You have probably heard at some point that you should do a good portion of your running training in Zone 2. Yet every time you go out for a jog, you notice your heart rate spiking above that zone. What gives? And what should you do?
As a running coach, there are a few reasons this can occur. But first, it’s important to understand the purpose of Zone 2 running and what it actually represents (which isn’t just an arbitrary heart rate).
Disclaimer: This post was written by Chrissy Carroll, RRCA Running Coach, USAT Level I Short Course Triathlon Coach, and Certified Personal Trainer. It is for informational purposes only and should not be construed as training advice. Consult a doctor prior to starting any new exercise routine.
What is Zone 2 Running?
Many people think Zone 2 running is just a measure of low heart rate during a run. While that can be true, the actual definition of Zone 2 (in conventional running discussions) is the intensity at which your body is maximizing fat oxidation during exercise. This means when your body is using fat optimally for fuel.
Our bodies have nearly unlimited supplies of fat to burn for energy, whereas carbohydrate stores are much more limited. If you can train the body to become more efficient at a particular intensity, you enable yourself to burn higher proportions of fat for a longer time frame.
(Note that you don’t just switch exclusively between only fat or only carbohydrate; your body burns a mixture of these fuels during runs. Zone 2 running is predominantly burning fat for fuel though).
When we refer to Zone 2 being the intensity where maximal fat oxidation occurs, we’re thinking about a traditional five zone model. This can get a little confusing when looking at running literature, because many use a three zone model instead of a five zone model. In those papers, Zone 2 is actually closer to threshold intensity, whereas Zone 1 is closer to the easy aerobic intensity we’re thinking of here.
For the purposes of this article, assume we’re talking about Zone 2 as the comfortable aerobic zone on a standard five zone scale. (Not the three zone scale).
Generally, zones are set based on a percentage of max heart rate in order to estimate a low enough intensity that you’re maximizing fat burning rates. Often, Zone 2 is noted to correspond to around 60-70% of max heart rate.
However different research methodologies and running experts believe Zone 2 may actually fall anywhere between 60-85% of max heart rate. For example, this study notes that easy and moderate continuous running falls between 62-82% of max heart rate (again, noted as Zone 1 in this study because they are using the three zone rather than five zone scale – confused yet?!).
Setting zones based on max heart rate can be inherently problematic, as a runner’s true zones can vary based on their training and genetics.
Some guidelines set zones based on the percent of lactate threshold heart rate rather than max heart rate. For example, Joe Friel sets running Zone 2 based on 85% to 89% of lactate threshold heart rate. This can be a bit more accurate, because you’re using actual field test data.
Over time, doing more running in Zone 2 will help your body become more efficient at using oxygen at a particular intensity level, making a run at that level feel easier than it did before. This is called improved running economy, and research has linked running economy (even at low intensities) to better performance.
OK, now that we’ve gotten that defined, let’s take a look at some of the problems people run into with Zone 2 training….
Problem 1: Your zones were set by your age.
Many people (and watches) default to setting their zone-based training on common formulas. The most popular is the Fox formula which determines max heart rate using 220 minus age, and then people may set zones accordingly.
The problem is that heart rate formulas based on age aren’t always accurate. In fact, most age-predicted formulas have a large margin of error. A 2020 study looked at formulas and compared them to a participant’s actual max heart rate derived from exercise testing. The study concluded “All equations used in this study show poor agreement between measured max heart rate and age-predicted max heart rate.”
At a population level age-predicted maxes may give us general helpful data, but there is a lot of inter-individual variability that can make it less accurate with any one particular athlete.
For example, let’s say you’re a 45 year old runner. The Fox formula (220-age) gives you a max heart rate of 175 beats per minute. But in actuality, your max may be 195 beats per minute or may be 155 beats per minute, depending on genetic differences.
If your true max heart rate is different than your age-predicted max, then any zones set by the age-predicted formula will be off. In the example above, the age-predicted max was 175 beats per minute, so a Zone 2 heart rate estimated by the popular guideline of 60-70% of max heart rate would be 105-122 beats per minute.
But what if your true max was closer to 195 beats per minute? In that case, your actual zone 2 (at 60-70%) would fall to 117-137 beats per minute.
This doesn’t even take into account the fact that the true Zone 2 range could be anywhere from 60-85 percent of max heart rate, giving even larger ranges and more variation based on estimated versus true data.
Hopefully you can see how these variations can cause people to think they’re in a different zone, but in actuality they could very well be in their Zone 2.
Similarly, the most popular heart rate based training program for runners, MAF, also uses an age based formula of 180-age to determine the top of your aerobic training zone. In the 45-year old runner described above, the top of their aerobic zone would be 135 beats per minute. This is actually more in line with the runner’s actual data above in this example, but can still fall subject to some inaccuracies based on genetics.
