system
Given the following post, respond as if you're commenting on Reddit:
human
Why does a muscle get stronger?

I know *what* happens when a muscle gets stronger. Buy *why* does that happen? 
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This question was asked by my father who, for years, has been unsatisfied with the answers he's been getting. Can someone finally answer him?
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Edit: Thanks guys! He finally got an answer he was looking for! I can't reply to everyone and thank you, I didn't expect to get this many responses, so thank you all, on both his and my own behalf!
gpt
Enhanced strength, either through thicker muscle fibers or increased neuromuscular efficiency, is the body's natural adaptation to having a certain stimulus, or stress,  placed on it (in this case a heavy load or resistive force). The body adapts to be more ready for that same level of stress, should it happen again. In order to continue getting stronger the body must be progressively and consistently stimulated with an appropriate amount of stress. Loads of different magnitude, moved at different speeds, over various durations of time, can cause a wide variety of muscular and neuromuscular adaptations. Strength is only one of them.