I have two reasons for riding my bike on a regular schedule. The less important reason is to be ready for long distance fun rides with my friends. The more important reason is for my health. What is the advice of the Medical Community for how much aerobic exercise (e.g. bicycle riding) I should do? Pretty universally, the Medical Community recommends a minimum of 150 minutes a week of "Moderate" exercise or 75 minutes a week of "Vigorous" exercise. These can be mixed and matched, e.g. one can meet that recommendation with 50 minutes of Vigorous exercise and 50 minutes of Moderate Exercise. This is a minimum recommendation. The ideal is twice that, 300 minutes a week of Moderate Exercise or 150 minutes a week of Vigorous Exercise in any combination. The key issue for me is the definition of Moderate and Vigorous Exercise. Originally, I kept things simple by assuming that all my cycling constituted Moderate Exercise. However, after looking at the guidance provided by various branches of the medical community as to what constituted Moderate and Vigorous, I felt like that was a very conservative assumption, that much of my cycling was at the Vigorous level. About 2012 I started collecting various heart rate zone systems. One of them was one that defined Moderate and Vigorous exercise in terms of heart rate, and it came from a source I trusted, the Mayo Clinic. That system said that Moderate Exercise is 50 to 70% Maximum Heart Rate and that Vigorous Exercise is 70 to 85% Maximum Heart Rate. If I use an estimate of 180 for my Maximum Heart Rate, the Moderate exercise zone goes from 90 to 125 beats per minute and the Vigorous from 126 to 153 beats per minute. Because so much of my riding was at a heart rate in the Vigorous zone, and because my recovery rides, easy rides from a training perspective, qualified as Moderate Exercise, I found it easy to reach the recommended amount of aerobic exercise. Recently, I went back to the Mayo website to confirm some aspects of this classification and I found that they had revised their recommendations; they now use a different set of zone definitions. Using that new system and my maximum heart rate of 180 beats per minute results in Moderate exercise being 125 to 147 bpm and Vigorous being 147 to 164 bpm. This is a huge difference. This means not only are my Recovery Rides no longer Moderate Exercise and thus no longer count towards my 300 minutes but also that a significant fraction of my Zone 2 rides don't count either. Further, very little of my riding qualifies as Vigorous. To be honest, even before seeing these new recommendations I had been worrying that the old set of zones seemed a bit too easy. However, the second set of zones definitely seem much too hard. Not only would they make it difficult to impossible for me to meet the health guidelines, I would have to deviate significantly from the carfully thought out and extensively tested training plans I am currently following to even try to do so.
What accounts for the enormous change in the guidance the Mayo Clinic offers to Americans trying to figure out how best to exercise? This change is the result of Mayo switching from Percent Maximum Heart Rate to Percent Heart Rate Reserve for calculating the boundaries of of Moderate and Vigorous exercise. What is heart rate reserve? It is the difference between resting heart rate and maximum heart rate. For me, my maximum heart rate is 180 bpm and my resting heart rate is 70 bpm so my heart rate reserve is 110 bpm. The way the new method works is that heart rate reserve is multiplied by appropriate percentages and then the resting heart rate is added back. Using the percentages in the latest recommendations from Mayo, Moderate and Vigorous exercise are 50% to 70% and 70% to 85%, respectively, then the boundaries for the new system are:
(50% x 110) + 70 = 125 bpm
(70% x 110) + 70 = 147 bpm
(85% x 110) + 70 = 164 bpm.
But where did Mayo get these percentages? Why should they be the same as for the old Maximum Heart Rate system? Spoiler alert: I think the whole thing was the result of a misunderstanding by the Mayo Clinic and should be ignored. Who am I to accuse the Mayo Clinic of "a misunderstanding" especially after describing Mayo in this very post as a source I trust? Clearly I have to defend that charge, and in addition, I have to accept responsibility for my part in this confusion. I do consider the Mayo Clinic one of the most prestigious and trustworthy institutions in the medical establishment. The problem is, the medical establishment is really good at some things and less good at others, nutrition and exercise being areas in which the medical community is less good. My mistake was thinking that the Mayo Clinic would be an exception to these limitations, I put more trust in them with regards to exercise than I should have. The first thing that should have given me pause is their suggestion of using an age-determined estimate for maximum heart rate. In their old recommendations, they suggested the classic estimator of:
Maximum Heart Rate = 220 - Age
For me, that would be 220 - 74 = 146 bpm. Given that I routinely hit heart rates of over 160 bpm and that, about a year and a half ago, I reached a heart rate of 176 bpm, this is absurd. For their new recommendation, they suggest a more modern formula:
Maximum Heart Rate = 208 - (0.7 x Age)
For me, that would be 220 - (0.7 x 74) = 156 bpm, a bit better but still absurd.
Wikipedia notes that although these kinds of formulae are fairly good at estimating the average heart rate of a population, individual athletes can have maximum heart rates that are 30 bpm higher or lower than that average. For me that would be 116 to 176 for the first formula and 126 to 186 for the second. The fact that Mayo did not discuss or allow for this should have alerted me to their lack of sophistication. In fact, even back in 2012, I rejected the advice of Mayo on how to determine maximum heart rate but then failed to be as skeptical of the rest of their recommendation. It was only when their advice changed so dramatically that I called the entirety of their recommendations into question. What was the basis for their new recommendation?
