 There's a tremendous amount of strength needed to do these sorts of things because as your body goes further and further, you need every muscle to try to keep you upright. It's why you see some people that get to an end of a marathon and they're kind of hunched over because their core has kind of given up because they haven't built up that strength everywhere. And so it's always interesting for me because the most sore thing at the end of these races is actually my arms from using my trekking poles to pull me up and over these hills again and again and again.