|My experience riding at different intensities. HR Zone (Heart rate zone) was related to BPM (Heart Rate in Beats Per Minute) as described in Joe Friel's "Cycling Training Bible." BPM and MPH (Speed in miles per hour) were measured using a Garmin Edge cycle computer and heart rate monitor. Power (in Watts) was estimated from speed using the website at http://bikecalculator.com/. Four intensity models were compared: RPE (Relative Perceived Exertion) was based on my subjective RPE estimated as described in Thomas Chapple's "Base Building for Cyclists". RNP (Relative Normalized Power) was calculated from my Power estimate as described in the text below. Similarly, calculation of HR (heart rate) is described in the text as well. The final column, labelled Gillen et al., used the heart rates for Moderate and Vigorous exercise as well as their relative intensities from the Mayo Clinic website and the relative intensities of Moderate vs. Intense exercise from the Gillen et al. paper. Based on the description of Intense exercise in that paper, I determined that it corresponds to HR Zone 5c. All intensity estimates in the final four columns were normalized to the intensity of a ride in HR Zone 2.|
I have spent more time on this blog post than any other I have written, and thus it is discouraging that this is also the post about which I am most uncertain. Preparing this post took so much time because of the amount of reading and thinking it required. My uncertainty comes from the fact that I argue that the entire exercise community is wrong about how to measure intensity, and I have very little standing from which to make such an extravagant claim. So, after all that apologizing, what is intensity?
Bicycle rides can be characterized by volume (e.g. how many hours a ride lasts) and intensity (e.g. how fast the ride was, all things considered.) Together, these factors determine load1, how much the ride improves fitness and causes fatigue. These two things are not necessarily the same, so for the purposes of this post, I am going to consider only how intensity affects fitness.
Volume seems like it should be relatively simple to measure and understand, even quantitatively; it would seem logical that a two hour ride would have twice the volume of a one hour ride. However, it is not certain that the second hour of a ride provides the same fitness benefit as the first did2, perhaps it provides more benefit, perhaps less. However, most coaches assume that, for the purposes of measuring volume, the first and second hours are equivalent, and for the purposes of this post, so will I. If quantifying load is problematic, quantifying intensity is much more so. In the first place, there are many different things people measure to get at intensity; speed, power, and heart rate for example. In the second place, there are many different ways people calculate intensity from such measurements. It seems obvious that a 20 MPH bike ride is more than twice as hard as a 10 MPH bike ride, but how much harder?
In addition to speed, power, and heart rate listed above, the volume of oxygen that is used per minute (VO2), the level of lactate in the blood, and relative perceived exertion have also been used as measures of intensity. Relative perceived exertion is the subjective measure of how tiring a ride feels and turns out to be a very useful measure despite its subjectivity. Speed is relatively easy to measure but is affected by hills, wind, type of bicycle, etc. Power is conceptually similar to speed except that it requires an expensive ($500 to $1,000) power meter to measure and is not affected by hills, wind, type of bicycle, etc., so is easier to interpret. In addition, it takes more than twice the power to go 20 mph than it does to go 10 mph, so power may come closer to reflecting intensity than does speed.
In contrast to speed and power which measure what is happening externally (to the bicycle) some of the other measures determine what it happening internally (to the cyclist). Heart rate is one such measure. Compared to power, heart rate can be measured inexpensively, $50 will purchase a perfectly adequate heart rate monitor. It is my impression that heart rate (beats per minute or bpm) is the most common way endurance athletes measure effort (with the possible exception of relative perceived exertion.) It is important to note that different measurements are, in fact, measuring different things. Heart rate, besides estimating load on the heart itself, seems to integrate a wide variety of bodily stressors, things such as heat, cold, fear, fatigue, etc. Blood lactate may be the best measure of fatigue of the leg muscles themselves, especially as they go anaerobic. VO2 measures the extent to which leg muscles are using their aerobic energy system.
How do the experts quantitate ride intensity? In "The Cyclist's Training Bible", Joe Friel suggests assigning a subjective "hardness" to each training ride, a number between 1 and 10, and to use that as a measure of intensity, a measure similar to Relative Perceived Exertion. As a (preferred) alternative, Friel recommends a software package, TrainingPeaks, which (among other things) estimates intensity. Based on what I read on the TrainingPeaks website, I believe the measure of intensity they use is Relative Normalized Power (RNP). The Power part of that is straightforward; it is what is measured by a power meter. The Relative is necessary because generation of 200 watts of power might represent much more intensity for me than it does for you. To make power relative, the power of a ride is expressed as the ratio of that power to Functional Threshold Power, the power used in a 30 minute time trial. Normalized means that the power is averaged over a training session. In "Base Building for Cyclists", Thomas Chapple recommends using Heart Rate Zone as a measure of intensity; a bike ride ridden in Heart Rate Zone 5 would have five times the intensity as one ridden in Heart Rate Zone 1.
