length of the canon bone is changed. The lengthening 

 of this radius is, therefore, of no particular benefit to 

 strength. But we will see later that the length of the 

 canon bone has an appreciable importance in the length 

 of the stride (formation of the third step of the gallop) ; 

 the canon bone of the hind leg of the sprinter must, there- 

 fore, not be too short. 



Excessive lengthening of the phalangian lever would 

 be ruinous to the tendons. Too great shortness, must 

 however, not be sought after in this region. See on this 

 subject the note on page 17, which explains the close 

 relation existing between the rotation of the phalanges 

 and that of the legs. 



We are then permitted to conclude that in the interest 

 of propulsion the femur should be long compared with 

 the tibia, that the canon bone must not be too short. This 

 double condition imposes upon the tibia a relatively re- 

 duced length. The twin muscles which stretch over the 

 rear part of the tibia will then be relatively short, which 

 is not a disadvantage if they are strong. Inserted in the 

 crest of the calcaneum these muscles act very quickly 

 upon a short lever, whose motion is almost immediately 

 stopped. They need, therefore, strength, not length. 



Before closing this analysis of propulsion, it must be 

 noted that the movement begins at the same time as the 

 straightening of the lumbar arch by the extrinsic muscles 

 which change the coxo-femoral angle. 



This action is progressive ; the coxo-femoral angle 

 opens hy a ^2° from position o to position i, then 

 quickly by 4/^° from position i to position 2, and con- 

 tinues to open slowly by 5^° from position 2 to position 3. 

 It is precisely at this moment that the tibio-metatarsal 

 and the metatarso-phalangian angles assume their pro- 

 pulsive role by opening the former by 24°, and the latter 

 by 63°. 



42 



