which work is done, and the second the rate 

 at which heat is liberated during shortening. Resting muscle in the steady- 

 state condition at 20°C has a basal metabolic rate (bmr) of heat loss, 

 dq' bm /dt, of about 2 cal per kg of muscle per minute. The rate is 2.5 times 

 higher at 30°, 2.5 times lower at 10°C. Extrapolated to man (the example 

 is Hill's 16 ) — 30 kg of muscle at 37°C — the value of that part of the bmr due 

 to muscle alone is about 18 Cal/hr, about 25 per cent of man's total bmr. 

 During action, i.e., during a single twitch, the muscle gives out a contraction 

 heat of about 3 cal/kg of muscle. For a fast muscle which twitches in 0.1 to 

 1.0 sec, therefore, the rate of heat loss, P%. n would be 180 to 1800 cal per kg 

 per min — up to many times the bmr (~14 cal per kg per min). 



Because the contraction heat is independent of rate of shortening, the rate 

 of heat loss, P^ n increases linearly with increasing speed of shortening. But 

 the power expended to do work, (i.e., P w ) is zero if no load is lifted (v = 

 v ): it is also zero if the load is so heavy that the muscle can just sustain 



max / * ' *■* 



but not lift it {v = 0); and it goes through a maximum value for intermediate 

 loads. Figure 10-11 illustrates this behavior of P w and P^ r The top curve 

 gives the total power expended by the muscle. 



The key to all this activity in muscle is the molecule myosin. But before 

 discussing myosin itself, we must first understand the structure of muscle 



v/v mox ► 



Relative Speed of Shortening 



Figure 10-1 1. Power as a Function of Fraction 

 Shortened (after Podolsky, 1961). 



