From "Intensity Training 2016" by John Hughes, modified from Allen, Hunger and Andrew Coggan, Ph. D. (2006) "Training and Racing with a Power
Meter." VeloPress, Boulder, CO.
Some comments on Coach Hughes' training Zones:
1) Sweet Spot is not an independent training zone, but rather an alternative to parts of Zones 3 and 4.
2) This is a practical, not theoretical set of zones. In that context, this is a 7 zone system with "Sprints" being Zone 7. Because it is difficult and unnecessary to measure heart rate (and to a lesser extent power) during an all out sprint, Hughes does not give metrics for this zone.
The main purpose of training zones is to help coaches communicate levels of intensity to their athletes. The coach has a training plan that they believe will maximally benefit their athlete given that athlete's strengths, weaknesses, schedule, and goals, and that it is just a matter of getting their athlete to execute that plan. In this context, all that is required is that the zones are sufficiently fine grained so that specification of a zone adequately limits the variability in the athlete's intensity, that from ride to ride a zone specifies the same level of intensity, and that there is some practical way for the athlete to measure what zone they are in during their training. Perhaps the two most common ways of accomplishing this last goal are by using heart rate (HR) or by using power output (measured by a power meter attached to the bicycle.) Relative Perceived Exertion (RPI) is a third metric than can be used as well.
The value of the metrics defining training zones are not the same from athlete to athlete. Zone 2 might run from an HR of 130 to 140 for one athlete and from 100 to 120 for another. It is the job of the coach to locate those boundaries for each of their athletes. To do that, coaches sometimes use metrics in addition to heart rate, power, and RPI: VO2 (sometimes including VCO2) and blood lactate are two common examples. (I will be explaining these metrics below.) The idea is that the coaches are looking for specific physiological boundaries that can occur at different relative heart rates or power outputs in different athletes, and these other metrics can help them find those boundaries. Once found, such boundaries can be associated with specific values of HR or power which then can be used by their athletes to track the intensity at which they are riding.
I have no complaint with any of the above. Where I depart from common practice is that sometimes coaches try to use training zones to monitor training load (accumulated fatigue) of their athletes. For example, some training books suggest that training load = length of ride x training zone number. For example, a 30 minute ride in Zone 4 would be assumed to produce the same amount of fatigue as a 60 minute ride in Zone 2. I have previously blogged a critique of this way of estimating training load and today's post is background for a followup to that critique. One reason coaches and athletes might be misled to use training zones in this way because the common metrics used to define these zones track each other pretty closely. In theory, coaches could set training zones to anything they like, but in fact most common training zones are set up so that each zone represents a similarly-sized range of heart rates and power levels and so the zones themselves seem to fit this same pattern. Thus, it is easy to believe that this pattern is more universal than it actually is. The question I will address in today's post is why do these metrics track each other so closely?
To attempt to explain why the standard metrics used to track training zones tend to be similarly spaced, I will be explaining in some detail the physiological meaning of these metrics. This explanation is quite long and fairly technical, I needed to make it that way to provide the foundation for future posts. However, if all you want is the bottom line, here it is:
The metrics used to delineate training zones are all related to power and energy, either the watts measured by a bicycle power meter or the calories and oxygen consumed by an athlete to generate those watts. That is why they track each other so closely. In my future post, I will argue that it does not necessarily follow that other outputs like training benefit or fatigue will do the same.
Power
The simplest of the intensity metrics to understand is Power. It is how much energy an athlete's muscles apply to the pedals each second. Uphill, downhill, windy, none of it matters, power is power. Of course, it might be much harder for one athlete to generate 300 watts of power than for another, so training zone boundaries measured by power will vary from athlete to athlete. One way coaches try to correct for that is to not measure power in watts, but rather relative to power at some defined level of intensity, for example, the power than can be maintained for 30 minutes, known as Functional Threshold Power (FTP.) Thus, many tables of training zones use %FTP to specify training zone boundaries.
Calories to Power
It is commonly known that the power an athlete's muscles apply to their bicycle's pedals comes from the food they eat. The amount of energy in food is measured in calories. Energy and power are not the same thing but they are closely related: power is the energy is used per unit time. Bicycle power meters measure power in units of watts. 100 watts of power is more or less what is needed to move a bicycle forward on a level road at about 15 miles per hour. If the conversion of food calories to watts was 100% efficient (which it is not) generating those 100 watts would require eating 86 calories per hour. In fact, the efficiency of the conversion of food energy to bicycle motion is more in the ballpark of 20% so you might expect that to ride a bicycle at 15 miles per hour would burn approximately 400 calories per hour. If you go onto one of the many websites that estimates calories burned while bicycling, you will find they vary a lot from site to site but that they give numbers which are in that ballpark^.
