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.
Two factors contribute. Firstly, arteries (blood vessels that deliver blood) can expand or contract all around you body to divert blood where you need it. When you work out, your muscle needs blood, so you divert blood from somewhere else (often the GI tract (stomach and intestines)) into the muscle, so the muscle seems bigger. Second, working out causes minor damage to you muscles, and it grows back stronger after healing, which is how your muscle grows. The damage also causes inflammation, which causes water from the blood to leave the blood vessels and hang out on the muscle tissue, also increasing in volume.
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