Measurements of dynamic changes in oxygen uptake (VO2 kinetics) during recovery from exercise may describe regulatory control of oxygen transport and utilization and have greater reliability and less inherent risk than assessment of maximal oxygen uptake (VO2max) in patients for whom exercise is limited by pain, excessive fatigue, dyspnea and motivation. The purpose of this pilot study is to evaluate the effect of exercise intensity on the time constant describing changes in VO2 (tauVO2) during recovery from one minute of constant work rate exercise. Five normal healthy volunteers ages 18 and older will perform a progressive maximal aerobic exercise test, using a cycle ergometer, to determine VO2max and lactate threshold (LT) estimated by gas exchange. Each subject will also complete a series of maximal constant work rate tests and submaximal constant work rate tests at 80% LT and 50% of the difference between LT and VO2max (50% delta). Breath by breath variability and VO2 span will be used to determine the number of constant work rate test repetitions, for each subject for each exercise intensity, needed to establish confidence in tauVO2. A mean response profile of VO2 recovery kinetics for each exercise intensity will be analyzed using non-linear regression to determine tauVO2. To examine the effect of exercise intensity on tauVO2, one way analysis of variance will be used to determine whether differences exist among maximal and submaximal (80% LT and 50% delta) time constants. We hypothesize that there will be no significant differences among time constants for VO2 during recovery from maximal and submaximal constant work rate exercise lasting one minute. The results of this study are expected to provide increased understanding of the measurement of VO2 kinetics during recovery.