What causes muscle soreness after a tough workout?

Muscle soreness after intense exercise, known as delayed onset muscle soreness or DOMS, is primarily caused by microscopic tears in muscle fibers. This happens especially during eccentric movements, where muscles lengthen under tension, like the lowering phase of a bicep curl or running downhill. These tiny tears trigger an inflammatory response as the body sends blood and immune cells to repair the damage. The soreness typically peaks 24 to 72 hours after exercise. Contrary to old belief, lactic acid is not the culprit, as it clears from muscles within an hour of finishing exercise. The repair process ultimately makes muscles stronger and more resilient, which is why progressive overload leads to adaptation over time.