I thought lactic acid pain was something that only occured during intense workouts when your body could no longer meet the oxygen demand of the muscles, so they switch to anaerobic respiration to keep working. This creates lactic acid as a by product, which causes the burning sensation. This builds up quickly and intensely, and the body doesn’t keep it up for long, because you are pretty much gassed at the point that it starts. That pain also ends almost immediately after you stop and the muscles can get enough oxygen again. Lactic acid burn is very intense and goes away very quickly, which seems to mean the body can get rid of it fast, so I am not sure why anyone assumed that it somehow stuck around to start hurting again the day after a workout when you really start feeling those sore muscles. I guess this was a lay person belief? It seems like a scientist would have thought that if it was lactic acid that caused the pain, it would have been the source of the microtears and inflammation in the muscles, and that might have been a little harder to refute than detecting the lingering presence of lactic acid and blaming that for the pain.