If you feel like your zones may be off, the solution here is simple: set zones based on actual data.
Of course, getting a lab test to determine the proportions of fuels burned or lactate levels or your max heart rate isn’t practical for 99% of runners. But doing field tests can work pretty well as a proxy measure. Once you have an idea of your max heart rate or lactate threshold heart rate based on field tests, you can set more accurate zones for your training.
I recommend using a lactate threshold field test and setting your zones based on that, with Zone 2 falling roughly around 85-89% of lactate threshold heart rate. It’s fairly simple to do and can give you good data. Of course, even field testing isn’t perfect – for example, if you field test on a day you’re stressed out or it’s hot out, you may end up with different data than if those things weren’t a factor.
Problem 2: Your watch is struggling with cadence lock.
Watches that measure heart rate do so using optical sensors. When you run (especially if your watch is a little loose), the bouncing motion can cause light to enter into those sensors with each step. The result is something called cadence lock, which is where your heart rate on the watch isn’t truly your heart rate, but rather the steps per minute that you’re running.
For example, let’s say your Zone 2 heart rate should be around 135 beats per minute. You feel like you’re running really easy, but your heart rate is displaying 165 beats per minute. If your watch is dealing with cadence lock, it may be latching onto your steps of 165 per minute, rather than your actual heart rate.
Try pausing and taking your heart rate during a run to see if it’s accurate. If not, there are three potential solutions to cadence lock:
- Wear your watch a little higher and more snug on your arm (this works well for me).
- Use a chest-based heart rate monitor for more accurate data.
- If number 1 didn’t work and you don’t have access to number 2, try taking your heart rate manually during easy runs to figure out if you’re around the general vicinity of your zone 2 heart rate.
Problem 3: You’re truly running too hard.
Some runners tend to run the same pace for almost every run – doing their easy runs too hard, and their hard runs too easy.
If you have an accurate measure of your heart rate and your zones, and notice you’re always considerably higher than Zone 2, then try:
- Slow your running pace down until your heart rate falls in or close to Zone 2.
- Add walking breaks in to bring your heart rate down into Zone 2.
Most of your training should fall into the easy aerobic zone (Zone 2). Research shows that even elite athletes do 70-80% of training at quite low intensities.
However, not all training should be exclusively in this zone. Think of the 80/20 rule of running: about 80% of your training should be in this easy range, but about 20% should be challenging to push neuromuscular adaptations and turnover rate (think track intervals, hills, or tempo runs).
Problem 4: Your runs feel easy but go above Zone 2.
It is completely possible that for some people, running of any kind would currently push you over your Zone 2 heart rate. You’d have to stay walking continuously to achieve that heart rate. So should you not run at all when training for a race?
As a coach, I obviously wouldn’t recommend that. My (perhaps unconventional) wisdom is for people to stop worrying so much about heart rate zones if they are struggling with this while training for a race. This is especially true if you are a newer runner.
Instead, run by how you feel. If you’re thinking of an RPE of 1-10, you should feel around a 4-5 on an easy aerobic run. You should be able to talk to someone running next to you comfortably (but you probably can’t sing comfortably). You should wake up the next morning feeling like you could easily go out for another run again.
If you are meeting those benchmarks but your heart rate isn’t where it “should be” based on zones, then I’m of the mentality that it doesn’t really matter for many folks who simply want to cross the finish line and have fun at their race.
Now if you are an experienced distance runner that’s reached a plateau with other training types, and are truly trying to build your aerobic capacity – then maybe taking a break from race training and spending some time working on base building with a lot of Zone 2 training could be beneficial. But this is really dependent on your overall goals, genetics, and other training factors.
Key Take Home Points
Zone 2 training can be a useful part of a runner’s training plan, but there are many factors that can affect the accuracy of your zones. If you want to train in Zone 2:
- Recognize that while popular articles suggest Zone 2 is at 60-70% of max heart rate, it can actually fall between 60-85% depending on your fitness level and genetics.
- Consider setting your zones using real field data, rather than age-based calculations.
- Make sure your heart rate monitor is giving you an accurate reading.
- Pay attention to how you feel, whether you can talk, and your recovery time to tease out if your runs are truly easy.
If you’ve done all that and still can’t manage to stay in Zone 2 while running, consider whether that matters to your overall training goals.
If it doesn’t, go out and run at what feels easy to you and forget about the heart rate monitor.
If it does matter, spend some time slowing down your pace and possibly incorporating walking breaks. Use that strategy for most of your runs, while still incorporating occasional speedier workouts (think of that 80/20 rule). See if that helps you build up the aerobic engine.
Either way, enjoy your run training and have fun out there! 🙂
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