November 17, 2025 • 12 mins
EP41 - Strava Relative Effort - A Deep Dive (ft AI Insights)
Original: https://the5krunner.com/2025/11/17/strava-relative-effort-guide-tss-2025/
Mark as Played
Transcript
Episode Transcript
Available transcripts are automatically generated. Complete accuracy is not guaranteed.
(00:00):
OK, so let's try to unpack this.It's a fundamental challenge
really for any athlete who's even a little bit nerdy about
their training. How do you actually compare
effort? You know, you go out and do a
brutally hard 5K race, you feel like you're going to pass out.
Then the next day, maybe some relaxed 3 hours Sunday bike ride
that just felt easy. They're both training, but which
(00:21):
one cost you more physiologically, I mean, and how
do you even begin to weigh that across different sports like
running versus cycling versus swimming?
It is, you know, the ultimate apples to oranges problem in
training. It really is because external
things like your pace or your speed or just how long you were
out there for, that's only half the story.
(00:41):
Just half the picture. Exactly.
To really get it right, to optimise your training, you need
some kind of common currency forthe internal stress the the
physiological cost to your body.And that's where Strava's
relative effort or RE comes in. That's.
Precisely what it was designed to solve.
So our mission today is a deep dive into this metric.
(01:02):
We're going to get under the hood of that little RE score you
see on your dashboard. Yeah, look at its roots.
Exactly. Look at its roots in exercise
science, how it's calculated andyou know critically the
practical pros, cons and all thelittle caveats.
We've pulled from a lot of technical analysis, Strava's own
engineering blogs and a lot of practical commentary from
(01:22):
endurance experts like the 5K Runner who's done a tonne of
work on this. He really has.
He was one of the first to pointout its connection to the
classic Trimp model. So at its core, what is it?
Relative effort is basically Strava's single number that
tries to quantify your total cardiovascular load.
That's it. It uses your heart rate data.
That's the key internal measurement.
(01:43):
Or if you don't use a monitor, it'll let you manually punch in
your perceived exertion, your RPE, to get that personalised
score. OK, so let's go back to the
blueprint then. This isn't just some number
Strava made-up. It has, like you said, deep
roots in exercise science, specifically something called
training impulse. That's right, The whole
physiological basis for REE is an adaptation of the very well
(02:06):
established training impulse method.
We call it TRIM. It's a validated framework that
calculates strain by giving a really heavyweight to the amount
of time you spend in your various heart rate zones.
So the higher the zone, the morepoints you get basically.
Exponentially more, and as the 5K runner pointed out, RE is
fundamentally based on Bannisters widely used trim
formula. That makes a tonne of sense.
(02:27):
You get way more credit for timespent in the red, you know, Zone
5. But trim in its original form
sometimes had this, this duration bias, right?
And this is where Strava kind ofmade a key change.
They shifted the focus to intensity.
That is the defining feature of RE.
It's why it's such an improvement over what they had
before. Strava basically tuned the
formula to make sure that a short maximal effort session,
(02:50):
say a 20 minute all out HIID workout, can actually generate
the same rescore as a much much longer lower intensity session.
Like that two hour easy zone 2 ride?
Exactly, and that's so importantbecause it properly reflects the
huge physiological debt and the recovery you need after high
intensity training. And this was a necessary change
(03:11):
because RE replaced that older metric which I'm sure a lot of
our listeners will remember the suffer score.
Oh, the suffer score. Yeah, back in April 2018, I
remember my easy Sunday rides would always score ridiculously
high compared to it. You know, the crushing track
workout. So why was the suffer score so
bad at rewarding intensity? Well, you experienced exactly
(03:34):
its failure point. The old suffer score had two
massive weaknesses. 1st, a huge sport bias because of how your
heart rate just tends to spike more during weight bearing
exercise. Runners would score way, way
higher than cyclist for what felt like the same effort.
So if you were a triathlete. It was almost useless for
comparing your run load to your ride load.
So my 10K run would look way harder than my 40K bike time
(03:55):
trial even if they both felt like a Max effort.
Precisely. And the second flaw, what you
just hit on was the duration bias, the suffer score.
Just loved it when you were out there for a long time.
So your 4 hour easy zone 2 ride might score 150, but a short,
brutal 30 minute interval workout might only score 40.
(04:16):
RE completely flips that. Now that 30 minute HII workout
might score 100 and the long easy ride might also score 100.
It finally creates A levelled playing field for things like
running, cycling, swimming, whatever, as long as you have
heart rate data. Which leads us right into the
biggest pro that personalization, that universal
comparability. The system works because it
(04:37):
knows you. Exactly.