VO2, VCO2, and VO2max
How can the number of calories being consumed by exercise be measured? The calories in food are turned into energy the body can use by reaction with oxygen to generate carbon dioxide (CO2) and water (H2O). Oxygen consumed is much easier to measure than calories. The units used to measure oxygen consumption is VO2. VO2 is the volume of oxygen absorbed by the body, the amount breathed in minus the amount breathed out (not all the oxygen breathed in is absorbed.) The same equipment can often be used to measure CO2 at the same time, the volume of CO2 expelled, a metric named VCO2. It is useful to measure both VO2 and VCO2 because the ratio of the two depends on how much fat and how much carbohydrate the athlete is burning. As the intensity of a ride increases, more carbohydrate is burned and the ratio of fat to carbohydrate and thus the ratio of CO2 expelled and O2 absorbed go down. This ratio is one of the physiological states coaches can use to determine training zones. The ever popular metric, VO2max, is just the highest level of VO2 an athlete can achieve when they are cycling as hard as they can. Although it is possible to purchase a user grade device for measuring VO2, it costs as much as a high end bike so VO2 is usually measured in a gym or medical lab.
METs and %VO2max
METs are a unit that is used much more by the medical community than by the exercise community. Fundamentally, it measures exactly the same thing as VO2. One issue with both these measures is that a larger person will use more calories and oxygen than a smaller person. To correct for that, METs are almost always expressed per kilogram (kg) of body weight, and VO2 often is as well. This is far from a perfect correction, however. How much of the body weight is fat vs muscle matters and size has different impacts for different kinds and intensities of exercise. As a result, there is a lot of talk in the exercise community about reconsidering this correction, but as of today, dividing by body weight is usually what is done.
Another problem with both METs and VO2 is that they ignore levels of fitness. Riding at 15 mph might be very fast for a completely untrained person but very slow for a professional cyclist. Even though METs and VO2 are the same thing, the medical community, who are the main users of METs, almost always ignore this. The exercise community correct for this by not using VO2 itself, but by using the ratio of the VO2 at various intensities to VO2max, a number known as %VO2max.
A final practical difference between METs and VO2 is that the medical community is interested in even low levels of exercise whereas the exercise community is not, so the values reported for %VO2 max do not extend into these low intensities of exercise.
Another way to think about the low level of intensity problem is to ask, what is the level of intensity at the bottom of Zone 1? Theoretically, it is the cyclist sitting on the couch, watching TV. In practice, it is considerably higher than that. If that were not true, then Zone 1 would be much larger than the other training zones. In fact, if Zone 1 were to be made the same size as the other zones, there would be room for two more zones below Zone 1 (as I discuss below in Putting It All Together.)
Heart Rate
The oxygen the athlete breathes in is transported from their lungs to their muscles by their blood which is pumped by their heart. The amount of blood which is pumped by each beat of the heart can be increased by training, but on any given day, each beat of the heart pumps the same volume of blood, so that (in some cases, to some degree of accuracy) heart rate is determined by the oxygen consumed by an athlete's muscles and thus watts measured by their power meter.
There is enough truth in the above story to be very useful, but it is a simplification. Anyone who has ever used heart rate to determine training intensity zones knows that heart rate can vary enormously at a constant level of effort (intensity.) For example, emotional stress affects heart rate dramatically. Similarly, at constant level of power output, an athlete's heart rate will be higher on a hot day than on a more temperate one. Training has a huge impact on heart rate. One of the more useful statistics for monitoring training is the decrease in heart rate that occurs at a constant level of power output. Finally, there is the phenomenon of decoupling. On a long bike ride at a constant effort, heart rate will be steady at first, but as the ride goes on will start increasing. When used to define ride intensity, heart rate has to be used with caution and with awareness of all these factors, but despite that can be extremely useful. The point is, heart rate turns out to be just another way of looking at this same energy in/energy out system.
