Muscles can contain “fast” twitch fibers, and “slow” twitch fibers. Fast fibers give you more power at the cost of using a lot of energy very fast. On the other hand slow fibers can maintain their contraction for prolonged periods of time.
So for example the bicep muscle typically contain a lot of fast twitch fibers, while the muscles of the back in charge of your posture contain more slow fibers. There are other reasons why a certain muscle might require more energy but I want to keep it eli5.
Endurance training typically results in increased slow fibers while muscle building results in more fast fibers.
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