RE is entirely personalised. It's based on your individual
Max heart rate, your Max hour, and your custom zones.
So if you and a friend who mayberuns way faster than you both go
run your hardest possible marathon, your RICE scores would
actually be pretty similar. Because the effort relative to
your own limits was the same. It's an equalisation metric,
that's the key. And that daily RE score doesn't
(04:59):
just sit there, it feeds into a much bigger system for managing
your long term training. This is where we get into
chronic and acute load, yes. The daily RE score is the
fundamental impulse for Strava'smain cracking tool, the fitness
and freshness graph, and that system uses the Impulse Response
model, which was pioneered by Bannister and later really
(05:20):
applied extensively by Andrew Coggan.
So let's breakdown those 3 termsbecause they are so important.
What are the components of that forecast?
OK. So first you have fitness,
that's your chronic training load.
Think of it as your long term capability, what you've built up
over weeks and months. Then there's fatigue, which is
your acute training load. So that's the strain from the
(05:40):
last say 7 to 14 days, OK. Your recent work.
And finally, form is just fitness minus fatigue.
Form tells you if you're fresh enough to actually perform well.
A powerful way to manage your risk and for, you know, daily
use. I think the weekly relative
effort range is probably the most actionable part of the
whole system. Oh absolutely.
That shaded band is just calculated using a three-week
(06:02):
moving average of your RE totals.
It's basically a really easy to read proxy for the acute chronic
workload ratio, or ACWR. Which just means if you want
sustainable progressive training, you should aim to stay
in that green band. That's the idea.
Staying green means your acute load is building gradually on
(06:22):
top of your chronic base. If you go above range into the
orange or red, that's a signal. Oh, overload.
Block a deliberate overload block and the system is
basically warning you, hey, you need to follow this up with
recovery and going below range in the blue.
That's for your recovery weeks or a taper before a big race.
So the goal isn't to live in thegreen band forever.
(06:42):
No, not at all. The goal is strategic
manipulation. You want those peaks and valleys
to force your body to adapt. That's super compensation.
OK, so RE is simple, it's standardised, it's personalised.
But now we have to talk about the downsides.
The very simplicity that makes it great also makes it
vulnerable. It relies completely on one
thing. Your heart rate.
Your heart rate. And that is the core limitation.
(07:03):
Because it's measuring internal load, it's really susceptible to
what we call confounders. These are external factors that
can Jack up your heart rate without actually increasing your
mechanical work. Oh I've seen this.
I did an easy recovery run during a massive heat wave 1
summer and Strava thought I'd just run a half marathon PR.
My HR was just through the roof from the heat.
That's a perfect example. The main confounders are
(07:25):
environmental stress, so heat and humidity, dehydration,
getting sick, psychological stress from work or life, and
stimulants like caffeine. Sure, if you run in 90° heat,
your heart rate spikes because your body is trying to cool you
down, not just move you forward.So the RE score gets inflated.
And since the entire system is built on how much time you spend
(07:45):
in these different zones, the accuracy of your Max HR has to
be absolutely critical. It is paramount the whole system
stands or falls on having an accurate Max HR if you're just
using those generic age based formulas like 220 minus age.
So many people do. So many people do, but they can
be off by 10, even 20 beats per minute.
(08:07):
And if your Max HR is wrong, your zone 4 and zone 5 are
wrong, which basically makes thewhole RE score pretty much
meaningless. Now you mentioned already
prioritises intensity to fix theold suffer scores flaws, but
some people especially in the ultra endurance world, they
argue that might have been a bitof an overcorrection for what
they do. That's the other side of the
coin, yeah. By waiting high intensity so
(08:29):
heavily, the model can sometimesunderestimate the sheer
cumulative fatigue from these extremely long low intensity
efforts. Like 100 mile bike ride or a
long mountain. High exactly an effort where
you're mostly in zone 2. Your heart rate curve kind of
flattens out, but you are accumulating massive muscle
damage, tendon stress, glycogen depletion.
So RE is great for cardiovascular stress, but for
(08:51):
ultra distances it kind of misses the local muscular
damage. It does that non cardio stress
just isn't fully captured by theheart rate curve, which is why
those athletes often rely more on external metrics like power
or pace which feed into something like training stress
score or TSS. And we should probably just
state this clearly before we getinto devices RE and fitness and
(09:12):
freshness and all that. It's not free.
Correct, these are all subscriber only features on the
Strava platform. OK, so let's get practical.
How do you actually use this live on your device?
Especially for Garmin users since they're just so dominant
in the endurance space. Well, live relative efforts
pretty widely available. It's built into Wahoo devices,
(09:34):
it's on Santo through Sunto Plus.
