Muscle fatigue is primarily caused by a lack of which substance and the production of which byproduct?

Muscle fatigue is primarily linked to a lack of oxygen and the accumulation of lactic acid as a byproduct of anaerobic metabolism during intense exercise. When muscles exert themselves beyond their aerobic capacity, they rely on anaerobic pathways to generate energy. This process does not require oxygen but leads to the production of lactic acid. As lactic acid builds up, it contributes to the sensation of fatigue and the inability to sustain muscle contractions effectively.

Oxygen is crucial for aerobic respiration, where glucose is broken down more efficiently to produce energy. However, in the absence of adequate oxygen, muscles shift to anaerobic respiration, which is less efficient and results in lactic acid production. Therefore, the correct relationship highlighting muscle fatigue is the depletion of oxygen and the rise of lactic acid levels in the muscle tissues.