634 MAN xxiv. 6- 



6. The posture and gait of man 



The gait of man differs from that of any ape in that the body can be 

 fully and continuously balanced on the two legs. This involves con- 

 siderable modifications throughout the skeleton and musculature (Fig. 

 408). The backbone, instead of the single thoracic curve of quadrupeds, 

 has an S shape, being convex forward in the lumbar, backward in the 

 thoracic, and again forward in the cervical region. The thoracic curve 

 develops before birth, but the cervical only as the baby holds its head 

 up and the lumbar as it begins to walk. The vertebral column, which 

 in quadrupeds is a horizontal girder, in man becomes vertical, carrying 

 bending and compression stresses along its length. This entirely alters 

 the arrangement of its secondary struts and ties. The bodies of the 

 vertebrae carry much of the weight and are massive, tapering in size 

 upwards. They are separated by well-developed intervertebral disks, 

 acting as elastic cushions. The weight of the head is balanced on the 

 backbone through the neck, and the thorax acts as a bracket from which 

 the viscera are suspended. The muscles of the back, the ties of the 

 vertebral girder, though arranged on the same general morphological 

 plan as in quadrupeds, now carry very different stresses and no long 

 neural spines or large transverse processes develop, since the girder is 

 not now of cantilever type. For the same reason there is no sharp 

 change in the direction of the neural spines at the hind end of the 

 thoracic region; the girder is now one unit, with bending stressing 

 along its whole length. 



The balancing of the body on the legs also involves many changes. 

 The muscles around the hip joint achieve this balance, and the changes 

 to allow this affect especially the gluteal muscles and the ilium and 

 sacrum to which they are attached, these being the extensor and 

 abductor muscles, which raise the body from the quadrupedal position 

 and prevent it falling medially when the weight is on one leg. The 

 buttocks are therefore a characteristic human structure. The adoption 

 of a bipedal position imposes entirely new requirements on the 

 musculature of the limbs. In quadrupedal progression the retractor 

 muscles are the main means of locomotion, drawing the leg backward 

 at the hips while straightening the knee. In man the propulsive thrust 

 is obtained mainly from the calf muscles and in particular from the 

 soleus, which runs from the tibia to the heel, the gastrocnemius, since 

 it tends also to bend the knee, being reduced. The quadriceps femoris 

 becomes very large, serving to keep the knee extended both while the 

 calf muscles develop their thrust and, as a check to the forward 

 momentum, when the foot touches the ground. 



