Final answer:
Lactic acid buildup in muscles is caused by anaerobic respiration when there is not enough oxygen available for muscle cells to produce energy aerobically. This occurs during intense exercise when demand for energy exceeds the oxygen supply.
Step-by-step explanation:
Lactic acid buildup in muscles is caused by anaerobic respiration, which occurs when there is not enough oxygen available for the muscle cells to produce energy aerobically. During intense exercise, the demand for energy is high and the body cannot supply enough oxygen to meet this demand. As a result, glucose is broken down through anaerobic respiration to produce ATP (adenosine triphosphate), a molecule that provides energy for muscle contraction. However, this process also produces lactic acid as a byproduct, leading to lactic acid buildup in the muscles.
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