Why do muscles get sore after a new workout?

When you perform unfamiliar exercises or push muscles harder than usual, you cause microscopic tears in the muscle fibers. This damage triggers an inflammatory response as the body sends blood and immune cells to repair the tissue, which is what causes the delayed onset muscle soreness, or DOMS, that typically peaks 24 to 72 hours after exercise. The soreness is not primarily from lactic acid buildup, as was once believed. Eccentric movements, where muscles lengthen under tension like lowering a weight, cause the most damage and soreness. As muscles repair and adapt, they grow back slightly stronger, which is why the same workout causes less soreness over time once your body adjusts to the new stress.