What causes muscle soreness after a vigorous workout? Muscle soreness after a strenuous workout, called delayed onset muscle soreness, occurs anywhere from 24-72 hours post workout. When you have a tough workout, the muscles undergo metabolism. In this process, lactic acid is released, causing irritation of the muscles and discomfort leading to soreness. Additionally, in the anabolic process, or the normal process of growth in the body, microscopic muscle damage contributes to soreness. Tearing and inflammation within the myofibrils, the contractile units of the muscle, activate the immune response by an influx of white blood cells, prostaglandins, and other nutrients to repair the muscle. The whole process can take days for muscle healing depending on the intensity of the workout.
So being sore is not a bad thing. Just remember that there must be microscopic damage to muscle fibers before there can be growth! (The physician and sports medicine).