CH. X.] MUSCULAR WORK 115 



doing this work probably less than a thousandth part of the muscle 

 has been consumed. 



Next let us consider the relationship between the work and the 

 heat produced. An ordinary locomotive wastes about 96 per cent, of 

 its available energy as heat, only 4 per cent, being represented as 

 work. In the best triple-expansion steam-engine the work done rises 

 to 12*5 per cent, of the total energy. 



In muscle, various experimenters give different numbers. Thus, 

 Fick calculated that 33 per cent, of the mechanical energy is avail- 

 able as work ; later he found this estimate too high, and stated the 

 number as 25 ; Chauveau gives 12 to 15 ; M'Kendrick 17. Thus 

 muscle is a little more economical than the best steam-engines ; but 

 the muscle has this great advantage over any engine, for the heat it 

 produces is not wasted, but is used for keeping up the body tempera- 

 ture, the fall of which below a certain point would lead to death, not 

 only of the muscles but of the body generally. 



So far we have been speaking as though the only active phase of muscular con- 

 traction is the period of shortening. It is, however, extremely probable that 

 lengthening is also an active process. This was originally mooted by Fick, who 

 pointed out that the fall of a muscle lever during the relaxation period is of variable 

 speed, and is obviously not due to the passive elongation of the muscle by gravity ; 

 the way in which this part of the curve is varied by such agencies as temperature, 

 and drugs like veratrine, also indicates that relaxation is an independent process. 



Isotonic and Isometric Curves. If, in recording the contraction of a muscle, the 

 load is applied vertically under the muscle, its pull upon the muscle varies during 

 the successive stages of a single contraction, owing to the inertia of the load. In 

 order to avoid this variation in tension, it is usual to apply the weight at a point 

 close to the fulcrum of the recording lever, so that when the lever is raised, the 

 weight remains practically stationary, and thus the error due to its inertia is avoided. 

 In order to apply the necessary tension to the muscle, the weight hanging on the 

 lever must be increased in the ratio of the distances of the muscle and weight from 

 the fulcrum. A twitch recorded under such circumstances is called isotonic, i.e., one 

 in which the tension remains constant throughout. If, on the other hand, the 

 muscle is fixed at both ends, and then excited, the resulting activity expresses itself 

 in a phase of increasing tension followed by one of decreasing tension. If the 

 alterations of tension are recorded, we obtain what is called an isometric curve. 

 This curve is obtained by making the muscle pull against a spring which is so strong 

 that the muscle can only move it to a very slight extent. This slight movement is 

 then highly magnified. The curve thus obtained resembles in its main features an 

 isotonic contraction, but its maximum is reached earlier, and it returns to the zero 

 position sooner. The flat top of the isometric curve described by the earlier 

 observers was due to the imperfection of the instruments employed. The tracings 

 of muscle curves given in previous illustrations (see figs. 118 to 124) were obtained 

 by the isotonic method, but it is probable that the isometric curve is a more faithful 

 record of the variations in the intensity of the contraction process than that yielded 

 by the isotonic method. The momentum or swing of a light lever such as is used 

 for obtaining isotonic curves will no doubt account for the extra upward movement 

 it executes. The whole matter has been keenly discussed, and the foregoing view 

 is that expressed by Kaiser. Schenk, on the other hand, maintains what appears to 

 be an improbable idea, that there arc really two kinds of change in muscle, which 

 account for the difference obtained by the two methods. 



