Delayed muscle soreness is fatigue muscle soreness. It is called delayed muscle soreness because the symptoms of muscle soreness do not appear when the cause of muscle soreness appears. Instead, the soreness symptoms appear gradually only after rest, sleep, and stopping excessive activities. This is mainly because the soft tissues of the human body, such as muscles and fascia, will obviously have a lot of lactic acid in the soft tissue interstices after excessive activity, labor, or a lot of exercise. Lactic acid is a metabolite produced by anaerobic muscle contraction, and this metabolite does not appear immediately because of anaerobic exercise, nor does it stop immediately because anaerobic exercise stops. Lactic acid is produced gradually after exercise and continues to be produced after exercise stops, thus triggering delayed muscle soreness symptoms.