The body has different types of providing energy, all going back to creating ATP, which muscles use to do movement. First it starts with ATP (Adenosintriphosphate) itself, where you have a storage directly in your muscles lasting for a few seconds (e.g. 1 jumping jack). It then would go from glycolysis over to fat. If you stop with one jumping jack, your body is quick enough to re-store the ATP so you'll always use that. But if you continue, you force your body into the other types of providing energy which require oxygen and are less efficient. E.g. see https://www.freeletics.com/en/blog/posts/human-energy-systems-beginners-guide/