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Dehydration synthesis and hydrolysis break down of muscle fibers uses and produces water. We aren’t perfectly efficient and need the extra water. Additionally there quite a bit of waste produced that need water to transport it out. Ions like sodium, calcium, and potassium are also moving quite a bit and can dehydrate if you are pushing yourself.

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Water increases in proportion to muscle creatine phosphate and glycogen content. Both fuels are used up during exercise, and the water leaves the cell. As your body replaces the fuels you depleted, water goes in with it to maintain the osmotic balance. Plus there are probably hundreds of other cellular processes activated that require water in some capacity.

Ok that makes a lot of sense. So, if you worked out hard enough, the muscles need more water than before, to maintain that balance, if I read you right.

Yes. Your body will replace the fuel stores, but to do that water needs to go in with it.

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Thanks for that very thorough answer. I got thirsty just reading it.