Monitoring Exercise Intensity

Perceived Exertion

What I’ve described previously for monitoring intensity of aerobic exercise using your breath is a simplified version of judging intensity by “perceived level of exertion” (also called “rating of perceived exertion”). Recently RPE has been shown to correlate very well with heart rate [1]. I think it’s possible to make too big a deal of this with a detailed scale, there are basically two important levels of exertion: one which is within your current comfort zone (“brisk but comfortable”), and one that stretches your limits by pushing somewhat beyond what’s currently comfortable. If you’re the type that tends to go too hard on recovery days following a hard effort, a third level is also useful- just “comfortable” or “easy”.

Some coaches and exercise scientists are not too big on using perceived exertion because it’s subjective. They like the idea of being able to apply a precise, repeatable stimulus in training, using a more objective measure like heart rate or power monitoring. Maybe it’s because I’m an amateur, but I don’t subscribe to that. I prefer perceived exertion because I’m “listening to my body”. Yoga teacher Baron Baptiste talks about going beyond what’s currently comfortable for you as “working with your edges” [2]. I like that concept: you push yourself to the edge of what is currently comfortable for you, then improve by going a little beyond it, whether it’s a slightly faster pace, more reps in strength training, or a little further in a stretch. By emphasizing “slightly” uncomfortable you steadily improve with much less chance of injury, as opposed to the more drastic “no pain no gain” approach. This method also automatically tailors the stimulus to the individual, which is recommended by more than one health agency for physical fitness [3]. As for repeatability, I find perceived exertion to be a pretty repeatable measure, at least for me: whenever I double-check something more objective like the amount of power I’m putting out, the amount corresponding to a perceived level of intensity is about the same (or hopefully going up slowly over time because training is making me fitter). And as we’ll see below, the most popular objective measure of training intensity, heart rate, has more subjectivity than is commonly recognized, and is often based on an inaccurate and unscientific formula. There are also caveats about using heart rate monitoring for special populations, such as children or older people, and people with various medical conditions. It also changes over time with continued exertion at the same effort level, due to “cardiac drift“. In each of these cases, it’s recommended to fall back on perceived exertion instead of heart rate [4].

Heart Rate Monitoring

If you do want to try something besides perceived exertion to monitor your intensity, the most widely used measure is heart rate. Some books, especially older ones, will show how to measure your heart rate without a monitor, usually by counting your pulse for 10 seconds then multiplying by 6. This is inaccurate, because you have to stop exercising to count your pulse, and your heart rate can drop a bit from it’s value during exercise. So if you want to try heart rate training, I’d recommend using a monitor. Basic ones are pretty inexpensive nowadays, and some exercise machines at gyms have them built in. Your heart rate while exercising is a pretty good measure of intensity. To use it properly, you must know comparison values like your maximum heart rate or heart rate at LT and your heart rate at rest. You can determine your resting heart rate (RHR) by checking your pulse upon arising in the morning. In addition to helping you estimate intensity, resting heart a good indicator of overtraining (it will start drifting up in the morning if you are overdoing it).

Power Meters

Power meters for cycling have been around since the 1980s. They are a very useful measure of intensity, but until pretty recently have been very expensive. Now options are available for a few hundred US dollars. The classic book on using them is Training and Racing with a Power Meter by Hunter Allen, Dr Andrew Coggan, and Dr Stephen McGregor.

Much more recently, power meters have become available for running also, there are detailed discussions of them at Velopress and at Runner’s World. Measurement of power for running is less straightforward than for cycling, it is either estimated from the motion of your center of mass, or from various measurements from a sensor inside your shoe.

Power is probably the most precise way to monitor your training. It is used heavily by professional athletes and by serious amateurs concerned about improving their performance. I personally stick to PLE because I’m not as worried about my performance per se as I am about indications that I am improving with time, like “it used to take me 10 minutes to climb that hill now I make it in 9:18”. This blog is concerned with healthy aging, part of which is motivation. I don’t think fancier measurements are needed for healthy aging, but if they help with your motivation, by all means use them.

Accuracy of Age-based Maximum Heart Rate Estimates

As we saw under heart rate monitoring, suggestions for exercise intensity in your training are often based on a percentage of your maximum heart rate. Unfortunately, many trainers and books cite the formula “220-your age” for estimating maximum heart rate. This can be easily off by more than 20 beats on the high or low side. This formula is often quoted without any warning about it’s potential inaccuracy, and in addition to the inaccuracy, it turns out it has little scientific basis [5,6]. In a more recent study the concept of a formula for predicting age related max heart rate was revisited [7]. Based on thousands of subjects, male and female, ranging in age from 18 to 81, the authors came up with a “best fit equation of

Max heart rate = 208 -0.7xAge.

They also went one step farther and did separate fits for men and women, coming up with

Max HR = 208.7 -0.73xage for men

and

Max HR = 208.1- 0.77xage for women.

