The Whoop first determines your maximum heart rate, then uses that number to establish your personal heart rate zones. It then monitors the duration of time you spend in each of your heart rate zones to calculate your strain.

Strain is a summarizing metric of the cardiovascular load - the level of strain your training takes on your cardiovascular system as based on your heart rate - achieved during an individual Activity or over the course of a day.  Strain is scored on a scale from 0 to 21. 

Strain Scale

In general, the Strain scale is divided into 4 categories, shown below:

Light Strain (0-9) - This strain category indicates room for active recovery with minimal stress being put on the body.

Moderate Strain (10-13) - This category indicates moderate stress is being put on the body, which helps maintain fitness.

High Strain (14-17) - This category indicates increased stress and/or activity which helps build fitness gains in your training.

All Out (18-21) - This category indicates all-out training or a packed activity day that put significant stress on the body and may be difficult to recover from the day after. 

This recording of Nellie Korda demonstrates the mental aspect of golf nicely.

When Holter monitor first came out in 1970's, one of interesting studies was

the recordings of interns' heart before and during and after case presentations 

at weekly Medical Grand Rounds at a University Hospital.

Their heart rate exceeded 170 per minute surpassing the heart rate during skiing

by the same interns at Vale, Colorado, which was under 170.

My mentor during my Cardiology fellowship used to joke about the fact that

interns and residents ought to be thankful by their leading a stressful life

which automatically conditions their CV system.