What is the primary cause of muscle fatigue during prolonged exercise?

The primary cause of muscle fatigue during prolonged exercise is the accumulation of lactic acid. During intense or prolonged physical activity, the body relies on anaerobic respiration to meet the energy demands when oxygen levels are insufficient. This process leads to the production of lactic acid as a byproduct. As lactic acid builds up in the muscles, it can lower the pH within the muscle cells, leading to an environment that is less favorable for muscle contractions. This accumulation contributes to sensations of fatigue and discomfort, impairing the muscle's ability to function effectively.

Although insufficient oxygen supply, dehydration, and exhaustion of glycogen stores can also contribute to fatigue, the immediate effect of lactic acid build-up is a significant factor that manifests strongly during intense exercise. Understanding this helps to grasp how the body responds to exertion and the physiological limits that can impact performance.