Multiple choice Which of the following changes occurs during strenuous exercise? Glucose is converted into glycogen. Glucose is converted into pyruvic acid. Starch is converted into glucose. Pyruvic acid is converted into lactic acid. - Reveal answer Fill a bubble to check yourself D Correct answer Explanation When sufficient oxygen is not present in the muscle cells for further oxidation of pyruvate and NADH produced in glycolysis, NAD+ is regenerated from NADH by the reduction of pyruvate to lactic acid.