Question

what is link between oxygen and fatigue

Answer 1

lack of oxygen makes you tired ?

Answer 2

Humans perform cellular respiration to breathe. This is an aerobic process, meaning it requires oxygen. When we exercise, most of us (except for those dang Cross Country runners) tend to overwork our lungs faster than we can take in oxygen needed for high levels of exertion. We then revert to an anaerobic (without oxygen) respiration which is largely inefficient in humans. One result of anaerobic respiration is lactic acid. Simply put, lactic acid tells the body to stop working out by that 'fatigued' feeling.