Maximum oxygen uptake (VO2max) is the key parameter for cardiorespiratory fitness and an important predictor of cardiovascular and general morbidity and mortality. In the German National Cohort (NAKO), a submaximal bicycle ergometer test is performed to measure cardiorespiratory fitness. This test can only be used to estimate VO2max. Therefore, a bicycle ergometer exercise test with respiratory gas measurement will be performed on approximately 500 participants at the NAKO study centres in Halle and Bremen in order to optimise the estimation equation of the submaximal test.