Strain

Heart rate

Every reading from local midnight to midnight, sleep included. Each reading covers the gap to the next one, up to 2 minutes.

Resting heart rate

Fitbit’s daily value from Google, else last night’s sleeping resting heart rate, else 60 bpm.

Max heart rate

The value in Settings; else the top of your Peak zone from Google; else 208 − 0.7 × your age.

Each minute earns points by how hard your heart works, as a share of your heart-rate reserve: the gap between your resting and max heart rate.

Below 50%

0 points a minute

50-59%

1 point a minute

60-69%

2 points a minute

70-79%

3 points a minute

80-89%

4 points a minute

90% and up

5 points a minute

The day’s points go on a log scale: Strain = 21 × ln(points + 1) ÷ ln(7,201), where 7,201 is a whole day at 5 points a minute, plus one. An hour at 70-79% with nothing else gives about 12.3. Doubling your points adds only about 1.6, so each point of Strain is harder to earn than the last.

0-9.9

Light

10.0-13.9

Moderate

14.0-17.9

Strenuous

18.0-21.0

All out

Strain needs at least 600 heart-rate readings, or 20 spread over at least 10 minutes; otherwise the day shows Not enough data. Today’s Strain is a running total until midnight. Each activity also gets its own Strain from the heart rate during it.

Heart rate misses effort that barely raises it, such as heavy lifting with long rests. The zone chart uses Google’s heart-rate zones for the day (Light, Moderate, Vigorous, Peak), or % of max heart rate on a day Google has none, while Strain uses its own steps of heart-rate reserve, so the two do not line up exactly.

1. noop: the open-source analytics engine Pulse's recovery, strain and sleep scoring is ported from
2. Edwards S. The Heart Rate Monitor Book. 1993 (heart-rate zone weights)