Overview
Apple’s Exercise Time is effectively an Apple Watch metric. For a long stretch, that meant every Grio member without a watch saw Daily Strain, the Target Strain Zone, and Strain Performance as a permanent dash, even though their phone had been counting steps every single day.
Grio’s current strain algorithm fixes that. It observes a day’s movement from either Apple exercise minutes or the phone’s step count, so an iPhone-only setup produces a real Daily Strain.
How it’s calculated
The channel rule
Exercise minutes remain the source of truth whenever the watch reported them. Only a day the watch never covered falls back to steps. That ordering is what guarantees the two channels can never double-count the same movement.
Turning steps into movement minutes
A step count is a volume measurement, not an intensity one, so it has to be converted into the same brisk-minute scale exercise minutes report. Grio does that with three code-owned bounds:
- A sedentary floor: the first couple of thousand steps of a day are the steps you take during routine tasks, like walking to the kitchen or around the office, and are not converted into movement minutes.
- A deliberately unhurried steps-per-minute rate: Brisk walking runs roughly 100–120 steps per minute, and the conversion rate Grio uses is far higher than that. It is not a cadence claim; it is a discount for the unhurried steps mixed into a day total.
- A daily cap: Step-derived ambient movement cannot exceed a bounded number of minutes, so an unusual step total cannot manufacture a training day.
Those minutes then feed the same light-intensity ambient term the exercise-minute path uses, and the rest of the calculation is identical. Ambient minutes always subtract the minutes already inside a recorded session, in both channels.
What did not change
- Active energy still never enters Strain: Deriving load from it would need your body mass and would inherit another vendor’s estimate.
- A day whose channels all stayed silent is still
unavailable: Silence is not a confident zero. - Observed zeroes: A measured zero is still an authentic rest day, not missing data.
- Frozen targets: Historical targets are never recomputed or relabelled after the day ends.
Edge cases and fallbacks
- A long strength session with few steps: Because ambient minutes always subtract canonical session minutes, an hour of lifting can cancel the day’s ambient channel entirely. The error runs conservative: it never double-counts, and a category-aware ambient rule would cost more than it buys.
- A phone left on the desk: Steps taken away from the phone are not observed. This is the fallback channel’s honest limit.
- Switching from phone-only to a watch: New dates use exercise minutes. Older dated results keep the meaning they had when you saw them.
How to read your score
Read a step-derived Daily Strain as movement volume, lightly weighted, because that is exactly what it is. It will track your day-to-day activity faithfully and will systematically understate:
- cycling, swimming, and rowing, which produce few steps;
- strength training, for the same reason;
- any hard effort whose intensity a step count cannot see.
A Target Strain Zone built on step-derived history is internally consistent, because it was calibrated on the same channel, so Strain Performance stays meaningful even though the absolute values sit lower than a watch would produce.
What to do with it
- Log the sessions a step count cannot see: a recorded workout with a declared intensity is read directly, so a manually logged cycling session lands as session load rather than disappearing.
- Carry the phone on walks: it is the whole measurement.
- Do not compare a step-derived week with a watch-derived week: They are different channels. Compare each against its own Target instead, which is what Strain Performance already does for you.
Troubleshooting & FAQs
- I bought a watch. Will my old step-only days be recalculated?
- No. A dated assessment is never upgraded by a later algorithm release, because that would move numbers you have already seen. The one exception is a date that never produced a score at all: a blank has no history to rewrite, so it is filled the next time the date refreshes.
- Are my steps counted on top of my watch's exercise minutes?
- No. Exercise minutes remain the source of truth for any day the watch reported them. Steps are read only for a day the watch never covered, so the two channels can never double-count the same movement.
- Why does a day with a few hundred steps read 0% instead of blank?
- Because the phone measured it. A measured near-zero is a rest day, and a rest day is a real reading. Blank is reserved for a day where no channel observed anything at all.