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a man looks at his heart rate monitor while working out

If you’re into heart-rate training (and if you care about fitness, you ought to be), you probably know all about maximum heart rate (MHR) and the fact that it can be an important measure in helping you optimize your aerobic workouts. You probably know less about your anaerobic threshold (AT), also commonly called the lactate threshold, despite the fact that it’s actually at the core of most heart-rate training methods.

Physiologists use the abbreviation AT to describe the tipping point where your body shifts from primarily burning fats (aerobic activity) to burning stored sugars (anaerobic activity) in order to provide fuel. Your AT corresponds to a specific range of heart-rate measurements, say, for example, from 150 beats per minute to 155, but the actual range varies from person to person.

Many zone-based fitness methods identify AT as a percentage of MHR (the highest number of beats per minute your heart can reach during exercise), but estimates of where AT occurs can range a great deal: Some experts suggest that it typically sits between 80 and 85 percent of MHR; others say 85 to 90 percent. Since most MHR-based cardiovascular training ultimately emphasizes staying within the zone where your heart is working aerobically, knowing your actual AT – the point at which you slide into the anaerobic zone – can make your training more precise and help ensure that you are spending your workout time wisely.

The point at which people cross the threshold varies according to their level of cardiovascular fitness. So how can you calculate your own AT? As noted, you can attempt to gauge it by extrapolating from your MHR, but because MHR is based on an average and calculating your AT involves using a second average, such mathematics are unlikely to yield a number with any real precision. Instead, consider one of the following ways to find your AT.

Best Method: Metabolic Testing

The best way to target your AT is to undergo metabolic testing. The test involves running on a treadmill or riding a stationary cycle with a mask over your face to capture and analyze the oxygen you consume and the carbon dioxide you exhale under varying levels of exercise intensity. Computerized equipment then analyzes the expired gases. The point at which the oxygen you consume equals the carbon dioxide you exhale marks your AT.

Good Method: Observed Exertion

You can get a reasonably good sense of your AT without being hooked up to a machine. A certified fitness professional can help you identify your AT through a combination of two experiments: the Talk Test and the Borg Scale of Perceived Exertion.

Talk Test

While jogging or cycling at an easy pace, you can be confident that you won’t exceed your aerobic metabolic zone as long as you can converse fairly normally (without speaking in short bursts). As you increase the intensity of your effort, however, you’ll notice that your breath deepens and it becomes more difficult to inhale and exhale exclusively through your nose. Although you can still converse, it requires more labor. This is your signal that you have reached the lower edge of your AT. Note your heart rate.

You could probably exercise at this rate for a long period, and even carry on a conversation, but you have stopped using purely aerobic metabolism and begun to use some of your body’s sugar stores for energy.

Pick up the pace and your oxygen demand increases. You begin to breathe through your mouth and exhale more forcefully. You can talk, but the words do not flow as naturally. You now have crossed over the far edge of your AT and your body can no longer supply all of the energy you need through purely aerobic metabolism. Note your heart rate once again.

The sweet spot for cardiovascular training and weight management lies between the two points you have just identified. This is where you are challenging your cardiovascular system to improve while also burning the most fat calories.

Borg Scale of Perceived Exertion

This is a more subjective test, but in combination with the Talk Test it can be useful in determining your AT. The Borg Scale ranks your feeling of exercise effort on a scale from 6 to 20, with 6 representing a very mild effort and 20 a very strenuous one. During the test, a trainer reviews the various zones, from very easy to very hard, and describes the sensations associated with each. You begin the test at a light level and then increase the speed, resistance or incline until you begin to feel your efforts are sustainable, but somewhat hard, corresponding to a rate of 15. A reading of 15 on the Borg Scale corresponds to your AT. Keep track of your heart-rate measurements at level 15 and you’ll have a good approximation of your AT.

Fair Method: Formula

Most people experience the best results by gauging their AT from the methods above. But in the absence of any testing protocol, you can estimate your AT using the following method developed by Phil Maffetone, a widely respected coach and sports physician. Just keep in mind that any method that relies on a formula introduces a larger margin for error.

