Strain is a measure of how much physical load you have taken on today, from 0 to 100. It is the one score that moves during the day.
How it is worked out
Strain is built from the minutes you spend in each heart rate zone, with harder zones counting for much more. A minute in your top zone is worth several times a minute in your easiest one.
Those weighted minutes are then put through a curve with diminishing returns, so climbing from 20 to 40 is far easier than climbing from 80 to 90. This stops one very long session from pinning the scale at 100 and making every other day look trivial by comparison.
Everything counts, not just recorded workouts. A day of walking around a city can produce meaningful Strain with no workout logged at all.
How Strain differs from Cardio Load
Strain is the zoomed-out view of your whole day. Cardio Load is the focused view of intentional training: it counts tracked workouts only and compares your recent seven-day load with what has been usual over six weeks.
An unrecorded walk can raise Strain but cannot change Cardio Load. Recording that same walk as a workout means it contributes to both.
There is no Strain target
Strain measures how much, not how well - so there is no daily target to hit. A high number on a hard training day is exactly right. The same number on a day you meant to rest is worth a look. A low number the day after a race is a sign things are going to plan.
Rather than a verdict, NOYU describes the day: Minimal, Light, Moderate or Hard. What that ought to be depends on your week, your goals and how you are recovering.
Why it changes during the day
Strain is a running total for the day so far. Check at eight in the morning and it will be low; check after an evening run and it will have jumped.
This is different from your sleep score and Readiness, which are set from last night and stay put. If you want to compare Strain across days, compare finished days.
Because it depends on activity syncing from your device, a workout that has not reached NOYU yet is not in the number yet. Most data arrives within minutes, but this is the score most affected by a sync delay.
Zero Strain and no Strain are different
If your device recorded your heart rate all day and you barely moved, your Strain is 0. That is a real, quiet day.
If your device recorded no heart rate zones at all, NOYU shows no score rather than 0, and tells you the data was missing. A day you did not wear your watch is not a day you did nothing.
If your device shows a different strain figure
WHOOP and some others publish their own strain figure on their own scale, often 0 to 21. NOYU shows that alongside yours as a comparison when it is available, but it does not use it.
The two are different formulas on different scales. Neither is wrong; they are not meant to match.
