Understanding how WHOOP measures strain

Awarded on a scale between 0.0 - 21.0, strain is one of WHOOP’s most commonly misunderstood metrics among newcomers: how come I’ve recorded two activities today each with a strain of 5.0 but my day strain is still less than 10.0? How much effort do I need to put in to get my current strain up to my daily target? Let’s take a look.

Strain progress is not linear

The more strain you earn - within an activity, or over the course of a day - the harder it is to earn more. Getting nerdy for a moment: it becomes exponentially harder to accrue strain the more you earn.

In order to let you compare activities against each other, this scale resets to zero within each activity. This makes sense, because otherwise you might record a 10.0 activity in the morning and then do an identical workout later that day only to get a much lower strain score the second time around. You’d have no objective way of comparing the two activities.

The flip side is that the maths doesn’t add up when you record multiple activities within a day. The key is to not try and make it: in their own right those activities were 10.0 strain efforts, even if in sum they’re less than 20.0. If you’d recorded them as one long activity, they wouldn’t have added up to 20.0 either.

Day strain resets at midnight

There’s another reason you can’t just sum your activity scores together for an accurate picture of day strain: the latter resets every day at midnight local time. This means that a workout spanning midnight will contribute to two different day strains but only a single activity strain score.

Visualizing the strain scale

The graph below might help get a better understanding of how strain accumulates, using estimates derived by Reddit user u/Jjpeters10 to whom all credit is due. You can choose to plot strain on the Y-axis (current) or the X-axis:

The graph approximates how much effort (a purely relative measure) you need to put in to hit a certain strain. Take the exact values with a grain of salt - particularly towards the lower and upper ends of the scale - and focus instead on the rough shape of the curve. You may spot a few things:

1. Hitting 5.0 strain takes very little effort: this explains sometimes waking up with a strain score closer to 5.0 than 0.0
2. It takes roughly 3x the effort to go from 5.0 to a 10.0, and another 3x that to get to a strain score of 15.0
3. Getting near the maximum strain score of 21.0 takes roughly 10x the effort of getting to 10.0 (in practice it’s probably even harder: the curve flattens out a lot more the closer you get to 21.0!)

For example, if it takes you 30 minutes of running to get to 10.0 strain, you can expect to need to run for about an hour to get to around 13.2 and roughly 90 minutes to hit 15.0, assuming a constant effort. Remember that the ‘effort’ scale shown above is relative: indicated values of 10, 50 and 100 mean nothing in an absolute sense and will vary from person to person. What they do tell you that 50 is five times harder than 10 and 100 is twice as hard as 50.

High strain in practice

The theory is all well and good, but what does it take to actually hit the upper echelons of the strain scale? Below is a graph of my own personal day strain distribution after 1,150+ days on WHOOP:

You can’t quite tell from that static image, but I’ve recorded a day strain of 20.0 or more only 30 times - or about 2% of the days I’ve had WHOOP. My raw data shows my highest day strain is only 20.7, which I’ve hit five times in three years:

So, what does it take to hit 20.8 or more? Well, Lachlan Morton managed it after riding 565km (351mi) and climbing 3,833m (12,575ft) in a single day at the end of his alternative Tour de France route - that might give you some idea. If you’ve ever hit a 20.8 or above, let me know and I’ll add you to this rather short list!

Explore your strain on fitIQ

FitIQ lets you visualise loads of training and activity metrics, including day and activity strain. But it also allows you to go further: you can set goals based on how much strain you want to average over a week, month, or a year, or set a maximum strain goal you want to hit once in a given period. I’m still chasing my target of a single 20.8 this year and have been for a while! You can also earn strain-related achievement badges and track strain-related streaks, too.

fitIQ is free to try and reasonably priced if you like what you see. Give it a go today and get a handle on your strain.