Heart Rate Training Zone (cont.)

Fat Burning vs. Cardio Mode?

Perhaps no other training "technique" is more gimmicky and misleading than the "fat burning" and "cardio" modes on the control panels of exercise equipment. They are based on the biology that at lower levels of exertion a higher percentage of fat is burned compared to carbohydrate. That's because:

    1. Fat is denser fuel than carbohydrate (9 calories per gram vs. 4 calories).

    2. It takes more oxygen to burn fat than carbohydrate because fat is denser.

    3. At lower levels of exertion, you presumably breathe in and deliver more oxygen to the muscles to burn fat.

All of the above may be true given the right circumstances, but there are problems with it when it comes to real-world exercise scenarios. First off, lots of fat is burned at all intensities within the aerobic training zone. Secondly, the terminology "fat burning" and "cardio" can confuse individuals into thinking that fat is burned only during exercise in "fat burning" mode and that no fat is burned in "cardio" mode. The fact is that you burn fat during both modes. But the major problem is that the fat-burning mode is typically too slow a workout for many people to maximize benefits. In fact, at the end of a fat-burning workout, you could end up burning fewer calories and less total fat than during a cardio-mode workout. Here's an example of what I mean.

Suppose a 150-pound moderately fit man walks on the treadmill for 60 minutes at 3.0 mph ("fat burning" mode). That's 300 calories for a 150-pound man (a 150-pound man burns 100 calories per mile whether he walks or runs). Since this man is moderately fit, he will burn approximately 60% of the calories from fat (180 calories) and 40% from carbohydrate (120 calories).

Now let's say the same 150-pound man walks on the treadmill for 60 minutes at 4.0 mph ("cardio" mode). That's 400 calories burned, with approximately 50% of the calories from fat (200 calories) and 50% from carbohydrate (200 calories). The percent of fat burned may be less at 4.0 mph than 3.0 mph because the exertion is higher and so theoretically less oxygen is delivered to the muscles.

If you examine the example carefully, you will notice that at the slower fat-burning mode the man does indeed burn a higher percentage of fat compared to cardio mode (60% v. 50%), but in cardio mode, he burns more total calories (400 v. 300) and more total fat (200 calories v. 180 calories). My suggestion is to ignore the fat-burning mode (unless you want a less intense workout). You're not going to burn more fat in this mode than in cardio mode, and it could end up being an inefficient use of your time. I suggest training as hard as you comfortably can without risking injury so that you maximize the calorie and fat burn and the overall cardiorespiratory training effect.

Calculating a Target Heart Rate Zone

I recommend the heart rate reserve method (HRR) for calculating a heart rate zone. Heart rate reserve uses the range from your resting heart rate to predicted maximum. Below is the formula and an example of the method for someone 29 years old, assuming a resting heart rate of 68 bpm and a training range of 70%. You can get other ranges if you plug in other values.

    1. 220-Age = HRmax

    2. Subtract resting heart rate from HRmax = Heart Rate Reserve (HRR)

    3. Multiply HRR times the percent that you want to train at

    4. Add back resting heart rate

Assuming a resting heart rate of 68 bpm, 29 years old, and a 70% training range:

    1. 220 - 29 = 191

    2. 191 - 68 = 123

    3. 123 x .70(%) = 86

    4. 86 + 68 = 154 bpm

In this example 154 beats per minute is 70% of HRmax. The American College of Sports Medicine recommends a training zone of 40/50%-85% of HRR for developing and maintaining cardiorespiratory fitness.

Errors in Predicting Maximum Heart Rate

Calculating target ranges in individuals over age 40 can be inaccurate because of errors in estimating HRmax due to considerable heart rate variability in older adults. This means that the popular equation to estimate HRmax, "220-age", may not be accurate in individuals older than 40 years. The error is probably due to the origin of the equation which was derived from volunteers who were most likely not representative of the general population.

In an important study published in The Journal of the American College of Cardiology in 2001, researchers examined data from 351 studies (18,712 subjects) and determined that the "220-age" equation underestimates maximum heart rate in older adults (the older the individual the more the error). In their conclusion, the researchers suggest the following equation to estimate HRmax in older adults:

208 - (0.7 x AGE)

Using this new equation, a 70-year-old's predicted HRmax is 159 bpm (208 minus 49). In the original "220-age" equation, it's 150 bpm (220 minus 70), or nine beats per minute lower. The problem with using a lower HRmax to calculate your training zone is that you end up with a training zone that might be below your actual capacity, which means that you could potentially miss out on fitness benefits. My suggestion if you are over age 40 is to try it both ways and monitor how you feel during the workouts. Stick with the workout intensity that leaves you feeling at least warm and slightly out of breath but does not put you at risk for injury. You should speak with your doctor if you have any questions about your target heart rate.

NOTE: There will not be any difference in HRmax when you factor in age 40 to the old and new equations. The difference in HRmax between the two equations only becomes significant when you factor in older ages.


Heart rate monitors are devices that measure heart rate in real time. They have grown wildly in popularity over the past 10 years partly due to the miniaturization and accuracy of computer chips. Many athletes use heart rate monitors during their workouts to determine if they are in the proper training zone. But heart rate monitors aren't just for elite athletes. I recommend a heart rate monitor if you like gadgets or think you might like the heart rate data and real-time feedback from your body that these devices provide. The standard design is a strap with a transmitter that you wear around your chest and a wristwatch with a receiver. The chest transmitter detects your heart rate during exercise and wirelessly sends the signal to the wristwatch display for you to see. You can purchase all the bells and whistles with functions like downloading the entire workout to your computer or alarms that let you know when you go too high or too low in your training zone, or you can go minimalist and purchase the basic model that just reads your heart rate. Nike, Polar, and Timex are just three reputable companies that manufacture heart rate monitors.