If we think of our knees and elbows as pendulums they will stay in motion without any additional energy
A pendulum is something hanging from a fixed point which, when pulled back and released, is free to swing down by force of gravity and then up because of its inertia. Inertia means that bodies in motion will stay in motion unless acted on by an outside force. We have several pendulums in our body. If we think of our knees and elbows as pendulums they will stay in motion without any additional energy when running. Running then becomes much more of a letting go process. I find this really useful when running up hill, letting my elbows be the pendulums helps me run up hill with far less effort.
If you are just walking this same applies but the end of the pendulums are your feet and hands.