In the short term, the muscles you’re working out are getting slightly inflamed because you’ve caused microscopic damages to them. The inflammation response is like any other. I’m not an expert but I think it’s partly an influx of certain fluids to aid in the repair of the tissues and carry dead cells and other waste away.
In the long term, your body decides to use proteins to build more muscle tissue rather than just rebuilding exactly what was lost, because it figures you’ll need more.
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