acts as a limiter, preventing us from accessing

all of our

or

. But the more we learn about how that limiter functions, the

more we learn how we can push it back just a bit, in some cases by

convincing the

that the body won't be in mortal danger by pushing harder.

Endurance and ultra-endurance sports serve as a great example.

Ultra-endurance was once

thought to be harmful to human health, but now we realize that we have

all these

that are perfect for ultra-endurance: no body fur and a

glut of

and long

legs compared to our

for shock

absorption. We

have an

that are

better for pushing

than for grasping tree limbs, and when we run, we

can turn our

straight. Our

primate cousins can't do that. They have to run like this. And we have

big old butt

muscles that keep us upright while running. Have you ever looked at an

ape's butt? They have no