1. Subtract your age from 180

2. Adjust this number by selecting one of the following categories:

a) If you have not exercised for more than one year or are recovering from a major illness (heart disease, any operation or hospital stay, etc.), or if you are age 65 or older, subtract 10.

b) If you have been exercising for up to two years at least four times a week, subtract zero.

c) If you have been exercising for more than two years, at least four times a week, add five.

d) If you are a competitive athlete, add 10.

For example, if you are 30 years old and began exercising moderately four times a week three years back, your number would be 155 beats per minute (180 – 30 = 150, then 150 + 5 = 155). This number represents your estimated maximum aerobic-exercise heart rate, or the rate at which you remain just below your AT. Once you have this number, consider trying one of the observed exertion tests described above to see how those AT results correspond to the formula-estimated figure.

How to Get Into Your Heart Rate Zones

Once you’ve established your AT, you can use it to identify and work within a range of different training zones, each of which provides specific fitness benefits. Although various heart-rate training and fitness experts divide the zones a little differently and label them in different ways, most identify either four or five zones, with AT occupying a middle-high point on the exertion scale. Most experts also agree on the following fundamentals:

  • Exercising well below your AT develops your aerobic capacity. It helps you burn unwanted fat and improve health parameters, such as blood pressure, cholesterol levels and blood sugar.
  • Exercising in the area at or just below your AT hones exercise endurance, boosts cardiovascular fitness and helps increase your aerobic range. It also burns a lot of calories and increases post-exercise calorie expenditure, meaning your body continues to burn calories for an extended time after completing the exercise.
  • Exercising at and above your AT boosts endurance and heart and exercise performance. It also increases your aerobic range and develops your ability to tolerate higher levels of lactate in the blood. Lactate, a byproduct of carbohydrate breakdown during anaerobic exercise, causes a burning sensation in the muscles, making this level of exertion difficult to sustain for very long.

Intensities for Everyone

Like MHR-based training zones, AT-based training zones correspond to varying levels of intensity, each of which serves different training objectives. Keep in mind that each zone, from the easy aerobic to strenuously anaerobic, develops essential physiological functions that contribute to good health and improved fitness. So you should never limit your training to a single metabolic zone, although you may exercise predominantly in one zone for a given time in order to reach specific training objectives.

Unless you are a beginner, are out of shape or have physical limitations (like a heart condition), you should regularly work at all levels of intensity to ensure well-rounded cardiovascular development. How much and how often you work in each zone will depend on your athletic goals and your current level of fitness (continue reading for a more detailed description of the zones and their primary applications).

In general, though, about 60 to 65 percent of your total workout time should focus on Zones 1 and 2; 30 to 35 percent on Zones 3 and 4; and about 5 percent on Zone 5. But you need not incorporate all zones into a single workout. In fact, you’ll get better results from designing a range of different workouts throughout the week – including long, easy ones in Zones 1 and 2, and shorter, tougher ones that occupy more time in Zones 3, 4 and 5.

How to Find Your Heart Rate Zones

ZONE 1: Warm-up

Heart-rate range: 60 to 70 percent of AT.

Training objectives: Encourage blood flow and burn fat.

What’s happening: This is almost a pure aerobic state, which you can maintain almost indefinitely without experiencing any rise in blood lactate.

Feeling: Comfortable to talk and breathe through your nose.

How long: 10-minute warm-up progressing to higher intensity zones for 45 minutes or longer. Depending on your fitness level, you may need to start with a shorter workout and build up to 45 minutes, or do several shorter workouts throughout the day.

Zone 1 basics: This is a good place to launch your fitness program if you are a beginner or returning to exercise after taking off several months or more. It develops basic exercise technique, endurance and an aerobic base. Ideally, if you are new to exercise, or returning to training after a long break, you should stay in Zone 1 for about six weeks before moving up in zones. Zone 1 is also a good place to recover from tougher workouts.

