Impetus

A man walks upright. For him it is strenuous to climb a steep hill, because he has to keep pushing his own vertical mass upward and cannot gain any momentum.

The rabbit is better off. His forelegs support his horizontal body and the great back legs do the work. They are more than equal to thrusting uphill the light mass in front of them.

Rabbits can go fast uphill. In fact, they have so much power behind that they find going downhill awkward, and sometimes, in flight down a steep place, they may actually go head over heels.

WATERSHIP DOWN (1972) by Richard George Adams