362 THE HUMAN MOTOR 



The corresponding expenditures of work per metre-kilogramme 

 for 80, 120 and 180 paces per minute are -088 kgm., -119 kgm. and 

 176 kgm. respectively. 



These figures are only useful for the purpose of comparison. 

 From the practical point of view the all-important factor is the 

 expenditure of energy as measured by the consumption of oxygen. 



275. Walking with a Load. The general nature of the step is 

 the same whether walking loaded or unloaded, although the 

 durations of the various movements are altered. The length of 

 the step is decreased while the periods of support, especially 

 those of " double support " are prolonged. The foot is placed 

 flat on the ground, and the contraction of the muscles of the 

 carrying leg is increased. 



The vertical oscillations are decreased, both owing to the above 

 reduction in the length of the step and also because the load 

 carried tends to increase the flexion of the knees. Other oscilla- 

 tions are influenced by the manner in which the load is carried. 

 If the load is borne on the head, the Centre of Gravity is thereby 

 raised and the muscles have to contend with an increased in- 

 stability of the body. Inconvenience on this account is much 

 less if the load is carried by the shoulder or the nape of the 

 neck. In the latter case, the body oscillates in an anterior 

 posterior plane and the muscles have to overcome the inertia of 

 the load. In the former case the effect of this inertia is but little 

 felt since the body adjusts itself to balance, the effect of the 

 load ( 261) and the Centre of Gravity remains approximately 

 in the same plane as before. It is not, however, possible, by 

 such adjustment, to balance a load exceeding one half, or at most 

 two-thirds, of the weight of the trunk, say from 16 to 22 kg. It 

 would therefore seem that the most economical method of human 

 transport is to carry loads of 20 kg. or so on the shoulder. 



For the same reason the soldiers haversack should rest on the 

 flank and not on the back. If on the loins, the movement of the 

 limbs are also less restrained. Some labourers carry on their 

 heads loads which, though heavy, are very compact. In such 

 cases the Centre of Gravity is not much raised and the 

 powerful muscles of the neck can maintain the equilibrium of 

 the head. 



276. Walking with Displacement of Resistance. When a man 

 pulls a load or pushes it before him, the periods of support and 

 double support are also increased. The body inclines so as to 

 bring its weight into play, and the Centre of Gravity is thrown 

 forward in the direction of the displacement. Also the lower 

 limbs are slightly bent and the vertical oscillations are reduced 

 ( 331). 



