GREGORY, QUADRUPEDAL LOCOMOTION 281 



weight of the body a formula which translated into the terms of our 

 Fig. 3 would be as follows : 



M X BA' = W X B'A' 



But Knorz, Henke, Ewald and others, as quoted by Haycraft (loc. cit.), 

 showed that the effective distance of the muscular force M is not BA', 

 but BB', and that we should rather conceive the foot as a lever of the 

 first order with the pivot at B', the "power" at B and the "resistance''" 

 (offered by the reaction of the ground upon the foot) at A. In that 

 •case, the moments around B' are as Jollows : 



M X BB' = W X B'A' 



Hence, other things being equal, the longer the foot, the greater will 

 be the moment of resistance to be overcome by the muscles of the calf. 



If the angle B'AD' be increased, as when the foot assumes a more 

 vertical position, the effective distance B'A' decreases; that is, the mo- 

 ment of resistance decreases as the foot becomes more vertical. In other 

 words, the "load" diminishes as the calf muscles contract. It has been 

 shown b}^ Fick and others (quoted by Haycraft, loc. cit., p. 246) that 

 when the force of- muscular contraction is opposed to a diminishing mo- 

 ment of resistance, the muscle is capable of performing more total work 

 (force X distance) than when the resistance is constant. Consequently, 

 the diminishing resistance, conditioned by the raising of the foot, ena- 

 bles the calf muscles to perform their work under the most favorable 

 conditions. 



SUMMAEY OF MECHANICAL AND PSYCHIC FACTORS IN POWER AND SPEED 



The speed of the distal end of a "long bone" of the limbs will depend 

 upon (1) the nearness of the point of insertion of the principal muscles 

 to the joint or axle, (2) the smallness of the angle of insertion of the 

 muscles, (3) the position of the muscle fibers with reference to the long 

 axis of the muscle, and (4) the speed of contraction of the muscle itself. 

 If a long muscle and a short muscle were isolated for experiment, it 

 might prove that the short muscle would contract faster than the long 

 one, but, in nature, a single movement of a long bone is produced by the 

 simultaneous and coordinated action of muscles of varying length. Thus, 

 in the act of extending the whole arm from the fully flexed position, 

 the relatively short, broad thoraco-scapular and scapulo-humeral muscles 

 contract in the same time as do the relatively long extensors of the fore- 

 arm, irrespective of their lengths. The speed and force of contraction 

 naturally depend partly upon the strength of the stimulus and partly on 