And for Garmin, yeah, you just need a Connect IQ capable
device. And there's a really important
technical step for Garmin users,right, to make sure what you see
on your watch matches with, Strava says later.
Yes, this is key. You have to download the Scrava
Relative effort CIQ app. It's super popular, but more
(09:54):
importantly, you must go into your settings and make sure that
the heart rate zones in your Garmin Connect IQ profile file
exactly match the HR zones in your Straba profile.
If they're different, the numbers won't line up.
The calculation on your watch won't align with the final score
that gets generated after you upload.
They have to match. And what about indoor training?
You're on a smart trainer or a treadmill, No GPS.
(10:15):
For stationary activity, a heartrate monitor alone usually isn't
enough. You need some kind of external
work metric to go with it. So the speed sensor.
The speed sensor, a power metre for the bike or a foot pod for
running. The system needs to see that
your internal load, your HR, is actually tied to you doing some
work. Otherwise your heart rate could
just be drifting up because you're getting hot and the score
(10:37):
would be invalid. Interesting little detail here.
Even if you're not a paid subscriber, you can still see
the metric while you're working out.
That's true. Free users can see they already
score live on their Garmin via the Connect IQ app.
But it's it's ephemeral. It'll vanish once the workout is
done. It won't appear in your activity
details online or in the app. So to synthesise all this,
(10:59):
Strava's relative effort is a simplified, multi sport, pretty
actionable metric that makes sophisticated load management
accessible for the everyday athlete.
It is. It's an accessible gateway into
periodization, but for the listener who wants to take this
whole concept to the next level,we'd suggest monitoring
something called decoupling. Decoupling.
(11:20):
It sounds complicated. How does that tie back to RE?
It's basically the comparison between your internal cost and
your external work. So you take your internal load,
which is your RE, and you compare it against an external
metric like your pace or if you use a power metre, your training
stress score TSS. And what are you looking for?
You're looking for a disconnect.If you notice, your relative
(11:42):
effort is disproportionately high compared to your output, so
you're working a lot harder for the same speed or power.
That decoupling is an early warning sign.
A sign of what? It signals accumulating fatigue
or maybe significant heat strain, or just a drop in your
efficiency. So your body is basically
working harder than it should beto get the same result, which
means what you really need isn'tmore training.
(12:04):
It's recovery. It's recovery that really
transforms RE from just a score you can brag about into a high
level diagnostic tool for coaching yourself.
OK, so let's try to unpack this.It's a fundamental challenge
really for any athlete who's even a little bit nerdy about
their training. How do you actually compare
effort? You know, you go out and do a
brutally hard 5K race, you feel like you're going to pass out.
Then the next day, maybe some relaxed 3 hours Sunday bike ride
that just felt easy. They're both training, but which
(00:21):
one cost you more physiologically, I mean, and how
do you even begin to weigh that across different sports like
running versus cycling versus swimming?
It is, you know, the ultimate apples to oranges problem in
training. It really is because external
things like your pace or your speed or just how long you were
out there for, that's only half the story.
(00:41):
Just half the picture. Exactly.
To really get it right, to optimise your training, you need
some kind of common currency forthe internal stress the the
physiological cost to your body.And that's where Strava's
relative effort or RE comes in. That's.
Precisely what it was designed to solve.
So our mission today is a deep dive into this metric.
(01:02):
We're going to get under the hood of that little RE score you
see on your dashboard. Yeah, look at its roots.
Exactly. Look at its roots in exercise
science, how it's calculated andyou know critically the
practical pros, cons and all thelittle caveats.
We've pulled from a lot of technical analysis, Strava's own
engineering blogs and a lot of practical commentary from
(01:22):
endurance experts like the 5K Runner who's done a tonne of
work on this. He really has.
He was one of the first to pointout its connection to the
classic Trimp model. So at its core, what is it?
Relative effort is basically Strava's single number that
tries to quantify your total cardiovascular load.
That's it. It uses your heart rate data.
That's the key internal measurement.
(01:43):
Or if you don't use a monitor, it'll let you manually punch in
your perceived exertion, your RPE, to get that personalised
score. OK, so let's go back to the
blueprint then. This isn't just some number
Strava made-up. It has, like you said, deep
roots in exercise science, specifically something called
training impulse. That's right, The whole
physiological basis for REE is an adaptation of the very well
(02:06):
established training impulse method.
We call it TRIM. It's a validated framework that
calculates strain by giving a really heavyweight to the amount
of time you spend in your various heart rate zones.
