Why Your Muscles Ache the Day After
Muscle soreness after exercise is caused by microscopic damage and inflammation, not lactic acid buildup.
When you engage in physical activity, especially intense or unfamiliar exercise, your muscles often feel sore the next day. This is commonly called delayed onset muscle soreness, or DOMS. The primary cause is microscopic tears in muscle fibers, particularly during eccentric movements like running downhill or lifting heavy weights. These tiny injuries trigger an inflammatory response as your body repairs the damage, leading to pain and stiffness.
Contrary to popular belief, lactic acid isn’t the main culprit. While lactic acid accumulates during exercise, it’s cleared from muscles within hours, not days. Instead, the soreness stems from the body’s effort to rebuild stronger muscle tissue. The inflammatory process involves immune cells and chemicals like cytokines, which signal pain and initiate healing.
Factors like exercise intensity, new activities, and individual physiology influence how sore you feel. Even gentle stretching or walking can cause soreness if you’re unaccustomed to it. The discomfort typically peaks 24–48 hours post-exercise and subsides as repair processes complete. This natural response is part of building strength and adaptability, though it’s not a sign of harm.