I can’t answer for hand, but people with [flat feet](https://en.m.wikipedia.org/wiki/Flat_feet) suffer from [**fallen arches**](https://en.m.wikipedia.org/wiki/Arches_of_the_foot). So the part of the foot between the ball and heel is an arch.

And the [**ball**](https://en.m.wikipedia.org/wiki/Ball_(foot)) of the foot is the fleshy part that "touches" the floor in stilettos.