The relationship between muscle fatigue and lactic acid buildup is crucial in understanding exercise physiology. During intense physical activity, the body may switch to anaerobic metabolism due to insufficient oxygen supply, leading to the accumulation of lactic acid in muscles. This accumulation can cause a burning sensation and ultimately contribute to fatigue and decreased performance. While lactic acid was once viewed solely as a waste product, recent studies indicate that it can also serve as an energy source for the body. Proper recovery and conditioning can help mitigate the effects of lactic acid during workouts.