Tuesday, June 14, 2016

Intervals and Common Sense

The 1969 Great Western Bike Rally, a celebration of moderate exercise.

"There are sadistic scientists who hurry to hunt down errors instead of establishing the truth." - Marie Curie

In my last post, I was a sadistic scientist. There were reasons I for that, but in this post, I'd like to take a more constructive, common-sense approach to understanding the relationship between different kinds of exercise and long term health. Specifically, I would like to address the following questions:
  1. How sure are we that exercise improves long term health?
  2. Is VO2max a good biomarker for the impact of exercise on long term health?
  3. Do the short term impacts of different exercise plans on VO2max accurately predict their long term impacts on VO2max?
  4. What is the value of High Intensity Interval Training (HIIT)?

Last Time on the Zombie Cyclist:

In my last post, I discussed an excellent paper (Gillen et al.) which argued that three 20 second sprints produces the same long term health benefits as 45 minutes of moderate intensity cycling. I used that post to illustrate the limitations that go with a biomarker study (a study where you investigate not what you are interested in, but something easier to measure that you believe is linked to what you are interested in.) In the case of Gillen et al., what they were interested in were the relative long term health benefits of different exercise regimens. Unfortunately, these take decades to measure, so what they studied instead was the effect of different exercise regimens on VO2max, glucose tolerance, and the mitochondrial content of muscle. They examined changes in these three different factors produced by 12 weeks carrying out one of three different exercise plans. The three plans were no exercise (the control group), 45 minutes of moderate intensity cycling done three times a week, and three workouts a week built around three 20 second "all out" sprints. Both exercise groups improved by about the same amount, leading them to conclude that the much shorter sprint workout was as good as the longer moderate exercise workout. At the end of that post, I stated that, based on rigorous scientific criteria, I found the paper interesting but not entirely convincing. As a scientist, my formal reservations about the conclusions of Gillen et al. were two-fold:
1) The three biomarkers chosen have not been demonstrated to be reliable proxies for long term health.
2) It has not been demonstrated that the results observed after 8 and 16 weeks of exercise will continue for the decades relevant to long term health.

My intuition tells me that the biomarkers chosen probably are a good stand-in for whatever effects of exercise contribute to long term health. However, I am less certain that the benefits of Gillen et al.'s specific exercise plans will continue long term. As I will argue below, I personally think more varied exercise plans might a be better long term plan.

Does Exercise Improve Long Term Health?

The influence of exercise on longevity and health has been extensively studied and the benefits appear to be substantial. Men in their 50's have a 42% chance of living to 85 if they exercise regularly compared to 23% if they don't. (For comparison, the numbers for not smoking are 37% vs 18% if you do and of not being obese are 35% vs 13% if you are.) Yes, yes, we have all heard about the curmudgeon who never exercises and drinks a quart of whisky and smokes two packs of cigarettes a day and is 97 years old; it can be done, 18% is not 0%. But there is no escaping the fact that your odds are a lot better if you take care of yourself. Such studies are (almost?) always observational, and so there is the theoretical possibility that the people who choose to exercise make that choice because they are different to begin with, that it is this pre-existing difference that is responsible for their longevity, and that they would have lived to a ripe old age even if they had not exercised. This is not as far-fetched as it may seem. Some people are healthier than others for a whole range of reasons. Healthier people live longer and feel better. Because they feel better, they might be more likely to exercise. This kind of ambiguity is present in all observational studies. That duly noted, I think the benefits of exercise are broadly accepted nonetheless, and that this acceptance is due to the large number of studies that have been done, the consistency of all the evidence, the substantial size of the effect, and the intrinsic plausibility of the conclusion. Thus, I will answer this question with a strong YES.

Is VO2max a Good Biomarker for Exercise?

VO2max varies enormously between people, but most of this difference is genetic. No matter how much you exercise, you can never catch up with someone who has "good genes." (The good news is that VO2max is not the most important determinant of who is a fast cyclist, and the parameters that are, are more trainable.)

