If the muscles are working fast and furiously and there's not enough oxygen, metabolism occurs by an anaerobic (without oxygen) process. Instead of releasing carbon dioxide and water, lactate is produced in the form of lactic acid. This is typical of high-power activities such as sprinting.

The rate of demand for energy is high and lactate forms faster than the tissues can get rid of it. If enough lactic acid builds up, it was believed that a condition called acidosis developed. This is a sign of hypoxia or shortage of oxygen. Muscle soreness would occur a day or two later.

Today we know that an increased concentration of lactate does not directly cause acidosis. And it is not responsible for delayed onset muscle soreness. Acidosis from increases in lactate during heavy exercise does occur but it is from a separate reaction.

It's still true that when there isn't enough energy to supply the needs of the muscle, energy released in the form of ATP is produced quickly anaerobically. New research shows that the buffering systems of the tissues are overcome by the large amounts of ATP produced in a short amount of time.

This is what causes pH (a measure of acididity) to fall and creates a state of acidosis. This may not be the only reason acute muscular discomfort occurs after intense exercise. There may be other factors as well. Exercise physiologists are very busy studying this new understanding of muscle function. You can expect to see more news about it in the near future.