Why use xendurance as an athlete the harder you exercise and compete the more acid builds up in your muscles as your blood and muscle ph goes down into the dreaded acid zone the increased use of muscle glycogen for energy produces lactic and pyruvic acid acidity reduces muscle power and reduces physical and mental endurance you your colleagues and your coaches refer to this as lactic acid build up or lactic acid fatigue your body39s acid concentration increases as you produce lactic a