Final answer:
Muscle soreness after intense activity is commonly attributed to the accumulation of lactic acid in the muscles, affecting intracellular pH and enzyme activity, though recent findings suggest other factors such as microscopic muscle damage may also play a role.
Step-by-step explanation:
After intense activity, muscles often feel sore due to the accumulation of lactic acid, not because of the accumulation of NAD+, ATP, or carbon dioxide. During strenuous exercise or when oxygen levels are not sufficient, muscle cells can switch to lactic acid fermentation to produce energy. This process results in the formation of lactic acid as a byproduct. A high concentration of mitochondria in muscle cells usually produces ATP through aerobic respiration, but in the absence of oxygen, lactic acid is produced instead. This lactic acid can contribute to a decrease in intracellular pH, potentially affecting enzyme activity and causing muscle fatigue. Additionally, while lactic acid accumulation has historically been linked to muscle soreness, recent research suggests this relationship may not be as clear-cut, but instead, it might relate more to microscopic damage to muscle fibers during intense activity.