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.
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.
Viewing Strain in the App
The Strain overview screen in the app provides a summary of the day’s Strain and Activities. The overall score displayed represents the Strain you have accumulated over the course of your entire day.
To view Strain associated with individual workouts, select the activity button inside the Strain circle.
To compare your current day's Strain statistics (Calories Burned, Average Heart Rate, and Max Heart Rate) to your previous 30-day average, select the flip box at the bottom of the screen.
How Strain is Calculated
Strain is calculated by the duration of time you spend in each of your personal heart rate zones, established from your max heart rate. Each heart rate percentile range has a different weight to how much Strain will increase. For example: if you walked casually throughout the day, your Strain score would most likely remain low; because despite being active, your heart rate (including cardiovascular load) did not elevate.
NOTE: The longer your heart spends at an elevated rate, the higher your Strain will be.
Strain starts to build when you go to sleep and each successive score requires more effort to achieve. This means that you could wake up with a Strain score anywhere from 0 - 4 upon waking; this is perfectly normal.
Your strain score is strictly based on your heart rate data and the amount of time spent in each heart rate zone. A 0-4 strain is a very common and normal strain level if you just waking up, not working out, or taking an active recovery day. This does not mean that you are not active, but that your heart rate is not very elevated. This is why you are likely to see a low Strain for the day.
Strain is personalized and accounts for differences in fitness and ability. Therefore, two people who complete the same run (distance and level) could get very different Strain scores based on those differences (and relative difficulty). An Activity's Strain is a measure of how hard your body worked, and not what your body did. Depending on your fitness level, a very hard workout for you may be a near maximal workout for someone else.
Day Strain measures the Strain you have accumulated over the course of an entire day.
While individual workouts receive Strain Scores to indicate solitary levels of cardiovascular effort (for a discrete period of time), Day Strain provides you with a full picture of the Strain you are putting on your body each day.
Day Strain is useful in determining what non-exercise activities are contributing the most to your accumulated Strain. This can help you optimize planning your days leading up to a competition or help you identify daily activities that may be contributing to elevated Strain. In addition, Day Strain takes into account Activities you may not consider to be workouts, such as your daily commute.
To learn more about Strain, check out our WHOOP Podcast No. 26: Understanding Strain.