Muscle fatigue occurs due to the accumulation of lactic acid. Lactic acid is produced in the muscles when they are working hard and there is not enough oxygen available. During intense exercise, the body relies on anaerobic respiration to produce energy, which results in the breakdown of glucose without oxygen. This process produces lactic acid as a byproduct. When lactic acid accumulates, it can interfere with muscle contractions and cause fatigue. The buildup of lactic acid can also lead to a sensation of burning in the muscles. It is important to note that while ATP (option 1) and ADP (option 2) are involved in muscle contractions, they do not directly contribute to muscle fatigue. Carbonic acid (option 4) is not involved in muscle fatigue at all.