10 A. V. HILL VOL. 4 (1950) 



Under conditions, therefore, of maximum efficiency, the energy is liberated in about 

 the following proportions: 



Heat of activation Work Heat of shortening 



or maintenance 



40 40 20 



At the other extreme, with zero load and rapid shortening, the situation may be this : 

 Heat of activation Work Heat of shortening 



40 Nil 49 



(The heat of activation is the same in both cases.) 



The fact that the external work may be so large a fraction of the whole energy 

 liberated in excess of the activation (or maintenance) heat naturally makes one ask 

 whether the heat of shortening may not itself really be work degraded into heat in 

 overcoming some internal resistance to shortening : in that case energy would be liberated 

 in two forms only, heat of activation (or maintenance) and mechanical work. For two 

 reasons, the supposed internal resistance cannot be of a viscous nature: (i) the heat of 

 shortening is independent of the velocity of shortening, and (2) the heat of shortening 

 per cm is the same over the whole range of possible shortening (if it were due to over- 

 coming viscous resistance it would be inversely proportional to the length). The sup- 

 posed resistance must be constant, and must reside in lines or filaments parallel to the 

 axis of the muscle, it cannot be a volume effect. An obvious objection to the theory 

 of a constant {e.g., frictional) resistance a parallel to and inherent in the contractile 

 elements is that there should then be a constant difference 2a between the load at which 

 a muscle just shortened and the load at which it just lengthened: experiment "showed 

 (Katz^") that no such difference exists. The objection would be valid if a muscle were 

 a single contractile element, with a parallel constant resistance. In fact, however, a 

 muscle fibre is very long relative to its thickness, and its diameter is by no means con- 

 stant throughout its length. There is no reason to suppose that its maximum force is 

 the same everywhere. If not, in an isometric contraction the stronger regions would 

 tend to shorten at the expense of the weaker regions, and the constant resistance would 

 hinder shortening at one point and lengthening at another (possibly a very convenient 

 arrangement in a system of non-uniform strength). With a large number of such elements 

 in series an increase of load would stretch the weaker elements, a decrease of load would 

 allow the stronger elements to shorten: and the difference of load between observable 

 lengthening and shortening would be small. The objection, therefore, is not really valid. 



A stronger objection, raised in 1938^^, is that there are indications that the heat 

 of shortening changes sign when shortening becomes lengthening ; and the heat generated 

 in overcoming a frictional resistance does not change sign when the direction of motion 

 is reversed. The difficulty is to get muscles to lengthen reversibly except at very low 

 speeds. Possibly the use of dogfish jaw muscles (Levin and Wyman^^) which stand 

 stretching well would allow more positive conclusions to be reached. One thing is 

 certain, namely that the work done in making a muscle lengthen does not reappear 

 completely as heat : Some of it is absorbed, presumably, in driving chemical reactions 

 in the endothermic direction. The subject is being investigated afresh by improved 

 methods. 

 References p. 11. 



