Below we’ll discuss exactly what our strain metric is, how it’s calculated, factors that play into determining your strain for a whole day and for various activities, WHOOP averages for each, as well as the relationship between strain and recovery.
WHOOP strain is a measure of cardiovascular exertion that quantifies the amount of physical and mental stress you’re putting on your body. We track your strain on a 0-21 scale, both for your entire day and for specific workouts and activities.
Inspired by Borg’s Rating of Perceived Exertion, strain numbers can be loosely broken down as follows:
LIGHT (0-9): Minimal stress put on the body, room for active recovery
MODERATE (10-13): Moderate stress on the body, generally good for maintaining fitness
HIGH (14-17): Increased stress and activity level, ideal for making fitness gains when training
ALL OUT (18-21): Significant stress, often overreaching, likely very difficult to recover from the next day
When your heart rate begins to rise, you start building strain. The higher your HR gets and the longer it stays elevated, the more strain you accumulate. In this sense, your strain is calculated based on the length of time you spend in various heart rate zones (percentages of your max heart rate).
Beyond exercise and other physical exertion, anything that gets your heart pumping will boost your strain. Regular daily activities like work, commuting, running errands, and parenting may all cause strain. So can stress, anxiety, excitement, or feeling nervous–even being a sports fan is strenuous.
When WHOOP notices a spike in heart rate and movement for a sustained period of time, it will automatically detect your activity or workout and give you a strain value for it (you can also add other activities manually and see their strain).
Your total day strain provides you with additional insight into the cardiovascular load your body takes on outside of exercise. For example, a presentation at work or a busy afternoon with the kids might raise your day strain above average, even on a day you don’t work out.
If you have multiple workouts in a day, the strain of your activities does not add up to your total day strain. The WHOOP strain algorithm is logarithmic, meaning the higher your strain gets the harder it becomes to build more.
As illustrated by the top graphic above, it is much easier to go from a 0 to 10 strain than it is to go from a 10 to 20. Hypothetically, running a marathon might lift your day strain to 20.4 or 20.5, but then running a second one that day could only increase it to 20.6 or 20.7.
The strain average for all WHOOP members is right around 11.0 per day. To no surprise, the number tends to decrease with age. Below you can see what daily average strains are by age and gender:
When it comes to individual workouts, more intense aerobic exercise leads to higher strain than less intense activity. The average strain for 1 hour of running is roughly 12.0, while an hour of walking is about 6.5.
Because strain is a product of your elevated heart rate, cardio-based workouts increase it faster than weightlifting or other strength-based training that doesn’t impact your HR as significantly (although the toll they take on your body is still reflected in your daily recovery). The average strain for 1 hour of functional fitness is 10.1.
Strain is calculated from your personal HR metrics and accounts for your individual fitness level. Since it quantifies how hard your body works and not what your body does, two people who complete the same activity may have very different strains.
For example, a 90-minute hike that registers a 10 or 11 strain for the average person might be just a 5 or 6 for a highly conditioned athlete. Along the same lines, as your fitness improves you’ll start to see lower strains for that same activity.
The higher your recovery is, the more strain your body is ready to take on.
You may notice strain accumulate faster on days when you have a low recovery, because your body is not as prepared to handle it. A workout routine that usually gives you a 9.5 might be a 10.5 when you’re in the red.
The Strain Coach suggests an optimal amount for you to take on each day based on your recovery. It’s a recommended level of strain intended to let you maintain fitness and still adequately recover the following day.
If you go above this amount you are “overreaching” (good for making fitness gains, but likely detrimental to your next-day recovery), and if you stay below it you are “restoring.”
In turn, the amount of strain you take on today then affects what your recovery will be tomorrow.