A graded exercise test means that the client starts to work out at a low intensity, and then the intensity is gradually increased every few minutes.  The heart rate is measured at each new intensity or stage, and then the heart rate data are used to estimate the VO2max.  For most graded exercise tests, the goal is to have the client work at two workloads in which the heart rate levels out (i.e., reaches "steady state") between 110 beats per minute and 85% of HRmax.  (HRmax is estimated by this equation: 220 - age = HRmax.  For a 20-year-old person, HRmax would equal 220 - 20 = 200 beats per minute.)  It is in this range that heart rate and VO2 have a linear relationship.  The range may vary slightly, so check with the test specifications before conducting the test.