Overview
A heart-rate zone chart splits a workout into five intensity bands so you can see where the effort actually sat. Grio shows one on session detail. It is worth being precise about what it is, because zones are widely assumed to be the input to training-load scores. In Grio they are not.
The zone breakdown is a view. Strain is a calculation. Both read the same underlying physiology (your heart-rate reserve), but they combine it differently, and mistaking one for the other is the main reason people find their numbers surprising.
How it’s calculated
Where the zones come from
Grio defines zones by percentage of heart-rate reserve, not by percentage of maximum heart rate. Heart-rate reserve is the span between your resting and maximum heart rate, so 70% of reserve is 70% of the range you actually have available:
| Band | Share of heart-rate reserve |
|---|---|
| Below Zone 1 | under 50% |
| Zone 1 | 50–60% |
| Zone 2 | 60–70% |
| Zone 3 | 70–80% |
| Zone 4 | 80–90% |
| Zone 5 | 90% and above |
The breakdown is produced on your device, in the foreground, from the session’s continuous samples. Those raw samples never cross into Grio’s servers: what the screen receives is a per-zone duration summary plus the time spent below 50% of reserve. When coverage is not sufficient, no breakdown is shown.
How intensity becomes strain
Session Strain does not sum weighted zone minutes. It builds a training impulse instead:
- Resolve the session’s intensity as a fraction of your heart-rate reserve: from measured average heart rate first, then from the intensity you declared, then from a moderate default.
- Multiply that by the session’s duration.
- Weight it by an exponential curve, so intensity counts for disproportionately more than time. Doubling the intensity is worth far more than doubling the duration.
That is why a 30-minute effort at a high reserve fraction can outweigh a 90-minute easy session, and why a lifting hour with long rests scores below a continuous run of the same length.
Ambient movement outside recorded sessions is added at a light intensity, for the minutes not already inside a session. Muscular load from strength work is a separate term rather than being folded into the cardiovascular one.
Edge cases and fallbacks
- No heart-rate samples: declared intensity is used; without that, a moderate default. Both are stand-ins, not measurements.
- No resting or maximum heart rate: published fallbacks are used, which widens the uncertainty on every derived percentage.
- Only an average heart rate: the average feeds Strain. It is never expanded into a zone distribution; Grio will not fabricate a shape it did not observe.
How to read your score
- Zones tell you what the session was: a long Zone 2 block and a spiky Zone 4 session can produce similar strain and are completely different workouts. If your training plan is written in zones, read the chart.
- Strain tells you what the day cost: it is the number to compare against the Target Strain Zone and across days.
- A low zone chart with high strain usually means duration. A high zone chart with modest strain usually means the hard part was short.
The common misreading is that more Zone 4 and 5 is always better. Zone time is a description of effort distribution, not a score to maximise, and Grio does not grade you on it.
What to do with it
- Set resting and maximum heart rate as accurately as you can: every zone boundary and every reserve fraction moves with them, so an inherited default quietly shifts your whole chart.
- Wear the watch for the session itself if you want the breakdown. Later manual edits can fix duration and category; they cannot recover samples that were never recorded.
- Declare intensity when you log manually: it is the difference between a session scored on your effort and one scored on a moderate default.
Troubleshooting & FAQs
- My zone chart shows 20 minutes in Zone 4, but Session Strain looks low. Why?
- The zone chart is computed from the session's continuous samples on your phone, while Session Strain is built from the session's average heart rate over its full duration. A session with 20 hard minutes and 40 easy ones has a moderate average. The two views are consistent: they are answering different questions.
- Why does my zone breakdown not appear for some workouts?
- The breakdown needs enough continuous heart-rate coverage from the session, and it is computed on the device in the foreground. Imported sessions without sufficient samples, and sessions opened while the data has not synced, show no breakdown rather than a fabricated one. An average heart rate alone is never expanded into a zone distribution.
- Which maximum heart rate does Grio use?
- It prefers the values your own data supplies and falls back to a published age-based estimate. Any age formula is a population average with a wide spread, so a zone chart is a good relative guide and a poor absolute one.