Why is muscle soreness after exercise often worst the next day or even the day after?

It's not due to 'lactic acid buildup,' but rather 'Delayed Onset Muscle Soreness (DOMS)' from microscopic muscle fiber tears! Many people believe post-exercise soreness is caused by 'lactic acid buildup,' but the lactic acid produced during exercise is actually metabolized by the body within 1 to 2 hours after the workout ends. The real reason you feel pain for two to three days is that exercise (especially eccentric contractions, like going downstairs or lowering heavy objects) causes countless tiny 'micro-tears' in your muscle fibers. Your body initiates an immune and inflammatory response to repair these torn muscle fibers. It takes 24 to 48 hours for white blood cells and inflammatory mediators to gather in large numbers at the injured site for cleanup and rebuilding. When the inflammatory response peaks between 24-72 hours and stimulates the surrounding nerve endings, that's when you experience the most intense soreness!