2024-05-04
I recently got into sports cycling after using a bike for daily commuting for over a decade. There’s a lot of new information to absorb, with HR (heart rate) zones being one of the most confusing topics I’ve come across. It’s confusing because there is a lot of contradictory information on the topic.
This blog post summarises my current understanding of all the information I could find on the web about HR zones.
I’m new to this topic and not much of an expert, so take everything here with a grain of salt!
HR zones are simply a way of splitting the continuous range of possible heart rates into a set of ranges. The most common systems have five zones, although other simpler systems have three zones. I’ll focus on 5-zone systems due to their popularity.
The zones semantically categorise the body’s response to physical exercise (in this case, cycling). Each zone corresponds to a categorical mode in which the body’s metabolism runs.
Because each individual is different (both genetically and in terms of fitness), the actual zone endpoints differ from person to person.
The 5-zone models include the following zones:
Zone 1: Active Recovery. The effort is very easy, with little pressure through the pedals. It’s used on a recovery ride after a hard training session or in short recovery periods between high-intensity intervals.
Zone 2: Endurance. Conversation is easy, but breathing starts to become heavier. This zone is called “endurance” because it’s the best zone for sustained endurance efforts that span many hours.
Zone 3: Tempo. Breathing becomes even harder, and concentration is required to maintain this effort level. I’m not confident I understand why this zone is called “tempo”. However, the name feels evocative of fast but not uncomfortably fast.
Zone 4: Threshold. Very difficult to hold a conversation. Breathing is very heavy but not yet strained. It requires mental determination to stay in this zone. This zone is called “threshold” because it’s at the threshold where the body can no longer clear lactate faster than it’s produced, causing levels to accumulate rapidly.
Zone 5: VO2 Max. Cannot speak and breathing is extremely laboured. Painful to ride in this zone for more than a few minutes. This zone is called “VO2 Max” because it’s the zone where the body is consuming oxygen at the maximum rate (volume per time) it can.
The main idea is that an individual can use a heart rate monitor during exercise to help them decide whether to go harder or back off on the effort level. HR zones are the framework that helps to make those decisions.
This can help to achieve specific goals. For example:
Training can be targeted to cause adaptations to the various “energy” systems in the body. The systems are the aerobic system, which powers long-duration efforts; the anaerobic system, which powers short and punchy efforts (< 3 minutes); and the phosphagen system, which powers very short but maximally powerful efforts (< 10 seconds).
Pace can be modified during an event or exercise session. For beginners, it’s especially easy at the start of a ride to go “too hard” and not have enough left in the tank to complete the ride. Suppose an individual stays in lower HR zones. In that case, exercising without excessive fatigue for a long duration (e.g., many hours) should be possible.
Post-workout analysis can quantitatively understand how much stress the body has been put under during an exercise session. This can help avoid overtraining.
In addition, data nerds (such as myself!) love having more data to look at and play with. Data is interesting.
This is where things started getting very confusing for me for a few different reasons:
There are multiple systematic ways to construct the zones.
Each system has many variants.
The amount of numerical “sloppiness” I encountered while researching the topic surprised me and added to my confusion. For example, some sources list adjacent zones as being between 50% and 59% and 60% and 69% (of some value). What about the 1% in between?
Heart rate zones can be anchored from two different measurements: maximum HR and threshold HR.
Maximum Heart Rate (MHR) is the highest physically possible rate at which an individual’s heart can beat.
It depends on genetics and age (it decreases with age). It is not an indicator of fitness or health, except at pathologically extreme values.
Two commonly used linear regression models estimate heart rate based on age.
The first is 220 - age, which is simple but has a low level of accuracy1.
The second is 211 - 0.64 * age, which is slightly less straightforward but
has slightly less inaccuracy2.
Maximum heart rate is easy enough to measure, so the linear regression models should be ignored other than as a sanity check.
One way to measure maximum heart rate is to ride intensively for ~10 minutes, followed by a 1-minute maximum effort in the saddle and a 30-second all-out sprint out of the saddle. The maximum heart rate is the maximum heart rate measured.
If you often perform max efforts, it’s possible to take the highest heart rate ever recorded instead of following a specific measurement protocol.
Once the maximum heart rate is measured, zones can be anchored based on that heart rate. This is where a big part of the confusion comes from because there isn’t a standard way to create heart rate zones based on maximum heart rate. I found seven different variants, each with different zone cutoffs.
I’ll describe 3 of the more interesting schemes:
Scheme 1. I don’t think this scheme has a name, but I think of it as the “10% per zone” scheme. It splits the range from 50% to 100% into five 10% slices, one per zone. It’s appealing because of its simplicity. It was the most commonly used scheme described when I was researching the topic on the web3 4 5.
Scheme 2 is similar, but increases the size of zone 1 from 10% to 15%, and compresses zones 4 and 5 to account for the extra 5% added to zone 1. Because zone 1 is bigger, everything is shifted upwards. It was the second most commonly described scheme I found when researching the topic on the net6 7.
