WHOOP Strain: what the 0 to 21 scale means and what it can and cannot see

WHOOP Strain is a daily and per-activity score on a 0 to 21 scale. WHOOP's developer documentation calls it "a measurement of the amount of stress on your body" and says the scale is "based on the Borg Scale of Perceived Exertion." It is the counterpart to Recovery: Recovery estimates how ready you are, Strain estimates how much you did.

The bands

Where the 21 comes from

The Borg Rating of Perceived Exertion is a scale on which people rate how hard exercise feels. Gunnar Borg's original version runs from 6 to 20. WHOOP has adapted the idea to a 0 to 21 range built from measured data rather than a personal rating. WHOOP's support material describes the score as non-linear: moving from a low Strain to a moderate one takes less extra effort than moving from high to all out. That mirrors how fatigue accumulates.

What goes into it

WHOOP's support material describes Strain as built mainly from cardiovascular load: how high your heart rate goes and how long it stays up, judged against your own heart rate zones, which come from your maximum heart rate. Newer WHOOP material also describes a muscular load component intended to capture strength training, which heart rate alone underestimates. Strain accumulates through the whole day, not only during logged workouts. The same session will usually produce a lower Strain as you get fitter, because your heart rate response falls.

What is proprietary

WHOOP does not publish how time in each heart rate zone is weighted, how muscular load is estimated or combined with cardiovascular load, or the curve that compresses the result into 0 to 21. Strain is therefore internally consistent for one person on one device, but it cannot be compared directly with training load scores from other brands, and it cannot be recalculated from outside.

How accurate is the heart rate behind it?

Strain rests on heart rate during movement, which is harder for optical sensors than heart rate at rest.

WHOOP was not among the devices in the two older studies, so they show the general limits of wrist-based optical heart rate rather than WHOOP specifically. The practical message: steady cardio is tracked well, intervals with sharp changes in heart rate less well, and lifting sessions are underestimated by any heart-rate-only model. ECG chest straps are the usual reference in these studies.

What Strain does not tell you

Strain measures load, not whether that load is enough for health. The US physical activity guidelines and the WHO ask adults for 150 to 300 minutes a week of moderate activity, or 75 to 150 minutes of vigorous activity, plus muscle-strengthening work on two or more days. A week of moderate Strain scores may or may not meet that, depending on how the minutes were spent. Strain also says nothing about technique, progression or the muscle you keep during weight loss, which is why a written training log still matters. The strength training guide covers the basics.

Why the calorie figure is the weakest number

The Stanford study behind the 20 percent figure tested 60 adults of varied age, size, skin tone and fitness on seven wrist devices while sitting, walking, running and cycling. Heart rate error was lowest during cycling and highest during walking, and was higher for men, for people with a higher BMI and for darker skin tones. Apple Watch had the lowest overall error for both heart rate and energy expenditure, and the Samsung Gear S2 the highest. Even the best device was off by more than 20 percent on energy. The 2020 review of 158 publications reached the same verdict across nine brands and also noted that Fitbit tended to underestimate heart rate.

Using Strain well

• Compare with yourself. A 14 for you is not a 14 for someone with a different maximum heart rate and fitness.
• Read it with Recovery and sleep. High Strain on a red Recovery day is a choice worth making deliberately, not by accident.
• Do not use calories from it as a food budget. Energy expenditure from wrist devices was off by 20 percent or more for every device in the Stanford study, and no brand was accurate in the 2020 review.
• Log your lifting separately. Sets, reps and load describe strength training better than a heart rate trace.

Logging it

WHOOP connects to Peptide AI directly through wearable integrations. Strain sits alongside the training log, where sets and loads are recorded, and the dose log. The cardio and Zone 2 guide covers how much aerobic work adults need.