Why does exercising cause the BTB to change colors faster?

Exercising increases carbon dioxide (CO2) production in the body, which leads to a higher concentration of CO2 in the blood. When CO2 dissolves in water, it forms carbonic acid, lowering the pH. Bromothymol blue (BTB) is a pH indicator that changes color in response to acidity; thus, during exercise, the increased acidity causes BTB to change color more rapidly.