154 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



curve). Hill (136) determined the area AEC (fig. i) 

 experimentally in a number of frog sartorius muscles 

 and found that the area bore a reasonably constant 

 relationship to the isometric tension developed by the 

 muscle, sucii that the maximum work (area AEC) ob- 

 tainable on stimulation at length OA was equal to 

 }-^ TL, where L was the unloaded length of the resting 

 muscle, and T was the isometric tension developed at 

 that length. Deviations occur at longer lengths (2). It 

 is apparent therefore that if one defines contractility 

 as the potential for performing work at a defined 

 length, then isometric tension developed at that length 

 is a fair reflection of contractility. 



By keeping the working diagram in mind one can 

 avoid the error of attributing increased work done to 

 increased contractility, whereas in fact changes in 

 work mav only reflect changes in the working condi- 

 tions of the muscle, in terms of the initial length or 

 load. Thus, as noted above, increases in the initial 

 length of the muscle are associated with increases in 

 the work done without any changes in contractility. 

 And of course under conditions of isotonic shortening 

 the work done will vary with the load in the absence 

 of changes in contractility. Great variations in cardiac 

 output with diastolic volume, such as those observed 

 by Patterson et al. (229), were indeed accounted for in 

 terms of the length-tension or work diagram of cardiac 

 muscle. 



Isotonic Measurements 



It has already been pointed out that work capacity 

 cannot be determined under conditions of isotonic 

 loading. It is also true that contractility cannot in 

 general be estimated by the extent of isotonic shorten- 

 ing. It was shown, for example, by Varga (301) in a 

 study of the effect of temperature changes on the con- 

 tractile properties of glycerinated muscle fibers that 

 the increase in shortening with increase in tempera- 

 ture reached a maximum plateau at a point where the 

 isometric tension development was only about half 

 maximum. Further increase in temperature above 

 this point resulted in an improvement of the muscle 

 as judged by further increase in isometric tension, 

 but this could not have been seen in the shortening 

 which had alreadv reached its maximum. 



Velocity oj Shortening 



It has been claimed that an increase in velocity 

 of myocardial muscle shortening as indicated by rate 

 of ejection of blood from the left \-entricle must mean 



an increased contractility. However, a consideration 

 of the work of A.\'. Hill (137) reveals the possibilit>' 

 that contractility as we have defined it and \elocity 

 of shortening may vary independently. He found that 

 if a muscle was allowed to lift a load the velocity of 

 shortening varied inversely with the load as shown by 

 curve mPo in figure 2.' Isometric tension for this 

 muscle is given by the point Pu (i.e., force where 

 N'clocity of shortening is zero); the intercept on the v- 

 a.xis represents velocity of shortening with no load. 

 The curve is described by the equation 



HP - Pn) 

 iP + a) 



(.) 



where r is velocit\ of shortening, P is load, Pu is iso- 

 metric tension, n is a constant with dimensions of 

 force, and A is a constant with dimensions of velocity. 

 The quantity Pq a is rather constant in various muscles. 

 In different types of muscles h varies widely, and it 

 increases with temperature. When P equals zero, ;■ 

 equals b{Pa a). A change in the velocity constant b, 

 without a change in Pu or a would give a curve which 

 went through point Pu but had a diff"erent intercept 

 on the ordinate. This would represent a faster muscle 

 without a change in contractility (as indicated by the 

 same Pa)- In general there is no necessary interrela- 

 tionship between the velocity constant b and the 

 force terms Pn and a. Not only are there diflferent 

 types of muscles in which contractility may be com- 

 parable but velocities are diflerent; but changes in a 

 given muscle with, say, temperature may be such 

 that in a certain temperature range b will increase 

 progressively, whereas Po (and probably a) will in- 

 crease, go through a maximum, and thereafter decline 

 (109). This short paragraph should suffice to indicate 

 that velocity of shortening and contractility may vary 

 independently, and experimental interventions may 

 affect either one or both.- 



Active State 



Ha\ins just excluded the velocity of shortening 

 from the definition of contractility, we must proceed 



' Velocity of shortening was constant over most of the short 

 distance shortened, but strictly speaking "v" is the initial veloc- 

 ity. See ref. (2) for a modification of the Hill equation which 

 applies over a wide range of shortened lengths. 



- It is evident, of course, that increased velocity of shortening 

 may contribute greatly to the stroke work of the intact heart. 

 For example, at high rates a rapid contraction cycle will allow 

 time for adequate filling in diastole so that despite the fast fre- 

 quency stroke volume and work can be maintained 1256). 