When I researched Heart Rate Reserve, I found that it is associated with a Norwegian exercise scientist by the name of Martti J. Karvonen. Karvonen himself proposed definitions for Moderate and Vigorous exercise which, like Mayo's new definitions, use Heart Rate Reserve, but uses significantly different percentages to define the zones: 40% to 60% for Moderate Intensity and 60% to 90% for Vigorous Intensity. For me, that works out to 114 to 136 bpm and 137 to 169 bpm, respectively. Interestingly, these ranges are close to the definition of Hughes Zone 2 and Zones 3 + 4, respectively, and they match the recommendations of both the American College of Sports Medicine (ACSM) and the Centers for Disease Control (CDC) which the recommendations of Mayo do not.
Perhaps Mayo was simply the wrong choice of an institution. Perhaps I should follow the ACSM either directly or via the larger entities that pick up their advice, organizations like the CDC. One reason not to do that is that both the ACSM and the CDC use one of those age related formulae (220 - age, 208 - 0.7 x age) to determine maximum heart rate, an approach in which I do not believe and which does not work for me. Besides Wikipedia, many coaches object to this approach as well. Why then is the medical establishment so in love with these formulae? I don't know, but if I had to guess, I would say the medical community would defend themselves by saying that direct measurement of maximum heart rate is extremely difficult and even dangerous. In order to not let best be the enemy of good, they went with the less accurate but much easier and safer formulae above. They might acknowledge that I am a patient for which these formulae do not work but then note that there are many patients for which they do and so they have to reluctantly sacrifice me in the interest of helping the majority of patients.
But what is the scientific evidence in favor of any of this? I have researched this and as best I can tell there is little to no scientific basis for assigning any heart rate ranges to the Moderate and Vigorous levels of exercise. The medical community believes there are lots of health benefits that result from aerobic exercise; the ability to complete everyday tasks, reduction of depression, for example but the main benefit that is studied is reduction of "premature" death. The word "premature" here might seem a little confusing but what it really boils down to is how long you will live. If you engage in aerobic exercise you will, on average, live longer. Estimates of how much longer vary wildly (5 months to 7 years) and that wild variation is both interesting and humbling but is beside the point for this post. What we are focusing on here are how Intense that exercise should be, and, given an Intensity, how much should one ideally do? The official guidelines suggest that, ideally, one should do 300 minutes per week of Moderate exercise or 150 minutes a week of Vigorous exercise to maximize lifespan. This is based on a wide range of research, but perhaps the most relevant is that which asks people how long and how intensely they exercise. So finally we get down to it: what measure of Intensity is used in such studies? The most common one is based on the kind of exercise: walking, jogging, running, swimming, cycling, etc. Walking is Moderate. Jogging or Running is Vigorous. And most interestingly, all Cycling is counted as Vigorous exercise! I truly believe that the authors of these studies are well aware of how crude a measure this is, and once again, that results from not letting best be the enemy of good. As imperfect as this classification is, it is the best data that is available. The reason there are no scientific studies of this kind that use heart rate to classify Intensity is because so few people who might be in these studies have heart rate data. I do not believe there is any scientific evidence supporting the assignment of heart rates to the medical Intensity zones of Moderate and Vigorous, the best one can hope for is common sense. My common sense tells me that the new Mayo estimates are far from correct. For now, I am going to use Coach Hughes' Zone 2 as my definition of Moderate Exercise and Zones 3 and above for Vigorous exercise. In addition, I will use the results of Gillen et al. to define an Interval Session consisting of six all-out 20 second sprints separated by two minute recovery periods as providing the same benefit as 90 minutes of Moderate exercise. I am far from certain that these definitions are accurate and I am open to revising them should I encounter reliable evidence suggesting better definitions, but I feel like they are the best I can do for now.
While researching for this post, I came across those who dismiss heart rate as an acceptable way to discover Intensity Zones and who advocate in favor of ventilatory threshold, respiratory exchange ratio, or blood lactate levels. If they are experienced coaches who have used this kind of data to improve the results obtained by their athletes, then I take them very seriously. If, on the other hand, they are advocating for using these measurements to establish the boundaries of Moderate and Vigorous exercise on purely theoretical grounds, then I will remain skeptical. If they want to convince me, they need to provide actual experimental evidence supporting their arguments. I don't believe there is any such evidence but very much hope to be proven wrong.
I would like to propose a contrarian hypothesis of my own:
"The bulk of the increase in longevity that comes from aerobic exercise comes from the fact that such exercise strengthens the heart muscle. Thus, heart rate is the perfect thing to measure to study the benefits of aerobic exercise. However, there are no Moderate and Vigorous zones, rather there is a continuum. The higher the heart rate during exercise, the faster the benefit is accrue and the relationship between heart rate and the benefit per minute is a smooth (if not necessarily linear) curve. It is your choice, cycle slowly for many hours or sprint all out for a few minutes, it's all the same."
Do I really believe this hypothesis? No, it is a straw man argument and, at best, an oversimplification. Very few people read this blog and I doubt that any of them have any influence in the exercise and health branch of biomedical research but if this post were being studied at the highest levels of the health establishment, what would I want them to do with this hypothesis? I would want them to design and execute experiments to test it, not as a true or false binary, but to ask the question if there is any truth to it at all, and if so, how much?
So where do I go from here? Nowhere, as it happens. In all my years of cycling, I have never changed how or how much I cycle to try to improve its health benefits. My hypothesis has been that the most important thing for my health is to keep cycling, so I do whatever keeps me motivated. Perhaps there has been some thought that "the more cycling, the better" and every scrap of research I have ever come across has supported that. Of course, that cycling needs to be sustainable, overtraining in the short run leads to less cycling in the long run so, especially as an old man, I need to avoid that, but with that in mind I will follow the advice of the famous medical research scientist, Eddie Merckx: "Ride lots."
