Adjusting to Your Monitor's Limits

Using a heart rate monitor can help with a workout program, but physiologists say it is important to understand the limitations of these devices.

The monitors measure heart rate accurately. To guide exercise programs, they typically establish training zones set as a percentage of the user's maximum heart rate -- 60 to 70 percent for basic fitness, for example, 70 to 80 percent for improving aerobic endurance -- or employ other protocols based at least partly on heart rate.

Establishing maximum heart rate is the ticklish part. Everyone has their own: a genetically determined "speed limit" that declines slightly with age. It can be measured in a lab but otherwise can only be estimated.

There are several common formulas for making that estimation, one being 220 minus your age. A monitor that you buy might use that or some other formula.

But for any individual, it is quite likely wrong, by as much as plus or minus 12 beats per minute, according to Mitchell Whaley, dean of the College of Applied Science and Technology at Ball State University. That means the zones set up by a monitor might push you too hard or not hard enough.

It is therefore important to take it easy at first. See if the zones set by the monitor square with how you feel. Working out at 60 percent of your maximum heart rate should seem fairly easy; does it? Does 75 percent feel invigorating or leave you gasping?

Typically, you can change the maximum heart rate calculated automatically by the monitor. After a few workouts, you should have a sense of whether you need to move it up or down a few beats.