Why do your muscles feel sore after a hard 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 lowering a weight 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. As the muscles repair themselves, they rebuild slightly stronger and larger than before, which is how training produces gains over time. The inflammation and fluid buildup in the tissue stimulate pain receptors, creating that familiar stiff, tender feeling. Contrary to old belief, lactic acid is not the cause of this delayed soreness.