Scheme 3 is the scheme that Strava uses when you don’t specify custom HR zones. It’s similar to scheme 2, but both zone 1 and zone 2 are large (15% each). Zone 3 is tiny (5%), and the remaining 15% is split between zone 4 and zone 5.
The percentages in the table below are percentages of MHR.
| Scheme | Zone 1 | Zone 2 | Zone 3 | Zone 4 | Zone 5 |
|---|---|---|---|---|---|
| 1 | 50% - 60% | 60% - 70% | 70% - 80% | 80% - 90% | 90% - 100% |
| 2 | 50% - 65% | 65% - 75% | 75% - 85% | 85% - 92% | 92% - 100% |
| 3 (Strava) | 50% - 65% | 65% - 80% | 80% - 85% | 85% - 92% | 92% - 100% |
Threshold heart rate (THR) is also known as Lactate Threshold Heart Rate (LTHR) and Functional Threshold Heart Rate (FTHR).
It’s the threshold between aerobic and anaerobic exercise and the point at which lactate begins to build up in the body because it’s being produced faster than it can be metabolised.
It’s the maximum heart rate that can be sustained for arbitrary amounts of time (e.g. over an hour).
Unlike MHR, a higher THR (relative to MHR) is indicative of higher fitness.
Like MHR, simple formulas can be used to estimate THR. For example, THR can be estimated as a percentage of MHR (common estimations are 10% and 15%). However, these are not accurate for individuals, so it’s better to measure THR instead.
There are two protocols for testing THR:
30-minute time trial.3 8 9 Warm up for 10 to 15 minutes. Then, do a 30-minute all-out but evenly-paced effort (a “time trial”). THR is the average HR for the last 20 minutes of the 30-minute effort. Only the last 20 minutes are taken to avoid the effects of cardiovascular drift.
Scaled double interval. Do an 8-minute well-paced max effort. Then, ride at a recovery pace for 10 minutes. Then, perform another 8-minute, well-paced, max effort. Find the average HR across each of the two 8-minute efforts and take the higher of these 2 averages. Multiply this by 0.93 to get THR.
The main challenge with both protocols is finding a place to safely perform them. Because you can’t stop or slow down during either protocol, this rules out most on-road locations (at least in urban areas).
There are two commonly used ways to create zones that are anchored off THR:
Andrew Coggan’s system lets up THR to be exactly in the centre of zone 4 (the “threshold” zone). In contrast, Joe Friel’s system sets THR at the upper bound of zone 4 and the lower bound of zone 5. Having THR at the centre of zone 4 makes a lot more intuitive sense to me.
The percentages in the table below are percentages of THR.
| Scheme | Zone 1 | Zone 2 | Zone 3 | Zone 4 | Zone 5 |
|---|---|---|---|---|---|
| Coggan | < 68% | 68% - 85% | 85% - 95% | 95% - 105% | > 105% |
| Friel | < 81% | 81% - 90% | 90% - 94% | 94% - 100% | > 100% |
Writing this blog post has clarified many concepts for me and helped me form an opinion about which system I like best.
I prefer THR anchoring to MHR anchoring because THR feels like a more robust measurement (it’s an average rather than a maximum) and corresponds to what seems to be a more important measurement. I spend a non-trivial amount of time near THR, but I rarely get near my MHR.
Coggan’s system feels more intuitive to me than Friel’s system. I like that THR is in the centre of zone 4, matching the semantic meaning of that zone.
https://www.bicycling.com/training/a34717708/heart-rate-training-cycling/ ↩︎ ↩︎
https://www.highnorth.co.uk/articles/cycling-training-zones ↩︎ ↩︎ ↩︎
https://stagescycling.com/en_au/content/power-vs-heart-rate-which-is-better ↩︎
https://au.wahoofitness.com/blog/the-ultimate-guide-for-tracking-your-cycling-heart-rate/ ↩︎
https://www.cycling-inform.com/how-to-use-heart-rate-monitor-and-zones-to-improve-your-cycling ↩︎
https://www.cycling-inform.com/how-to-work-out-your-cycling-heart-rate-zones ↩︎
https://www.bikeradar.com/advice/fitness-and-training/training-zones ↩︎ ↩︎
https://www.trainingpeaks.com/learn/articles/joe-friel-s-quick-guide-to-setting-zones/ ↩︎ ↩︎
https://www.cyclingweekly.com/fitness/training/training-zones-what-are-they-and-why-do-they-matter-180110 ↩︎
https://www.garmin.com/en-GB/blog/cycling-training-zones-guide/ ↩︎
https://endurancebikeandrun.com/blog/2020/04/29/how-to-use-heartrate-to-train-for-endurance-cyling ↩︎
https://manuals.stagescycling.com/en/dash-m50-l50/user-manual/settings/user-settings/heart-rate-zones/ ↩︎