Baseline Metabolic Rate
An athlete's calorie consumption does not start at zero. Even when they are sleeping or sitting on the couch watching TV their heart is beating, they are breathing, and they are burning calories. The other extreme depends very much on the fitness and genetics of the specific athlete. An average untrained person can burn calories about 10 times as fast when they are working as hard as they can as they do when watching TV. The best professional athletes can do much better, burning calories 25 times faster when working at their maximum rate than when at rest. The point is, when comparing calories in and power out, it is necessary to subtract this baseline. To further complicate things, different metrics of metabolic rate (intensity) have different baselines. Heart rate, for example, has a relatively high baseline compared to the highest possible heart rate, maximum heart rate (HRmax). In fact, many coaches and exercise scientists have switched from talking about heart rate (HR) to talking about reserve heart rate (rHR) which is just the measured heart rate minus the resting heart rate. For marking the boundaries of training zones, this makes no difference, but for understanding the relationship between heart rate and power, rHR is the better metric.
Blood Lactate
There are three kinds of muscle fiber, type I, type IIa, and type IIx. Between them, they use three sources of energy to generate ATP and creatine phosphate: anaerobic metabolism of glucose, aerobic metabolism of glucose, and aerobic metabolism of fat. Aerobic means with oxygen. Aerobic metabolism is what I talked about above, the reaction between food and oxygen to generate energy. Anaerobic means without oxygen. Below, I will be describing ways muscles can use glucose to generate energy that don't require oxygen.
Aerobic metabolism of fat and glucose do not result in lactate production. However they are slow. They are important during low intensity cycling but cannot keep up with the energy demands during high intensity cycling. Anaerobic metabolism of glucose is fast and thus allow for high intensity cycling. One downside of that is that it makes inefficient use of glucose and thus accelerates depletion of glycogen in the muscle fibers, and this depletion is a major source of medium-term fatigue. Another important source of fatigue is that anaerobic metabolism of glucose generates lactate that ultimately builds up in the blood, a major source of short-term fatigue. Between them, glycogen depletion and lactate accumulation limit how long high intensity cycling can continue.
Generation of lactate by anaerobic glucose metabolism and its release into the blood is a normal part of metabolism and not just by muscles. Many blood cells generate all their energy this way. Even at rest, muscle cells generate and release some lactate. Thus, there is a resting baseline level of lactate in the blood of about 1 millimolar (mM.) During low intensity cycling, most energy comes from the aerobic metabolism of glucose and fat, only a baseline level of lactate is released into the blood, and the blood lactate levels remain at this resting level. As cycling becomes more intense, more and more glucose has to be metabolised anaerobically creating lactate that is released from the muscle into the blood. During this medium intensity exercise a steady state is reached where at any intensity level there is an associated level of lactate in the blood. This phase continues up to a blood concentration of approximately 4 mM. Past that, metabolism of lactate cannot keep up with its production and as a result, there is no steady state, blood lactate continues to go up even at a constant intensity level. However, the rate of increase becomes faster as the intensity of cycling increases. In this third phase of intensity, lactate will relentlessly increase, more or less quickly depending on the intensity of cycling, up to a maximum of approximately 20 mM at which point it becomes impossible to continue. Thus, it is clear there are three fundamentally different intensity zones in terms of lactate. In Zone 1, lactate remains at resting levels and is not an issue in terms of fatigue. In Zone 2, lactate levels increase but the body can manage that lactate. As far as I know, the consequence of these steady state but increased levels of lactate on fatigue is not known. Finally, there is Zone 3 where lactate production exceeds the body's ability to manage it ultimately limiting the ability to continue at that intensity. This is why advocates of Polarized Training like Dr. Stephen Seiler often use a three training zone system based on these lactate levels. To avoid confusion, I will refer to these as Lactate Zones. As a final point I would note that blood lactate levels, like many other things (heart rate for example) vary significantly from person to person. Thus, the absolute values I have given here are typical but by no means accurate for any given cyclist. However, the general patterns are almost universal: no increase, increased but steady, steadily increasing*.
