What substance accumulates in our muscles after heavy exercise and can lead to muscle cramps?

The substance that accumulates in our muscles after heavy exercise and can lead to muscle cramps is lactic acid. During intense exercise, the muscles require energy quickly, and if there is not enough oxygen available to produce this energy aerobically, the muscles switch to anaerobic respiration, which produces lactic acid as a byproduct. Lactic acid can build up in the muscles, causing a decrease in pH levels and leading to muscle fatigue, discomfort, and cramping.