Strain is a summary metric of the strain undergone by the cardiovascular system during an individual Activity or over the course of a day. It is shown on a scale from 0 to 21.
All out workouts are 18 and above; strenuous workouts are typically 14-18; moderate workouts are typically 10-14; and everything below 10 represents degrees of light or minimal activity. The same scale applies to your entire Day’s Strain.
An activity’s Strain is a measure of how hard you worked, not what you did; as a result, different individuals will not necessarily receive identical scores for completing identical workouts. For example, a “very hard” workout for you may be a “near maximal” workout for someone else.
How is Strain Calculated?
Strain is a measurement, on a 21-point scale, which quantifies the total cardiovascular load undergone as a function of the total cardiovascular load you were capable of taking on in a 24-hour period. Unlike counting steps, Strain is personalized and accounts for differences in fitness and ability such that two people who complete the same run (with the same amount of steps) could get very different strain scores based on differences in the relative difficulty to complete that run. In the case of this run, the more fit athlete would get a lower Strain because he or she didn't have to work as hard to complete the run as the less fit athlete did.
Strain is calculated by the duration of time you spend in each of your personal max HR zones. Each percentile has a different weight to how much Strain will increase. If you only walk throughout a day, your Strain score will not increase, as it is essentially a resting heart rate.
Strain starts to build at the beginning of each cycle (when you go to sleep) and the score will get harder and harder to increase as it gets higher. If you don't do much activity outside of walking and/or working, it may not increase. Especially if you have done workouts that are strenuous in the past, as Whoop establishes thresholds for what hard work is to you personally.
To learn more about Strain listen to Podcast No. 26: Understanding Strain