Final answer:
Lactic acid build-up in muscle fibers can contribute to muscle fatigue during intense exercise by lowering intracellular pH, but it is not the sole cause of fatigue. The relationship between lactic acid and muscle performance is complex, and the acid can be recycled for energy by the liver.
Step-by-step explanation:
The correct statement about lactic acid build-up in muscle fibers is that lactic acid buildup can lower intracellular pH, affecting enzyme and protein activity, which may contribute to muscle fatigue during intense exercise. The build-up of ATP and lactic acid levels, as well as the exhaustion of energy reserves, are natural outcomes of physical exertion. The effectiveness of anaerobic activity, which often results in lactic acid production, can be improved with training. While lactic acid was once thought to cause stiffness, fatigue, and muscle soreness, more recent research suggests that muscle fatigue is a complex phenomenon and that lactic acid is not the only factor contributing to it. Once lactic acid is removed from the muscle, it can be circulated to the liver, where it is converted back into pyruvic acid and further catabolized for energy.