EX HA USTION AND DEATH OF MUSCLE. 



387 



If exhaustion is regarded as meaning diminution of the height to 

 which a muscle is able to lift a given weight, it is evident that the 

 assumption frequently made cannot be accepted, namely, that so long as 

 the time occupied in doing the work remains the same, exhaustion 

 increases with the sum of the work which has produced it ; in other 

 words, that it may be expressed as the product obtained by multiplying 

 the number of efforts by the work done in each. If we represent the 

 successive lifts in Kronecker's series by the vertical lines enclosed in 

 the triangular space ABC (Fig. 215), it is obvious that the sum of the 

 work done at the moment that any contraction (e.g. at D) takes place is 

 represented by the area A B D J, and that this area increases very 

 rapidly during the first part of the period represented by the base of the 

 triangle, very slowly towards the end. A muscle, for example, which 

 contracts and lifts a weight a hundred times at equal intervals, is only 

 half exhausted when it has made fifty efforts, though it has done three- 

 quarters of the work. 



Further evidence that the rate of exhaustion does not depend on the 

 amount of work previously done, may be obtained by comparing the 

 graphic records of two similar muscles after-loaded with different 

 weights. In these the lines of decline begin at a level which is the 

 higher the less the weight. They are both straight lines, and their in- 

 clination is the same, 

 so that if they were 

 written one above 

 the other on the same 

 recording surf ace, they 

 would be parallel. As 

 however the difference 

 between the heights 

 to which a weight is 

 raised in two successive contractions in each series is always the same 

 so long as the interval is unchanged and the excitation is maximal, 

 it follows that when a muscle is heavily after-loaded, the contractions 

 disappear from the record earlier than when the after-load is less. The 

 earlier disappearance does not mean that the muscle is sooner exhausted, 

 but only that its actual length when excited and extended by the weight, 

 becomes greater than I a longer time before it is exhausted ; for, as has 

 been already seen, when the after-loading method is used, no contrac- 

 tion, in which the muscle fails to shorten to this length, is recorded. 



The application of these results to the study of muscular exhaustion in man 

 is attended with considerable difficulty. Professor Mosso has lately invented 

 an instrument (ergograph) by which graphic records of fatigue, resembling 

 those of Kronecker, may be obtained in human muscle. The muscle of which 

 the contractions are recorded is the flexor of the fingers, the nerve is the 

 median, the movement of a weight lifted by the middle finger being inscribed 

 on a suitable recording surface. The weight is hung upon the finger, and sup- 

 ported during the intervals between successive excitations, so that the muscle 

 is " after-loaded." In general the line of decline is nearly straight, but, as might 

 be expected, it is more variable than in Kronecker's experiments. Although it is 

 difficult to say whether the numerous observations made by this method have 

 materially contributed to the understanding of the nature of exhaustion, they 

 have proved to be of great value as a means of determining the influence of 

 psychical, pharmacological, and other conditions on the process. Another 



FIG. 215. 