ZONE 2: Aerobic Development

Heart-rate range: 70 to 90 percent of AT.

Training objectives: Build aerobic efficiency and maximize fat burning at a higher caloric rate.

What’s happening: A small rise in blood lactate occurs, yet the body can process it without buildup.

Feeling: You can still converse and may have the urge to go faster. Breathing deepens a bit.

How long: 30 minutes (beginner); 90 minutes or longer (advanced).

Zone 2 basics: Before developing your lactate tolerance or increasing your AT, you should fully develop your “aerobic base,” and this is a great zone for moving that effort forward. Noticeable improvement in this zone generally takes about six weeks of consistent training. But don’t worry: As you become more efficient and fit, you’ll be able to work out faster in every zone. For fit people, Zone 2 is also a good place for active recovery.

ZONE 3: Aerobic Endurance

Heart-rate range: 90 to 100 percent of AT.

Training objectives: Increase endurance. Here, you push your AT up to higher intensity by training your body to tolerate more lactate in the blood.

What’s happening: As intensity increases, not all lactate produced in the cells can be shuttled back into the metabolic cycle. Lactate levels in the blood begin to rise and, with time, fatigue sets in.

Feeling: Breathing becomes noticeable, but not too difficult; conversation is restricted to short sentences.

How long: 20 minutes (beginner); one hour or more (advanced).

Zone 3 basics: Building endurance at your AT through long intervals (four to 10 minutes) at threshold range will help raise your AT and slowly develop your body’s tolerance of lactate. This training adaptation also allows your body to burn fat more efficiently. The idea here is to rest between intervals with your heart rate recovering in your aerobic zone and then increase intensity again up to your threshold.

ZONE 4: Anaerobic Endurance

Heart-rate range: 100 to 110 percent of AT.

Training objectives: Increase athletic ability by improving lactate tolerance. Here you’re pushing AT and VO2 max (your body’s highest ability to utilize oxygen), challenging the heart to work longer, and increasing cardiac output (more blood is being pumped with each stroke, thus requiring fewer strokes per minute).

What’s happening: Your body switches into primarily using glycogen or carbohydrate to fuel its need for energy, and your blood-lactate levels dramatically increase.

Feeling: Breathing becomes heavy, difficult and uncomfortable. You may also experience “muscle burn” because of lactate buildup.

How long: Five minutes (beginner); 30 minutes (advanced).

Zone 4 basics: Run at an aerobic pace, then speed up or increase resistance until your heart beats at 110 percent of your AT range. Maintain this level for one to four minutes. Reduce the pace until your heart returns slightly below AT, but not completely back to the aerobic zone. Then repeat the interval.

ZONE 5: Speed and Power

Heart-rate range: 110 percent of AT to MHR.

Training objectives: Increase athletic ability by improving neurological response, exercise mechanics, speed and muscle power.

What’s happening: Your body is burning the last remaining fuel (glycogen) in your muscles and cannot sustain this maximum effort for more than a few seconds without succumbing to exhaustion.

Feeling: Extremely difficult and uncomfortable. You’ll feel breathless and may hear pounding in your chest. You’ll feel an intense desire to slow or stop.

How long: Typically several seconds.

Zone 5 basics: Your work here consists of sprints and very intense, short intervals (up to one minute). Although you can come in and out of Zone 5 several times during the course of a single workout, spending more than 10 percent of your total workout time in this zone increases your risk of injury. If you’ve done a significant amount of Zone 5 work, be sure to add recovery time (meaning a day or two of rest or working in Zones 1 and 2) before returning to Zones 4 and 5.

Progress Makes Perfect

By using your AT as a marker for aerobic exercise, you can base your training on a highly individualized fitness parameter, without having to guess about percentages of MHR. This lets you take advantage of every exercise session, knowing you are making the most of your time and energy. And as your fitness improves, so will your AT – one of the best indicators of cardiovascular health, fitness and overall vitality.

This article has been updated. It originally appeared as “The A.T. Factor” in the May 2005 issue of Experience Life.

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