By Anne Stein  MSNBC

A heart rate monitor is useful for anyone embarking on a fairly serious get fit regimen.

       THE HEART RATE monitor is a hot item right now, but its more than a fad its useful for anyone embarking on a fairly serious get fit regimen. Six-time Hawaii Ironman winner Dave Scott explains that youll need to find your anaerobic threshold, or AT (this is also called the lactate threshold heart rate) and work from there to properly use the monitor.
       Your anaerobic threshold is reached when your heart rate is at 80 percent to 90 percent of your maximum heart rate. The primary benefit is to increase the bodys ability to metabolize lactic acid. The intensity of this exercise level is hard, and if youre just wanting to be fit, rather than being interested in competition, you dont need to spend much time in the anaerobic zone. If you keep training in this zone, youll be able to sustain harder efforts for longer periods of time at lower heart rate levels   in other words, youll be able to run faster and longer more efficiently   because the body wont have to work as hard as when youre less fit.
       How to find that anaerobic rate? After a long warm-up  the time depends on which sport youre doing  settle into a pace, from 10 to 30 minutes, and go very hard. At the end of that period, take your average heart rate over the past five to 10 minutes that will be reasonably close to your AT. Typically running and cross-country skiing will have the highest heart rate, whle cycling, swimming, rowing and in-line skating will have ATs that are six to 10 beats lower.
       When you train, says Scott, its only necessary to train in that AT zone 15 percent to 20 percent of the time.The bulk of your workout should be about 20-25 beats below your AT.
        For example, if your AT is 155 beats per minute, the bulk of your workout should be at 130-135. Spending time at this lower heart rate will enhance your aerobic efficiency. Your blood volume goes up metabolism increases oxygen level to the muscles increases and breathing rate slows, among other benefits of working on your aerobic efficiency.
       If you go above 155, youll start producing lactate and your workout will feel hard  your breathing will be labored and itll be tough to carry on a conversation. If you always work at a hard load (your AT), the cumulative effect of training will be damaging. You want to progress, overload and recover, explains Scott. If you always overload, your body doesnt recover. You get stronger by training, taking a rest and coming back again.
        As you become more fit, youll be able to work at a higher heart rate for a longer period of time and go faster. For example, if at the beginning of your season you can run an eight-minute mile pace for 15 minutes at a heart rate of 150, youd ideally be able, as time goes on, to run that same pace at 140, and more ideally, youd go faster.
The heart rate monitor allows your fitness to get better and gives instant feedback on whether youre overtraining.

       Over time, you'll push your anaerobic threshold up higher.
       A world class athlete has an AT thats around 10 percent lower than his/her maximum heartrate (what the heartrate is when youre going absolutely all out) The average persons AT is more towards 25 percent lower than his/her max heartrate.
       Consider the case of Michael Ryan. His ideal heart rate for racing  the rate he can go hard at for long periods of time  is about 176. When his heart rate dropped below that during his 15km running race, he stepped up his pace because he knew his body was capable of doing more. When he went up steeper grades, his heart rate went to the mid-180s, so Ryan knew he shouldnt push his body any harder. If he had, he risked a longer recovery time after the race and tiring out too much toward the races end.
        So the heart rate monitor allows your fitness to get better and gives instant feedback on whether youre overtraining. A caveat, however: Environmental conditions can affect your heart rate. Humidity and heat, for example, will bring it up, so youll need to slow down. As Scott says, you need to intermix how you feel, your heart rate and your speed to guide your workouts. One of these factors alone cannot be your only guide.
       One of the best on the subject is Precision Heartrate Training by Ed Burke, formerly of the United States Olympic Training Center.

Anne Stein, senior editor of Inside Triathlon magazine, lives in Boulder, Colo.