What causes muscle soreness a day after working out?

Muscle soreness that appears 24 to 48 hours after exercise is called delayed onset muscle soreness, or DOMS. It's primarily caused by microscopic tears in muscle fibers that occur during exercise, especially during eccentric movements where muscles lengthen under tension, like the lowering phase of a bicep curl or walking downhill. These small tears trigger an inflammatory response as the body repairs and rebuilds the damaged tissue. Fluid accumulates in the muscles, and various chemical signals activate pain receptors in the area. This process is actually part of how muscles grow stronger over time. Contrary to older beliefs, lactic acid buildup is not the cause, as it clears from muscles within an hour or so after exercise ends.