Knowledgeable readers may be raising their eyebrows at this point. Advocates of a blood-lactate-based three training zone system do not talk about lactate levels in the way I have above. Rather, they talk about how specific levels of blood lactate are associated with specific levels of intensity. When they plot a graph of blood lactate concentration as a function of watts generated on a bicycle trainer, they see that it goes up very slowly at first and then at some point starts going up faster. This transition is called the first inflection point and is the boundary between Lactate Zone 1 and Lactate Zone 2. As intensity continues to increase, there is a second increase in the rate at which lactate increases with intensity. This is called the second inflection point and is the boundary between Lactate Zone 2 and Lactate Zone 3. There are two reasons for the difference between how I explain lactate levels and how most coaches and exercise scientists do. The first is that, as I have previously noted, biology is never black and white. I talked about how at low intensity, blood lactate does not increase. That was a simplification. Rather than generating no excess lactate at low intensity, lactate generation starts gradually rather than all at once as I described it. So at these low intensities, in Lactate Zone 1, there is still some increase in blood lactate as intensity increases. The second reason for the difference in how I express things compared to coaches and exercise scientists has to do with the way exercise scientists typically conduct their experiments. Virtually every experiment I have seen has involved an intensity ramp where intensity is increased every few minutes and during each intensity period, a blood sample is taken and lactate measured. In Lactate Zone 3, where I claim lactate is not at equilibrium but rather is relentlessly increasing what is really being measured is how much lactate has accumulated at a specific intensity after the specific number of minutes used in the experimental protocol. Though reported as a lactate concentration, that value is really a rate, how fast lactate is being accumulated at that intensity and thus how much lactate has accumulated during those few minutes. The rate of lactate accumulation times the time the cyclist has been at the intensity being investigated results in the level of lactate that is reported. If the time at each intensity in the experimental protocol were increased, the level of lactate measured would also increase, so the levels reported are dependent on the details of the experimental protocol.
In Lactate Zone 2, the body is able to clear all the lactate generated by the muscles as fast as it is generated. How does the body do that? There are two general mechanisms. The first is to burn it as fuel. The heart is particularly good at that. The second mechanism is to convert it back into glucose. This happens mostly in the liver and in the kidneys. That glucose is then returned to the blood to be used by muscles and other parts of the body. These same two processes also occur when exercising in Zone 3, but they cannot occur fast enough to keep up with lactate generation by the muscles so lactate levels increase over time.
Oxygen Debt and Afterburn
Anaerobic metabolism of glucose is immensely wasteful. Oxidation of one molecule of glucose yields 38 molecules of ATP. Anaerobic metabolism of glucose to generate lactate yields only 2 molecules of ATP. Worse, conversion of that lactate back into glucose by the kidneys or the liver requires 6 molecules of ATP for a net loss of 4 molecules of ATP for each molecule of glucose used anaerobically by the muscles. From where comes the energy to create that ATP? From the oxidation of fat by the liver and kidneys. When exercise is over, the body has to get rid of all the lactate left in the blood, and that takes oxygen. ATP and creatine phosphate levels in the muscle are likely depleted, and oxidation of fat or glucose must occur after the end of exercise to replenish those. (These are just two examples, there are probably others.) As a result, oxygen use remains elevated after exercise has finished, a process that coaches and athletes refer to as "afterburn." When riding in Lactate Zone 1 or Lactate Zone 2, the oxygen used to burn carbs and fat is delivered more or less as it is being used. When riding in Lactate Zone 3, however, oxygen consumption lags behind energy generation. The athlete's body accumulates an oxygen "debt" that is "paid back" during the afterburn period, a period that can last for hours.
Relative Perceived Exertion
From a theoretical perspective, relative perceived exertion could be related to almost anything. It is a highly subjective, complex metric. All we know about it is that it is produced by subconscious processes in the brain from unknown data and using unknown algorithms. It is interesting, then, that when measured, the relative perceived exertion reported by most athletes is is fairly similar to heart rate and power. Why the subconscious brain of an athlete makes this association between the rate at which calories are being burned and how hard the effort that is burning those calories feels is a mystery of evolution. I won't be discussing it in this post but training effect, short and long term fatigue, and health benefits, things that our conscious brain think are much more important than calories, are
not closely related to calories burned. Thus, it is a good thing that the way training zones are usually used does not require them to be proportional to fitness, health, or fatigue, because (as I have
discussed in the past and will be discussing again in the future) they are not.
Putting It All Together
In the above graph, I have replotted the training zone data from the chart at the top of the post. The value plotted for each zone is the value at the upper boundary of that zone. Thus, Zone "0" is really the lower bound of Zone 1. As noted above, Coach Hughes does not really establish a lower boundary for Zone 1. In theory, Zone 1 extends all the way down to watching TV while sitting on the couch. For purposes of the graph above, what I did was to set a lower boundary for Zone 1 such that it would have about the same width as the other zones. When I did that, I found that I needed two more zones to get to the couch, which are labelled "light" for light exercise and "rest" for resting, or no exercise at all. I adjusted Coach Hughes heart rate data to use relative heart rate (rHR) rather than absolute heart rate. When I did that, I was astonished at how similar the graphs for heart rate and power were.
