Here's why:
* Long-distance running requires sustained energy: During prolonged exercise, your body's primary energy source, glucose, is broken down quickly. However, your muscles eventually run low on oxygen, which is needed for the complete breakdown of glucose (aerobic respiration).
* Anaerobic metabolism kicks in: To keep going, your muscles switch to anaerobic metabolism, which doesn't require oxygen. This process converts glucose into pyruvate, which is then converted into lactic acid.
* Lactic acid build-up: Lactic acid builds up in your muscles, leading to muscle fatigue and soreness. This is a natural consequence of intense, sustained physical activity.
Important Note: Lactic acid fermentation is a normal process that helps you keep running even when oxygen is limited. It's not inherently harmful, though it can cause temporary discomfort.
