The gold standard for finding your maximal heart rate is a treadmill stress test in a lab, but you can simulate one on your own with a heart-rate monitor. At a track, do a warmup mile or two, followed by a mile at tempo pace, then gradually increase your speed over 400 meters before running a final quarter all out. "After every 100 meters during the last 400, look at your monitor and accelerate," says Atlanta-based coach Roy Benson. The highest number on your monitor will be close to your maximum heart rate. Once you know your true rate, you only need to update it every five years to see the effect of aging on your max heart rate.