If you recall the sections above on Blood Lactate and Oxygen Debt and Afterburn it might seem surprising that rHR and power continue to follow each other all the way to the top of Zone 6. I argued that, rHR increases as demand for oxygen increases, and that, in Zones 5 and 6, muscles are using glucose to generate power in the absence of oxygen, so why does rHR continue to rise? I have two guesses, both of which I believe to be true. First, the heart pumps blood for reasons besides just supplying oxygen to an athlete's leg muscles. Some of those reasons might be to move lactate from the muscle to other parts of the body and to provide oxygen to organs like the liver which are generating energy by oxidizing fat to be used to convert lactate back to glucose. The second reason why rHR might parallel power through the top of Zone 6 is that Coach Hughes' Training Zone system has 7 zones, but does not give rHR data above Zone 5 nor power data above Zone 6. (For purposes of the graph, I estimated the rHR data for Zone 6.) It is only in the highest zone, Zone 7 (which Hughes designates not as Zone 7 but as "Sprint") that anaerobic power production really takes off. Hughes defines the top of Zone 6 by a power output of 120% of FTP. Race data shows athletes both professional and amateur reaching peak power outputs of 400% to 500% FTP (an output they can only maintain for 5 seconds or so.) There is no reason for Hughes to delineate the upper boundary of his "Sprint" Zone (Zone 7) because all the athlete needs to know is to go "all out", but that said, the absence of that boundary prevents us from seeing if the large predicted deviation of rHR and power in fact occurs even though common sense tells us that it must. Although there is significant athlete to athlete variation, for virtually all athletes, HRmax, the highest heart rate it is possible to reach, is between 120% and 135% the HR at lactate threshold, far lower than the 400% to 500% power that can be generated compared to the similar FTP.
Of the three commonly used zone metrics plotted, clearly Relative Perceived Exertion (RPE) seems to be the most different. This is despite the fact that, to make it more comparable to the other metrics, I adjusted the values of RPE to be the percentage of the RPE at the top of Zone 4. (It is not at all clear if, given what RPE is, doing so makes any sense.) Given how opaque and subjective RPE is, what is interesting is not that it is somewhat different than power and heart rate, but how similar it is. That said, it is relatively easy to rationalize away even that difference. Because Coach Hughes is a coach, the RPE values he suggests are for a trained athlete. Such an athlete would feel no exertion whatsoever during "light" exercise. An untrained person might have a different experience resulting in different values for RPE at the same heart rate and (relative) power output. I will leave it as an exercise for the reader to imagine how adjusting RPE for such an untrained person would cause them to approach those for rHR and Power.
In Conclusion
The goal of training plans developed by coaches is to increase the various aspects of fitness of their athletes without generating more fatigue than their athletes can handle. To accomplish that goal, the training plans specify different amounts (minutes) of exercise at specific intensities. Calorie consumption is a secondary consideration, if it is a consideration at all. As it happens, the tools available for measuring intensity all measure calorie consumption or the energy output fueled by those calories. This is not a problem as long as it is recognized that what is being measured (heart rate, power output, ...) is not a measure of fitness or fatigue and that the relationship between intensity defined by energy and power will be different than intensity defined by fitness and fatigue.
^ 360 cal/hr: https://www.welovecycling.com/wide/2020/05/14/how-to-convert-watts-into-calories-burned-on-the-bike/
568 cal/hr: https://captaincalculator.com/health/calorie/calories-burned-cycling-calculator/
540 cal/hr: https://gearandgrit.com/convert-watts-calories-burned-cycling/
360 cal/hr: https://mccraw.co.uk/2012/10/14/powertap-meter-convert-watts-calories-burned/
* (This paragraph is based on "Many Factors to Consider When Collecting, Analyzing, and Interpreting Blood Lactate Measurements" in "Physiological Tests for Elite Athletes" Second Edition by Australian Institute of Sport (Author), Rebecca Tanner (Editor), Christopher Gore (Editor) ISBN-13: 978-0736097116, ISBN-10: 0736097112)
