Strain is a summary metric of the cardiovascular load achieved during an individual Activity or over the course of a day. It is shown on a scale from 0 to 21.
All out, near maximal workouts are considered 18 and above; strenuous workouts are considered 14-18; moderate workouts are 10-14; and everything below 10 represents degrees of light or minimal activity. The same scale applies to your entire Day's Strain.
Viewing Strain in the App
The Strain overview 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 the Strain associated with individual workouts, tap the button under your score to access each workout. You can also view each day’s Strain statistics (Calories Burned, Average Heart Rate, and Max Heart Rate) relative to the average of the last 30 days by tapping on the flip box at the bottom of the screen.
How Strain is 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. Strain is personalized and accounts for differences in fitness and ability such that two people who complete the same run could get very different strain scores based on differences in the relative difficulty to complete that run. That means that an Activity’s Strain is a measure of how hard you worked, not what you did. For example, a “very hard” workout for you may be a “near maximal” workout for someone else.
Strain is calculated by the duration of time you spend in each of your personal max heart rate zones, established from your max heart rate. Each percentile has a different weight to how much Strain will increase. The more elevated your heart rate, and for longer duration, the higher your Strain. For example, if you walked casually throughout the day, your Strain score would probably stay low, because even though you were active, your heart rate (and therefore cardiovascular load) did not elevate.
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, and this is perfectly normal.
Day Strain is a measure of the Strain you have accumulated over the course of an entire day. While individual workouts receive Strain Scores to indicate your level of cardiovascular effort for that 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 very useful in determining what non-exercise activities are contributing the most to your accumulated Strain. This can help you better plan your days leading up to a competition, or can help identify activities during your day 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 listen to Podcast No. 26: Understanding Strain