Delayed onset muscle soreness (aka DOMS) is what you are referring to from being sore 1-2 days following exercise. Like previous people stated, we do get “micro tears” in our muscle during intense exercise. These tears have to be rebuilt. Often, the onset is within 24-48 hours and can last several days depending on how intense the workout was. A prolonged and much more serious version of this, rhabdomyolysis, is an injury to muscle tissue that results in muscle death. The muscle is broken down, enters the bloodstream and can cause kidney failure.
The mitochondria in your cells are constantly processing glucose (food) into energy. When there’s more energy needed than the mitochondria can produce then the glucose, which has already been slightly broken down by the cytoplasm (cell jelly), gets all backed up and must be reassembled, but your cells can’t make glucose so they make it into lactic acid which causes the soreness and burning in your muscles.
Believe it or not, we don’t really know. There is the micro damage theory but that is not proven and don’t line up with expectations. Like after aerobic activity that isn’t particularly damaging from a trauma perspective one might have severe DOMS. Someone lifting a lot of weight where micro trauma would be expected could have little to no soreness.
There is the chemical theory, that lactic acid is built up in the muscles causes the soreness, this is also imperfect because when tested people with similar levels of lactic acid had varying degrees of soreness from none to extreme.
This is one of those things that is surprising that we don’t know, the scary reality is that with all of our medical advances we know fairly little about how the body actually works.
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