Figure 8.

Anatomical protection mechanisms to avoid hyperextension in mammal quadrupeds and non-bipedal primates (macaque, left) in comparison with humans (right). Ventral transverse processes determine tensile stress in the intertransverse ligament in extension, which limits the vertebral rotations. Lumbar vertebrae commonly possess a styloid process which acts as an osseous block to facet sliding in extension. In humans, these protection mechanisms were lost; transverse processes are dorsally located, and the styloid process is absent. Furthermore, facet joints have fewer encompassing shapes and do not strongly interlock in extension.