FortiorCardio
Heart rate zones explained
Heart rate zones split your effort into five ranges, from easy to all-out. Fortior sets them from your own numbers and shows how many minutes each cardio session spent in every zone.
What heart rate zones are
A heart rate zone is a range of beats per minute that roughly matches how hard your body is working. Most systems use five zones. The low zones are comfortable and sustainable for a long time, and the high zones are short, hard and uncomfortable.
Zones help you check that an easy day was really easy and that a hard day was hard enough, which average heart rate alone can't show. They are built from a personal anchor rather than a fixed table of beats per minute, because two people of the same age can have very different heart rates at the same effort.
The five heart rate zones explained
| Zone | Talk-test feel (rough guide) | Typical use | Share of threshold heart rate | Share of heart rate reserve |
|---|---|---|---|---|
| Z1 | Very easy, you can chat freely | Recovery and easy aerobic work | Below 81% | 50% to 60% |
| Z2 | Comfortable, full sentences | Steady aerobic base, long easy runs and rides | 81% to 89% | 60% to 70% |
| Z3 | Moderately hard, shorter sentences | Tempo effort | 90% to 94% | 70% to 80% |
| Z4 | Hard, a few words at a time | Threshold effort | 95% to 99% | 80% to 90% |
| Z5 | Very hard, brief efforts only | Short, near-maximal intervals | 100% and above | 90% to 100% |
Some coaches call Zone 3 the grey zone because it is harder than easy running but short of threshold. Whether it belongs in your week depends on your goal.
How Fortior sets your zones
Fortior chooses an anchor in this order:
- Threshold heart rate, when it has one. Fortior looks through your recorded activities for sessions of at least 25 minutes whose average heart rate reached 90% of your maximum or more. It takes the highest of those averages as an estimate of your threshold heart rate. This is a field estimate, not a lab test.
- Heart rate reserve, otherwise. Reserve is your maximum heart rate minus your resting heart rate. Each zone starts at a set share of that gap above your resting rate.
The two numbers behind heart rate reserve come from your own data:
- Maximum heart rate: the highest maximum heart rate across your recorded activities in the past year. If no activity has one, Fortior uses an age-based estimate: 208 minus 0.7 times your age.
- Resting heart rate: your resting heart rate from Apple Health when it has a recent reading. Otherwise Fortior uses a rough stand-in drawn from the lowest average heart rates in your recent sessions, and 60 bpm if nothing else is known.
A worked example
Take someone with a maximum heart rate of 190 bpm and a resting heart rate of 55 bpm. Their reserve is 190 − 55 = 135 bpm, and each zone boundary is 55 plus a share of 135:
| Zone | Share of reserve | Upper boundary | Range Fortior shows |
|---|---|---|---|
| Z1 | Up to 60% | 55 + 0.60 × 135 = 136 | Under 136 bpm |
| Z2 | 60% to 70% | 55 + 0.70 × 135 = 149.5 | 136 to 149 bpm |
| Z3 | 70% to 80% | 55 + 0.80 × 135 = 163 | 150 to 162 bpm |
| Z4 | 80% to 90% | 55 + 0.90 × 135 = 176.5 | 163 to 176 bpm |
| Z5 | 90% to 100% | Maximum, 190 | 177 bpm and above |
A run at 158 bpm is Zone 3 for this person. If Fortior later finds a qualifying session (at least 25 minutes, average heart rate of 175 bpm, which is above 90% of 190), the zones re-anchor to that threshold estimate: Zone 2 then runs from 142 to 155 bpm and Zone 5 starts at 174.
Both anchors are estimates. An observed maximum can sit below your true maximum if you rarely go all-out, and an age-based maximum can miss an individual's by 10 beats a minute or more. When the estimates move, your zones move with them, and Fortior counts your recorded sessions against the current zones.
How time in zone shows on a cardio session
Open a cardio activity and the Heart Rate Zones card lists Z1 to Z5, each with its range in beats per minute, the minutes you spent there and a bar sized against your longest zone. A line under the heading names what the zones are built from, such as heart rate reserve with your resting and maximum numbers, or your threshold heart rate. Above the bars, Fortior says how much of the session had heart rate readings.
- Fortior credits the time between two readings to the zone of the earlier reading.
- With a continuous recording, such as a watch workout or a chest strap, gaps longer than 30 seconds are left out and reported as time without continuous readings.
- Some sources, such as Apple Health outside a watch workout, report only every minute or so. Fortior then estimates the time between readings and says so on the card.
- A session with only an average heart rate has no time-in-zone breakdown. Fortior doesn't build one from an average.
Seeing your mix of easy and hard training
In Progress, the Cardio category adds the same view across weeks. Time in Zone totals your minutes in each zone for the date range you pick. Intensity Distribution groups that time into low (Zones 1 and 2), moderate (Zone 3) and high (Zones 4 and 5) and labels the mix:
| Label | What the mix looks like |
|---|---|
| Polarized | At least 75% low, no more than 10% moderate, and some high time |
| Threshold | More than 30% moderate |
| Pyramidal | More than 60% low, 20% to 30% moderate, under 20% high |
| Unstructured | None of the patterns above |
Research on endurance athletes generally describes polarized and pyramidal splits as common among well-trained people, and many coaches build weeks around a mostly easy base. Treat the label as a description of what you did, not a grade.
What zones can't tell you
- Zones from Fortior can differ from your watch's zones or another app's, which may use other formulas.
- Heart rate lags effort. In a short interval it can peak after the effort ends, so brief hard efforts can look easier than they were.
- Heat, poor sleep, caffeine and stress can all move heart rate at the same effort.
- Long sensor gaps are unknown time, not time in a zone.
In the app: open Train, choose Cardio, and select an activity to see its Heart Rate Zones card. For trends across activities, open Progress and choose Cardio, then look at Time in Zone and Intensity Distribution. Zones need heart rate data from the activity.
Heart rate zones and thresholds are estimates for training, not medical measurements, and Fortior is not medical advice.