276 ANNALS NEW YORK ACADEMY OF SCIENCES 



RAPIDITY OF STRIDE 



Kapidity of stride, the second major factor of speed, is determined by 

 the rate of oscillation of the limbs, especially of the proximal segments. 

 The conditions determining rapidity of stride are discussed below (p. 



281). 



Mechanical and Physiological Kelations of Powek and Speed in 

 the Locomotive Appakatus 



contractile foece of locomotive muscles 



The contractile force of a muscle {%. e., its ability to overcome inertia 

 at a given instant) is proportional to the number of its contractile fibers, 

 when these fibers are parallel to the direction of contraction. The force 

 of such a muscle is therefore proportional to the sectional area of the 

 muscle. 21 In the case of weights lifted vertically by isolated muscles, 

 the work (W) performed is measured by the product of the muscular 

 force (F), multiplied by the distance (D) through which the load is 

 lifted 22 CW=F X D)- This distance is proportional to the length of 

 the muscle, 23 for the "shortening" of a muscle is proportional to its 

 length. Hence the total work performed will be proportional both to the 

 length of the muscle and to its sectional area, and hence to the mass or 

 the weight of the muscle. 24 



The work performed by a long muscle is greater than that of a shorter 

 one of the same sectional area. 25 Long and slender muscles such as the 

 sterno-mastoid and the sartorius of man exert a small power over a long 

 range; short and thick muscles such as the pectoralis major, the glutseus 

 maximus or the temporalis develop a relatively great power multiplied by 

 a short range. 26 The contractile force of muscle per unit of sectional 

 area is much less in cold-blooded than in warm-blooded animals. 27 It is 

 lessened by disuse and extreme fatigue and is increased by exercise, and 

 hence is dependent upon the nervous system and general systemic condi- 

 tions. 



The contractile force is inversely proportional to the number of con- 

 nective tissue fibers mingled with the striped muscle fibers. Hence 

 muscles grade into tendons and ligaments. When a muscle is stretched, 



21 Haycraft in Schafer's Text book of Physiology, p. 242. 



22 IUd., p. 245. 



23 IUd., p. 244. 



2 * IUd., p. 246 ; also Marey, p. 62. 1874. 



23 Haycraft, p. 246. 



20 Marey, p. 62. 



27 Haycraft, p. 243. 



