When my class 9 biology teacher Mrs. Khurana drew the structure of lactic acid and said, "This is what causes muscle fatigue.", I had learned how to draw the structure of lactic acid. Post-workout pains were always attributed to the “bad kitty” (Reference: South Park) lactic acid after that. I studied biology and biochemistry for years to follow, and always blamed lactic acid deposition for muscle pain after workout.
15 years later, I relearned my physiology when the advisor said, "Lactic acid is a myth, it is the leaky calcium channels." It seems the tremendous pressure you subject muscles to during short-duration, heavy exercise is what makes them leaky. Over time, the situation gets better because two things happen. We produce more calcium channels, and the calcium channels become more resilient. That is why we ache more when we start working out, but do not feel that much pain after a while. Over time, our body has produced more calcium channels, and they have strengthened themselves. Of course, I am paraphrasing what he said.
Whatever it is, right now my ribs and stomach muscles hurt so much that I am not in a state to care if it is the darned lactic acid or the leaky calcium channels. I will not care even if you tell me that I am suddenly producing excess male hormone testosterone or have generated a tail by mistake.