If the diet is deficient in one or more of the acids, the body must break down existing proteins to provide the amino acids.

The amino acids in body tissues and fluids that are available for new protein synthesis ... of the twoproteins provides sufficient amounts of all the essential amino acids ... of to meet physiological needs; essential amino acids must be supplied in the diet  enzymes that break down protein into peptides and amino acids.

What you should re write your question it doesn’t make sense

It’s a because it says during exercise so they are doing a athletic activity