What causes muscle soreness one to two days after exercise?

Muscle soreness that appears one to two days after exercise is called delayed onset muscle soreness, or DOMS. It is primarily caused by microscopic damage to muscle fibers, particularly from eccentric movements where muscles lengthen under tension, such as the lowering phase of a bicep curl or running downhill. This mechanical stress triggers an inflammatory response as the body repairs the damaged tissue. Fluid accumulates in the muscle, and chemical irritants released during inflammation stimulate pain receptors. The delay occurs because the inflammatory process and fluid buildup take time to develop. Contrary to older beliefs, lactic acid is not responsible, as it clears from muscles within an hour of exercise.