To me, all of the measures of intensity discussed above seem arbitrary; there is no rationale given for why intensity should be proportional to subjective feel, heart rate, or power. When I researched the issue, I was unable to find any evidence in support of any of these. The only experimental data I have found that speaks to intensity measurement is Gillen et al. (which I have previously blogged about not once but twice) and Gillen et al. gives a very different estimate of intensity than do any of the expert-recommended measures. According to this paper, 1 minute of high intensity exercise (HR Zone 5c) provides the same benefit as 45 minutes of moderate intensity exercise (HR Zone 2.) As an intermediate point, the consensus of the medical community is that 1 minute of vigorous exercise - HR Zone 4 - is equivalent to 2 minutes of moderate exercise - HR Zone 2. All of this is summarized in the table at the top of this post. In contrast, the metrics suggested by Friel and Chapple suggest that high intensity intervals (HR Zone 5c) has 3 to 4 times the intensity as moderate exercise. Thus, Gillen et al. disagrees significantly from the consensus of the exercise community.
Is it reasonable to override the opinions of highly experienced and successful coaches like Friel and Chapple based on a single scientific publication? Normally, I would be reluctant to do so, but in this case I suspect that, if pressed, Friel and Chapple would not cling to their estimates, and thus I am inclined to go with Gillen et al. There are two reasons I think Friel and Chapple may not be wedded to their metrics. The first is that, in the context of their books, these metrics are mentioned only once and in a rather off-hand manner; they do not constitute a significant part of any training plans. The second is that if you look at the high intensity interval plans that Friel suggests (Chapple's book only covers base training and thus contains no high intensity intervals), the length and number of repeats of these intervals is more consistent with the higher estimate of their intensity provided by Gillen et al. than it is with Friel's own metric. I suspect that neither Friel nor Chapple use the intensity calculations from their books to design training schedules for their clients. Rather, I would suggest they draw upon their experience and introduce rides of different intensity when their experience tells them their clients are ready for them.
The Value of Intensity Quantitation
If experienced coaches like Friel and Chapple don't feel a need for an accurate measure of intensity, why should we? For the purposes of training (which is what Friel and Chapple care about) I suspect we don't. There are so many factors affecting training (e.g. illness, stress at work, person to person variation) that it would still be necessary to constantly adjust training plans even if there were an accurate measure of intensity. Is it possible that at the highest levels of the sport, the Tour de France or the olympics for example, an accurate metric for intensity would allow coaches to bring elite athletes to a slightly higher peak of performance? Perhaps, but I am not even certain of that. My interest is more in the science, the understanding of how exercise affects our bodies. In fact, the impetus for this post was a paper by Stöggl and Sperlich.
The Stöggl and Sperlich publication argues in favor of polarized training, a training plan that emphasizes training at either very high intensity or very low intensity and minimizes training at moderate intensity. To provide evidence for that generalization, they compared four training plans that varied in the percentage of time spent at different intensities, and found that the plan that was the most "polarized" was the best. I wondered if there could be explanations other than polarization as to why the "polarized" plan did the best. For example, it is broadly accepted that training plans can be too easy, too hard, or just right. Without knowing a great deal more than I do about the athletes participating in the study, it is impossible to know where in that spectrum any specific plan falls, but conceptually, it should be possible to see if the four plans are equally hard, or if some are harder than others. However, when I attempted to determine this back then, I found that I could not because I did not know how to compare intensities. Have I now gotten past that barrier? As noted at the top of this post, I am very uncertain of the metric for intensity I am proposing here, but that said, I will answer with a hearty "maybe." To answer this question, I first converted their LOW, LT and HIGH levels of exercise to HR zones3, translated the description of the different protocols in Stöggle and Sperlich into a list of how many minutes of exercise at each HR zone each protocol contained. Using intensities of 1 for Zone 2, 2 for Zone 4, and 45 for Zone 5c, I converted this to a total load for each protocol. These were then normalized such that the lowest intensity protocol had a score of 1. The result was that THR (lactate THReshold, exercise at time trial pace) had a relative load of 1, HVT (High Volume Training, long slow miles) had a relative load of 2, HIIT (High Intensity Interval Training, all out sprinting) had a relative load of 3, and POL (POLarized training, a mix of all out sprinting and long slow miles) had a relative load of 5. Thus, the benefit of the different protocols was related to their intensity, the more intensity, the greater the benefit. Perhaps it is this higher intensity, rather than polarization, that made the POL plan the most beneficial.
I opened this post by apologizing for having the presumption to question the accumulated wisdom of the exercise community, given that I have very little standing from whence to do so. That said, I stand by my conclusions, at least in general. The reason I think the exercise community has gotten this wrong is that quantitative intensity is rarely used, and thus coaches and athletes don't notice how wrong current estimates are. Rather, workouts are designed based on experience and adjusted based on the subjective feelings of the athlete, both approaches which I agree are highly successful. The only time quantitative intensity matters is in a research study, where these two pillars of success are, by necessity, ignored. So, although an accurate estimate of intensity may not be necessary for day to day training, it may be necessary for research on training, and that research has a lot to offer the exercise community in the long run. Thus, I believe an effort to better quantitate intensity is warranted and that Gillen et al. is a good start towards that goal.
1) Actually, this is external load. Total load included both this external load as well as internal load that comes from non-training factors such as illness, stress, and lack of sleep as well as the leftover fatigue of training done during the prior days and weeks. In this post, I will only consider external load, and use the word load to refer to that.
2) Qualitatively, most training books will tell you that one-two hour ride provides different benefits than two-one hour rides, but beyond that qualitative assertion, I have yet to encounter a quantification of such a difference.
3) The levels of intensity in Stöggl and Sperlich were expressed as levels of lactate in the blood. I used the data contained in Belcher and Pemberton, International Journal of Exercise 5: 148-159, 2012, to convert lactate levels to heart rate zones.