If I ride a given route quickly, it will leave me more tired than if I ride it slowly despite the fact that I spend more time riding at the slow speed. That assertion is widely accepted by the vast majority of exercise scientists, coaches, and athletes. How to quantitate that is more controversial. For some purposes, speed (e.g. miles per hour) is used for such a quantitation. For example, many coaches recommend periodic time trials on a fixed course where the speed at which one can ride that time trial is a measure of Fitness. But because simple speed is affected by so many factors (e.g. hills) it is not usually used as a routine measure of Intensity, the generalized term used to quantitate the impact of different riding speeds. The most common measures used to routinely calculate Intensity are heart rate, power, and relative perceived exertion (RPE.) RPE is the simplest, it is just how hard a ride feels. That is the metric I used in last month's post to determine if I had been riding too much or too hard.
Power is something like a normalized version of speed in that it corrects for factors like hills that affect speed. It is measured using a power meter which is incorporated into the drivetrain of the bicycle. These are widely available if somewhat pricey. Power (like speed) needs to be calibrated for differences in the fitness and ability of different athletes. 150 watts of power might be at the upper end of the amount of Intensity I can manage while being an easy ride for a better cyclist, and the meaning of 150 watts would be different for me at the beginning of the season when I am not very fit versus at my peak fitness after I have trained for an important event. I have never owned a power meter and so I use speed on standard routes as something of a proxy for power.
Two months ago, when I posted about Zone 2 training and how the reporter Manon Lloyd used herself as a guinea pig to investigate Zone 2 training, something I did not mention is that when Lloyd developed a training plan with her coach, he specifically advised her to use both a power meter and a heart rate monitor. Why both? In many ways, power is considered to be the superior metric. It responds instantly to changes in Intensity and is independent of many factors that affect heart rate, things such as stress, caffeine, etc. However, power is rapidly affected by training and that changes are significant. At the beginning of her six weeks of training, if Lloyd rode at the top of Intensity Zone 2, she could generate 140 watts of power. At the end of her six weeks of training, she could generate 160 watts of power at the same level of Intensity. Thus, had she used only power to guide her training, she would have been training at too low an Intensity at the end of her training. Heart rate is also affected by training, but on a much longer time scale. Although not given in the videos, it is likely that Lloyd's Zone 2 heart rate at the beginning and end of her six weeks of training would have been similar or identical. In short, for the purpose of keeping her effort within Intensity Zone 2, power was the better metric over the course of one ride but heart rate was the better metric over the course of six weeks of training.
Although I use RPE as my primary measure of average ride Intensity, I use average heart rate as an important secondary measure. The goal of this post was to determine how reliable average heart rate is and specifically, to compare average heart rate of a ride to the speed of that ride on a standard route. I used two standard routes for that comparison, my Cañada route and my Alpine route. As noted above, the impact of training on ride speed would be expected to confound that comparison. As I train and become more fit, I would expect my speed a given heart rate to increase. To remove that confounding factor, I did my comparison over a period of time during which I felt that my fitness stayed relatively constant. The results for the Cañada route are shown at the top of this post. The results for the Alpine route are shown here:
Encouragingly, the results for the two routes were similar. I will focus here on the results for the Alpine route. This graph shows the results for 19 rides on the Alpine route from June 30, 2024 and October 24, 2024. It begins after I had gotten in shape for metric centuries, a period during which I was attempting to maintain my fitness but not trying to increase it. For each ride, I plotted the average speed of that ride on the vertical axis as a function of the average heart rate for that ride on the horizontal axis. The line is the best fit line through those points. The equation for that line (which can be ignored) is shown at the top of the graph as well as the value for R-squared (R2.) R-squared is a measure of how effective heart rate is at predicting ride speed. R-squared varies between 0 and 1, and if both heart rate and ride speed were perfect measures of ride Intensity, R-squared would be 1. The value measured for the Alpine route is approximately 0.7, very similar to that measured on the Cañada route. This is a pretty good correlation between speed and heart rate which means that both of them are pretty good measures of Intensity, but that one or both of them are imperfect measures. Besides Intensity, what might affect ride speed? The two things I can think of are fitness and wind. As noted above, I have done my best to control for fitness and my intuition suggests that wind is not a major factor. Besides Intensity, what might affect heart rate? Unfortunately, many things: caffeine, emotional state, and fatigue* to name but a few. This would suggest that speed is a better indicator of Intensity than is heart rate, so why not just use speed?
There are two reasons not to use speed instead of heart rate as a measure of Intensity:
- Speed is only a good indicator for one specific route. It is possible that I might be able to map some kind of equivalence for two commonly ridden routes like the Alpine and Cañada routes, though I worry that even this level of data processing could lead to false conclusions. Perhaps more importantly, many of my rides are ridden over a wide variety of routes, none of which I ride commonly enough to provide the data needed to map speed onto Intensity. That is why many cyclists use power meters.
- The speed corresponding to a given level of Intensity changes with training. The graph at the top of this post shows data for rides on the Cañada route between June 17 and October 26 of 2024. If I do a similar analysis for rides on the Cañada route between July of 2022 and October of 2024, the R-squared value drops from a respectable 0.70 to a useless 0.36. This is because variations in how much I was training and my resulting fitness varied substantially over that longer time period.
Where does this all leave me? In a pretty good place, I think. It is definitely true that heart rate is an imperfect measure of Intensity for any single ride. During the June through October 2024 time period, four of my rides on the Alpine route resulted in an average heart rate of 126 beats per minute. The speeds of those four rides varied between 10.8 and 11.5 miles per hour. However, the most important use I make of average heart rate applies not to single rides but rather to accumulation of fatigue over time, and for that purpose, these variations tend to cancel each other out so that average heart rate is a perfectly adequate metric. In that context, the R-squared value of 0.7 I determined from the comparison of speed and heart rate gives me confidence that recording my average heart rate is helpful to my training. But how do I use that to estimate accumulated fatigue? I plan to discuss that in a future post.
* Many coaches advise that when an athlete is fatigued, their heart rate will be lower than usual at a given level of effort. A rationalization sometimes given for that result is that the athlete's heart is "too tired" to beat any faster.
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