However, if you look at the data this is based on, it has a trend towards heart rate decreasing with age, but there’s a lot of scatter.

These new formulas are a little more accurate than the old one, but can still underpredict or overpredict max HR by 15 beats or so. A recent review of many attempts to come up with a formula to predict max heart rate concluded that no sufficient accurate formula exists to predict max heart rate from age alone [6]. In my opinion none is possible because of the large amount of scatter in the data. Exercise physiologist Dr. Fritz Hagerman, who has studied world-class rowers for 3 decades, has said that the idea of a formula to predict an individual’s maximum heart rate is ludicrous: Remember, if max heart rate just depends on age, then everyone at the same age should have the same max heart rate. But Dr. Hagerman has seen Olympic rowers in their 20’s with maximum heart rates of 220, and others on the same team, same age, and same ability, with maximum rates of just 160 [8].

[update 06/17/21]: A 2012 study [9] did an analysis using data on thousands of subjects that were known to be physically active. They came up with the prediction formula max heart rate = 211 − 0.64·age. This is only a few beats off for me. The authors claim it is accurate to within about 11 beats. It’s off by about 7 for me. I plotted the 220-age vs. the Tanaka formula 208-0.7*age, and the formula for active adults, 211 − 0.64·age. It appears 220-age is the most conservative as shown in the plot I created below. That may be why it is still used for cardiac testing purposes:

Many books have charts with elaborate training schedules based on various zones of intensity, all based on estimated maximum heart rate. I happen to know from more accurate tests that my max heart rate is more than 20 beats higher than 220 -age. I’d hardly break a sweat if I exercised at the levels from those charts. But more importantly, for some people the opposite is true and their maximum heart rate can be more than 20 beats lower than the formula predicts. If they were to exercise at the levels from the charts, their intensity could be too high, especially for anyone with a medical condition . It all may look very scientific, but it’s not too worthwhile if it’s based on an inaccurate number.

The other thing to keep in mind is that activity trackers (fitbit, etc), may be using formulas like these to estimate some of the results they are showing, like how many calories you burned during your workout. To put I mildly, take these with a grain of salt. Use it for motivation, but not for “great, I burned 317.6 calories, I can eat 3 of these cookies!”.

The “Fat Burn Zone”

As seen in the figure, the exercise intensity charts mentioned often show a “fat burn zone”, which is supposedly the exercise intensity at which you will burn the most fat during your workout. There are several things wrong with this concept, which is discussed at length on Clarence Bass’s website. The fact that his article is entitled “Forget The Fat-burn Zone” is your first clue as to where Clarence stands on this. First, if you have a properly functioning metabolism, discussed in my post on “Diet and Fat Adaptation”, it does not matter whether you burn fat or sugar during your workout. You create a caloric deficit while exercising. Your body will turn up your fat burning rate later when you are not exercising to make up for that deficit.

Second, though you burn a higher percentage of calories of fat from exercise when you’re in this zone, you burn a lot less total calories. Here’s an example: suppose you burn 300 calories in your session, 70% of it from fat, so you burned 210 calories from fat. If instead you worked out harder, you might burn 400 calories but only 60% from fat. That is 240 calories from fat, more than if you had been in the magic fat burning zone. And as I’ve emphasized previously, burning calories is not the most important thing about exercise anyway.

Third, this estimate of what heart rate the max rate of fat burning occurs at is based on average estimates which can be very incaccurate, because it is by no means one size fits all, it depends on factors like your diet and fitness level. Finally, the charts of fat-burn zone use the dubious age-based heart rate formula just discussed above.

References

1. Green, J, et al, “RPE Association With Lactate and Heart Rate During High-Intensity Interval Cycling”, Med. Sci. Sport Exer., 2006.
2. Baptiste, B, Journey Into Power, Fireside, 2001.
3. Pate, R, et al, “Physical Activity and Public Health, A Recommendation From The Centers for Disease Control and American College of Sports Medicine”, JAMA, 2000.
4. American College of Sports Medicine, ACSM’s Guidelines for Exercise For Exercise Testing and Prescription, Lippincott, Williams, and Wilkins, 2005.
5. Roberg, R, and Landwehr, R, “The surprising history of the ‘HRmax=220-age’ equation”, Journal of Exercise Physiology, 2002.
6. Kolata, G., Ultimate Fitness: The Quest For the Truth About Health and Exercise, Farrar Strauss Giroux, 2003.
7. Tanaka, H, et al, “Age-predicted Maximum Heart Rate Revisited”, Journal of the American College of Cardiology, 2001.
8. Kolata, G, “Maximum Heart Rate is Challenged”, The New York Times Health Page, April 23, 2001.
9. Nes, B, et al, “Age-predicted maximal heart rate in healthy subjects: The HUNT Fitness Study”, Scandinavian Journal of Medicine & Science in Sports, 2012.