LOCOMOTION 357 



The motion of the legs disturbs the equilibrium of the trunk 

 and causes it to oscillate. The centre line of the hips (the hori- 

 zontal line joining the coxo-femoral joints) rises and falls. The 

 period of this oscillation is the same as that of the step, while 

 its average amplitude is 4 cm. It swings forward towards the 

 leg which is being planted vertically on the ground, and swings 

 back towards the median plane of the body when the foot is 

 lifted. It receives, in the anterior-posterior plane, a certain 

 acceleration. All the oscillatory motions described in the pre- 

 ceding paragraphs ( 269) have the same period. 



The line of the shoulders moves in a similar manner. When 

 the body is at rest the lines of the hips and the shoulders are in 

 the same vertical plane. When walking they become out of 

 plane to the extent of 1-5 cm. In other respects these oscilla- 

 tions are similar to those above described, f 1 ) 



The arms swing during walking, a motion which is to a great 

 extent involuntary. When the right leg advances the left arm 

 swings forward, being slightly shortened by a small flexion of the 

 elbow. This shortening is augmented in fast walking, resulting 

 in frequent oscillations of short period ( 2). These motions 

 compensate for the torsion of the trunk, which latter can be 

 readily observed when a man is walking with folded arms. 



Marey and Demeny ( 2 ) found that the head shares in all these 

 oscillations and that it describes a left-handed curve. The 

 distance of the centre line of the head from the median plane is 

 approximately 2-5 cm. at the moment when the foot is placed 

 on the ground. At the moment of " double support " the above 

 distance is practically zero. The amplitude of the vertical 

 oscillations may be as much as 6 cm., but, as the trunk itself is 

 subject to inclination, this figure is not to be taken as the vertical 

 displacement from the centre of gravity of the body. 



This general centre of gravity is subject to oscillations in 

 various planes, and these movements have varying rates of 

 acceleration. We can see, in these facts, a cause of the diffi- 

 culties which very young children experience in keeping their 

 balance. 



The results of the investigations detailed in this chapter may 

 be summarised as follows -: 



In the example taken (walking at 121 steps per minute, the 

 average speed being 545 km. per hour) ; the centre of gravity 

 has a vertical oscillation of 4 cm. in amplitude, a lateral oscilla- 

 tion of 1-3 cm. and a displacement from front to back of 2-5 cm.( 3 ) 



( 1 ) Lamy (Nouvelle Iconogr. de la Salpetriere, vol. xviii., No. 1, p. 49). 



( 2 ) Marey and Demeney (Comptes Rendus Sciences, June, 1885, October, 

 1887). 



( 3 ) i.e., the mean oscillation, not the translation of the body. 



