That’s because you’re using more energy per time unit, and there’s only so much that you can sustain. The article is about energy per distance unit. So to trans

That’s because you’re using more energy per time unit, and there’s only so much that you can sustain. The article is about energy per distance unit. So to translate this: run the same distance faster, and you’ve burned about the same amount of calories, but you’re more exhausted from having to burn through the calories at a faster rate. However, from an efficiency perspective you got there faster at the same cost.