Final answer:
Under low oxygen conditions, muscle cells produce ATP less efficiently and switch to anaerobic glycolysis, which leads to the buildup of lactic acid, contributing to muscle fatigue and soreness.
Step-by-step explanation:
When an athlete's muscle cells are forced to operate without sufficient oxygen, a situation often encountered during intense exercises, the cells shift towards anaerobic metabolism. Muscle cells possess a high concentration of mitochondria, which are typically involved in aerobic respiration to produce ATP. However, in the absence of adequate oxygen, these muscle cells begin to perform anaerobic glycolysis, which results in the production of lactic acid. This process occurs because the cells still need to generate ATP to function, albeit less efficiently than they would with sufficient oxygen. As the intensity of exercise increases and the oxygen supply can't keep up, this lactic acid buildup is what contributes to the sensation of muscle fatigue and soreness after physical activity.