CARDIAC MUSCLE CONTRACTILITY 



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C 



A LENGTH 



FIG. I. Diagrammatic length-tension curve of muscle. [.After Abbott & Wilkie (2).] See text for 

 description. 



FIG. 2. Relation between velocity of shortening and the mechanical load in isotonic contractions. 

 [After Hill (137).] 



FIG. 3. Diagrammatic active state curve (solid line) and isometric tension curve (dotted line) of 

 muscle. [From Gasser & Hill (87).] 



.shortening tlie muscle would be capable ol lifting a 

 greater weight and at the termination of shortening 

 further shortening (and therefore work) could be done 

 if the weight suspended from the muscle were smaller. 

 The load which muscle, in a steady state of either 

 rest or activity, is capable of supporting at any length 

 is given by the length-tension curve of the muscle. 

 Therefore, applying the type of analysis useful for 

 any engine working in cycles, Hill reasoned that the 

 work capacity of a muscle was given by the area en- 

 closed by the curves relating length to tension of 

 resting and stimulated muscle. Such curves are shown 

 diagrammatically in figure i, adapted from a paper 

 by Abbott & Wilkie (2 ). OA is the length of the resting, 

 unloaded muscle; AB is the length-tension curve of 

 the resting muscle; and CED is the corresponding 

 curx'e for the tetanized muscle. The maximum work 

 starting from any given length which can be obtained 

 from the stimulated muscle is given by the area under 

 the curve CED. For example, the obtainable work 

 from a muscle of length OA is represented by the area 

 AEC. For lengths greater than OA, one must subtract 

 the work done in extending the resting muscle (area 

 under curve AB), so that the maximum work ob- 

 tainable from muscle activity is gix'en by the area en- 

 closed by the curves CED and AB. It can be seen, of 

 course, that the obtainable net work (total less that 

 done in stretching resting muscle) becomes greater as 

 the resting length is increased. This is true up to length 

 OF, the maximum obtainable work being repre- 

 sented by area AEC plus the cross-hatched area. 

 Beyond this length, the work increment done in 

 stretching the muscle (area Ao') becomes progressively 

 greater than the work increment obtained when the 

 muscle is stimulated (area Ad'). The maximal work as 

 a function of initial fiber length was measured for frog 



.sartorius by Doi (64), and it was found that maximum 

 net work increased with increases in initial length up 

 to 1 .7 times the unloaded resting length of the muscle; 

 beyond this the net maximal work declined. 



We may define contractility, then, as the capacity 

 for a muscle to do work, and this is given by a pair of 

 length-tension curves like those shown in figure i. It 

 should be pointed out, however, that muscle cannot 

 be thought of as a simple spring for which the differ- 

 ence between the resting and stimulated state is a 

 difference in the elasticity of the spring. The heat 

 changes in shortening, for example, are not those of an 

 inert spring, since the sum of heat plus work is greater 

 when muscle is allowed to shorten than when it is 

 held fixed, which would be thermodynamically in- 

 consistent with an elastic model (137)- Even the final 

 equilibrium state reached by a tetanized muscle and 

 represented by the cur\'e CED in figure i is not quite 

 independent of the path taken to get there. Thus, if 

 curve CED is constructed by allowing a muscle to 

 shorten isotonically to a certain length and then de- 

 velop tension Lsometrically to equilibrium (path A' 

 in fig. I ) it is frequently found that in the reverse se- 

 quence of isometric contraction followed by isotonic 

 shortening (path Y in fig. i) the muscle shortening 

 stops before reaching the previously determined 

 length-tension curve. [This has been observed over a 

 wide range of lengths by Buchthal & Kaiser (36), but 

 only above body length by Abbott & Wilkie (2). It 

 has been noted con.sistently in heart muscle by 

 Rosenblueth and co-workers (248).] 



Despite the reservations just noted, much informa- 

 tion can be obtained from a length-tension or work 

 diagram of muscle. An estimate of obtainable work 

 can be made from measurement of isometric tetision 

 at one length (i.e., one point on the length tension 



