For a long time many people believed intense exercise caused your body to produce lactic acid and that this, in turn, caused muscle soreness. This is not correct.
https://theconversation.com/why-are-my-muscles-sore-after-exercise-hint-its-nothing-to-do-with-lactic-acid-214638?utm_medium=Social&utm_source=Facebook#Echobox=1704689731