Your smartwatch HR Zones are wrong. Here’s how to fix them (Anaerobic and Aerobic Threshold Tests)
I write this in January, during what tends to be the start of a new year of training for many, which means it’s also time for speed work. I am coaching a client right now through a speed block, and we were discussing their Garmin data and how it shoes they have an “anaerobic deficiency”, even though they just completed a month of VO2 interval work. I asked: “what are your heart rate (HR) zones on your watch?”. They say what I expected: the HR zones are some preset value, based on % of max HR (booo hiss). The max HR zone guesstimate is absolutely the least accurate HR zone calculation on a smart watch. Why? The Garmin (or Coros, etc) is guessing your max HR: unless you get it measured you don’t know what it is, neither does Garmin, and this number is also something that’s mostly dictated by genetics vs your training - meaning - we can’t impact it nor can we measure it in training. So, please don’t use HR zones based on max HR.
Assuming you’re an athlete that likes to train based on HR zones with (or instead of) RPE (rate of perceived exertion), this article is for you. I think most athletes like to track both - even I like to make sure my HR zones are set up properly in my smart watch. Read on for how to fix this!
But first…why this matters.
If you’re setting out to do a Zone 2 long run, VO2 workout, or tempo run, it’s important that you know if you’re in the right HR zone, working the right energy system. Otherwise, your workouts may not be productive: you could be working too hard, or not hard enough. And of course, as much as us coaches get annoyed at the influence smart watches have over our athletes, the data your smartwatch will give you should be just a touch more accurate if you set up your HR zones properly. ;)
What should you do?
% of LTHR, or Heart Rate Zones based on % of Lactate Threshold Heart Rate will be the most accurate zone calculation to use, particularly because you can measure your LT HR fairly easy and make your zones pretty darn accurate through one (or ideally two) simple tests. So what are these tests and how do you complete them?
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Test #1: AnT: Anaerobic Threshold Test.
First, some definitions: your AnT is the point at which lactate begins to accumulate in your blood. Practically, that means if you’re operating above this effort level/HR, you will be forced to slow down - your body can’t keep up with the energy demand. This is a pretty hard effort, around your 10k pace (ish). You can get an approximate number (HR) for the AnT, and once you have it, this is what you put into your Garmin, Coros, etc to set your % of LTHR zones. (AnT = LTHR). If you Google search this you can find a number of ways to do the test: the fact this HR should be close to your 10k race pace gives you a clue. The test can be as short as 30 minutes. I suggest doing this on a treadmill so you have more control of your environment and can watch your HR, etc.
Note: you must do this test with heart rate monitor strap of some kind. Wrist-based heart rate monitors are not accurate enough.
The Anaerobic threshold (AnT) test:
Do a 10-15 minute warm up.
If you’re new to running overall, run a 10k at race pace or as fast as you can sustain at a 1-2% grade for 20-30 minutes. Take note of your average HR throughout: this is your LTHR/AnT.
If you’re focused on trail running, I find it helpful to do this uphill (because we rarely run pure flats): run as hard as you can sustain for 20-30 minutes at a 6% grade, and same as above, record your average HR. This is your LTHR/AnT.
The goal is to keep the pace steady if possible, but if you need to back off to keep you HR controlled that’s ok. Remember: this is a test to get your AeT Heart Rate, not pace!
Another note: This test is hard and should only be conducted when you are well-rested if the results are going to be meaningful. Your AnT changes based on your recovery status!
Once you have your AnT, you’re in good share to re-program your Garmin, Coros, other watches’ HR zones!
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Test #2: Aerobic Threshold Test (AeT). If you want to further refine your HR zones it’s critical to understand the limits of your Zone 2 as well, or your Aerobic threshold (AeT). This is the point at which your body switches from using fat as your primary fuel (which can be sustained for a very long time) to glycogen (or carbohydrates), which as you know, are in limited supply and requires refueling. When you hear people talk about “bonking”, they are operating above their Zone 2 or AeT, and they have run out of glycogen stores to sustain their training. Zone 2 is where you want most of your time to be in ultras - it’s your “conversational” running pace (where you could hold down a conversation comfortably, or breathe through your nose) - but it’s also a place most athletes don’t spend enough time in. I could on with this topic…but that’s for another article. :) Today, we’ll talk about how to find the upper limits of your Zone 2 / AeT.
As above, it’s best to do this test inside on a treadmill, and be sure to use a heart rate monitor strap for accurate data. This test taskes some trial and error unfortunately, so you may have to try it a few times is you’re really guessing. Most of us have a feel for our AeT - starting with a pace/HR you believe to be conversational will help get you there faster.
The Aerobic Threshold / Heart Rate Drift Test:
Start with a 10-15 minute warm up on a treadmill at a 1-2% grade. Watch for your HR to stabilize for ~ 3 minutes (it shouldn’t move up or down)
Continue at that pace and hold it steady for 40-60 minutes. If you see your HR start to increase significantly (e.g. increase more than 5 BPM from where you started), you are above your AeT and need to re-do the test at a slower pace.
Once the test is complete, look at your HR data in the first half of your post-warm-up run and second half. If your HR drifted more than 5% during the test int he second half, you are above your AeT, and you’ll need to re-test and a slower pace and lower HR. If your HR did not drift, you can do the test again until you do see drift. Somewhere in between those two is your AeT.
Once you have your AeT, you can put this in to your smart watch as well - at this point, your HR zones are well-programmed for your key workouts for the year!
Summary
If you’ve done these two tests, your HR zones should be set up like this (assuming a 5 zone system):
Zone 1 (Recovery): Below 85% of LTHR
Zone 2 (Aerobic): 85-89% of LTHR - this zone ends at the AeT HR we measure in test #2
Zone 3 (Tempo): 90-94% of LTHR - this zone begins 1 bpm above the AeT HR we measured in test #2
Zone 4 (Threshold): 95-99% of LTHR - this zone ends at the LTHR/AnT HR we measure in test #1
Zone 5 (VO2 Max): 100-102% of LTHR - this zone begins 1 bpm above the LTHR/AnT HR we measured in test #1
References for this blog/more details on the tests:
Ref 1 Evoke Endurance
Ref 2 The Uphill Athlete - anaerobic test
Ref 3 The Uphill Athlete - aerobic threshold test/HR drift test