induration. 1. the process of hardening or being hardened.
1 They're weak against the Ice and Water types in reference of erosion, which cause this elements to the ground. 2  They're immune against the Electric type because in real life, the ground is a natural sink of electrons.  The Bug type resists them because many bugs like worms live inside the ground and many of them eat it.
Now think aboutthe forces that the ground imparts on your every step as equal and opposite reactions; theseare the forces measured by a force plate. The force with the largest magnitude that theground imparts on your body is the vertical ground reaction force (VGRF), which is in they-direction.