Febuary: Heart

   The runner's mirage is always found in numbers, our most useful yardstick for athletic progress: miles logged, seconds shaved, workouts completed. To be sure, numbers represent honest and reliable measures of your efforts, Delusion brews when you begin believing that numbers confer some transcendent truth on your abilities, because they can equally exaggerate and deceive. Look no further than your heart-rate monitor. It is a beguiling device-only the modern running shoe has had a greater influence on training. After all, the human heart is a runner's essential engine. It miraculously ramps up as your muscles suck oxygen, its rhythm accelerating in step with demand, Strap a black band around your chest (or wrist), and you will learn the exact intensity of your workout without the distortions of wind, hills, mood, or injury. In terms of aerobic training, the heart-rate monitor certainly beats the stopwatch for accuracy.

   But recognize the mirage. If you know your maximum heart rate, usually calculated as 22O minus your age, you can hone your workouts to reap stunning gains. Here's the rub: That calculation is dangerously crude, since every human heart is unique (just ask those who love you). Elite athletes often have larger ventricles than the rest of us, which allow their tickers to pump more blood with fewer beats. Others have hearts capable of beating faster, a function controlled by the sympathetic and parasympathetic nerve systems, which are mostly a birthright, only vaguely improved by training.

   Here's another rub: There are silent omissions in the heartbeats your monitor counts. The key to efficient running is to pump as much oxygen to your muscles as possible. Tick-tock heart calculations fail to take into account the amount of iron in your bloodstream on any given day, the ways in which your immune system might be compromised, allergies, and anything else that affects the transfer of oxygen. Does your heart-rate monitor tell you that you're running brilliantly at 90 percent of your maximum rate? Maybe you have pneumonia.

   The great trouble with heart-rate monitors is a faster heart rate isn't necessarily more efficient. There comes a point in which your ticker pounds so fast that there isn't enough time between beats for it to fill with blood. This may explain why the parasympathetic nerve system will slow your heart when you're reaching its limits, either to improve efficiency or to prevent you from dying.

   Still, monitors have their virtues. If you plan to use one on a regular basis, lose the 22O-minus-your-age formula, and seek out your true maximum rate on a field setting or on a treadmill. (Remember that your maximum rate declines with age, so you'll need to retest from time to time,) Gradually pick up speed until your heart doesn't beat any faster. You'll want to do this over several workouts. This is a starting place for training but allow yourself to see progress in as many ways as you can.