Strain measures how much cardiovascular load you've taken on today, out of 100. It's the odd one out among your scores: the only one that moves during the day, and the only one with no baseline - it's a plain measure of load, before any comparison to your normal.
How it's worked out
Strain is built from the minutes you spend in each heart rate zone, with harder zones worth far more - a minute in your top zone counts several times a minute of easy movement.
Those weighted minutes then go through a curve with diminishing returns: climbing from 20 to 40 is much easier than climbing from 80 to 90, and 100 is approached rather than hit. Without that, one very long day would pin the scale and make every ordinary day look trivial next to it.
Your heart doesn't distinguish a logged session from a day spent hauling luggage around a city, and neither does Strain - all movement counts, workout or not.
There's no target
Strain measures how much, not how well - so there's no daily target to hit.
A high number on a hard training day is what you trained for; the same number on a day you meant to rest is worth a look. Instead of a verdict, NOYU describes the day - Minimal, Light, Moderate or Hard - and what your day ought to be depends on your week, your goals and how you're recovering.
A running total
Strain accumulates as the day goes: low at breakfast, jumping after an evening run. Your sleep score and Readiness are set from last night and stay put; Strain is the live one. If you're comparing Strain across days, compare finished days.
It also depends on activity syncing from your device, so a workout that hasn't synced isn't in the number. Most data lands within minutes, but this is the score most affected by a sync delay.
Zero and missing are different
If your device tracked your heart rate all day and you barely moved, your Strain is 0 - a real, quiet day.
If your device reported no heart rate zones at all, NOYU shows no score rather than 0, and says the data was missing. A day you didn't wear your watch is not a day you did nothing.
If your device shows its own strain figure
WHOOP and some others publish a strain number on their own scale, often 0 to 21. Where it's available NOYU shows it alongside for comparison, but doesn't use it. They're different formulas on different scales - neither is wrong, and they're not meant to match.
What to do with it
Watch the balance across the week rather than any single day. A stretch where every day reads Hard while Readiness slides underneath it is the pattern that precedes overreaching, and a week of Minimal days when you'd planned to train is its own signal. Both show up in the numbers before you feel them.