Most studies of exercise and aging work by assembling a group of people, determining how much they exercise, and then revisiting them some years later to see if they are still alive. There are two ways that "how much they exercise" is commonly determined. The first is to ask people. This is unreliable because people may not accurately remember or they may exaggerate. To get around these problems, the other way is to measure how much they have been exercising by measuring their VO2max. Because VO2max does increase with exercise, people who exercise will, on the average, have a higher VO2max. But this is only true on the average. Because VO2max is determined more by genetics than exercise, many people who exercise will have lower values for VO2max than many people who do not. However, since the whole study is based on statistics, the group who has a higher average VO2max will, on the average, have exercised more. The fact that VO2max does work as a stand-in for exercise despite the problem of genetic variability raises an interesting question; what if what is important for longevity is VO2max, not exercise? Because exercise increases VO2max, it will increase longevity, but will people who inherit a high VO2max but do not exercise live longer or shorter lives than those who exercise but have genetically low VO2max? I don't know the answer to this question, nor do I even know if the answer is known.

The scientific community believes that the important variable is exercise with VO2max being a biomarker for exercise, so for the remainder of this post, let's assume that is correct. What this may mean is that someone who inherits a high VO2max does not live longer as a result. This then begs the question, does the VO2max you get from high intensity interval training get you the same benefit as the VO2max you get from exercising at a more moderate level for a longer time? Without being the least bit sadistic, I am at least somewhat skeptical that it does. Does that mean Gillen et al. is wrong, or at least unreliable? One of the most important ways scientists avoid being mislead is to look at the same question as many ways as possible, and that is one of several reasons I like Gillen et al.; they did look at the effect of HIIT on VO2max but they also looked at the effect of HIIT on muscle mitochondria and on glucose tolerance. It is much less likely that they were mislead by all three of these biomarkers than by any one, making me significantly more enthusiastic about Gillen et al.'s conclusions.

Am I Sold on HIIT?

Yes and no. When I first started thinking about training five years ago, I was skeptical of the value of any kind of brisk training, intervals included. I now believe intervals, and short, fast (high intensity) intervals in particular are good for health and will improve my performance in most kinds of rides (group rides, long rides, etc.) Even five years ago, I rode intervals now and then because pretty much all training programs included them, but I was unenthusiastic and often skipped them. Going forward, I am going to make a real effort to include at least one set of intervals every week. Further, I had been favoring longer intervals and longer kinds of brisk rides (30 minute time trials) believing they were better for building endurance. I am now going to include shorter, faster intervals. A few weeks ago, for the first time, I tried 20 second intervals as describing in Gillen et al. and found I reached heart rates significantly higher than I usually do. I had been skeptical that such a short interval would last long enough to bring up my heart rate, but it turns out I was wrong.

So that is the yes part, what about the no? I am not prepared to replace all my other riding with high intensity intervals. Even if all I was interested in was health I would not do so. I believe that the mechanism by which exercise improves health is complex and that our understanding has just scratched the surface of that complexity. The exercise community has embraced the value of a diverse exercise program for millennia, and although I will always follow science when it successfully overturns tradition, I don't think the weight of scientific evidence about exercise and health is sufficient to do that here.


  1. "The exercise program that you do is always superior to the one you don't do." - my younger son
  2. If I were betting on the long term prospects of folks who do a small amount of very high intensity exercise versus a group that follows a more conventional moderate exercise program, I'm not sure how I would bet, but I definitely would bet against a third group that did no exercise at all.
  3. If there was a fourth group who mixed it up, did some long, slow bike rides, did some sprints, lifted some weights, and did some yoga, I would bet on them over any of the above three groups.
  4. I am who I am. I am probably not going to lift weights. I am definitely not going to do yoga. But I do plan to mix up my cycling; to ride long when I can, to ride intervals when I am inspired, and to keep doing 45 minute rides at a moderate level of intensity when I can think of nothing better to do. Do I think this is optimum? Nope. Do I think it is better than nothing? Yep. Do I think I will actually do it? I hope so.

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