So the higher the zone, the morepoints you get basically.
Exponentially more, and as the 5K runner pointed out, RE is
fundamentally based on Bannisters widely used trim
formula. That makes a tonne of sense.
(02:27):
You get way more credit for timespent in the red, you know, Zone
5. But trim in its original form
sometimes had this, this duration bias, right?
And this is where Strava kind ofmade a key change.
They shifted the focus to intensity.
That is the defining feature of RE.
It's why it's such an improvement over what they had
before. Strava basically tuned the
formula to make sure that a short maximal effort session,
(02:50):
say a 20 minute all out HIID workout, can actually generate
the same rescore as a much much longer lower intensity session.
Like that two hour easy zone 2 ride?
Exactly, and that's so importantbecause it properly reflects the
huge physiological debt and the recovery you need after high
intensity training. And this was a necessary change
(03:11):
because RE replaced that older metric which I'm sure a lot of
our listeners will remember the suffer score.
Oh, the suffer score. Yeah, back in April 2018, I
remember my easy Sunday rides would always score ridiculously
high compared to it. You know, the crushing track
workout. So why was the suffer score so
bad at rewarding intensity? Well, you experienced exactly
(03:34):
its failure point. The old suffer score had two
massive weaknesses. 1st, a huge sport bias because of how your
heart rate just tends to spike more during weight bearing
exercise. Runners would score way, way
higher than cyclist for what felt like the same effort.
So if you were a triathlete. It was almost useless for
comparing your run load to your ride load.
So my 10K run would look way harder than my 40K bike time
(03:55):
trial even if they both felt like a Max effort.
Precisely. And the second flaw, what you
just hit on was the duration bias, the suffer score.
Just loved it when you were out there for a long time.
So your 4 hour easy zone 2 ride might score 150, but a short,
brutal 30 minute interval workout might only score 40.
(04:16):
RE completely flips that. Now that 30 minute HII workout
might score 100 and the long easy ride might also score 100.
It finally creates A levelled playing field for things like
running, cycling, swimming, whatever, as long as you have
heart rate data. Which leads us right into the
biggest pro that personalization, that universal
comparability. The system works because it
(04:37):
knows you. Exactly.
RE is entirely personalised. It's based on your individual
Max heart rate, your Max hour, and your custom zones.
So if you and a friend who mayberuns way faster than you both go
run your hardest possible marathon, your RICE scores would
actually be pretty similar. Because the effort relative to
your own limits was the same. It's an equalisation metric,
that's the key. And that daily RE score doesn't
(04:59):
just sit there, it feeds into a much bigger system for managing
your long term training. This is where we get into
chronic and acute load, yes. The daily RE score is the
fundamental impulse for Strava'smain cracking tool, the fitness
and freshness graph, and that system uses the Impulse Response
model, which was pioneered by Bannister and later really
(05:20):
applied extensively by Andrew Coggan.
So let's breakdown those 3 termsbecause they are so important.
What are the components of that forecast?
OK. So first you have fitness,
that's your chronic training load.
Think of it as your long term capability, what you've built up
over weeks and months. Then there's fatigue, which is
your acute training load. So that's the strain from the
(05:40):
last say 7 to 14 days, OK. Your recent work.
And finally, form is just fitness minus fatigue.
Form tells you if you're fresh enough to actually perform well.
A powerful way to manage your risk and for, you know, daily
use. I think the weekly relative
effort range is probably the most actionable part of the
whole system. Oh absolutely.
That shaded band is just calculated using a three-week
(06:02):
moving average of your RE totals.
It's basically a really easy to read proxy for the acute chronic
workload ratio, or ACWR. Which just means if you want
sustainable progressive training, you should aim to stay
in that green band. That's the idea.
Staying green means your acute load is building gradually on
(06:22):
top of your chronic base. If you go above range into the
orange or red, that's a signal. Oh, overload.
Block a deliberate overload block and the system is
basically warning you, hey, you need to follow this up with
recovery and going below range in the blue.
That's for your recovery weeks or a taper before a big race.
So the goal isn't to live in thegreen band forever.
(06:42):
No, not at all. The goal is strategic
manipulation. You want those peaks and valleys
to force your body to adapt. That's super compensation.
OK, so RE is simple, it's standardised, it's personalised.
But now we have to talk about the downsides.
The very simplicity that makes it great also makes it
vulnerable. It relies completely on one
thing. Your heart rate.
Your heart rate. And that is the core limitation.
(07:03):
Because it's measuring internal load, it's really susceptible to
what we call confounders. These are external factors that
can Jack up your heart rate without actually increasing your
mechanical work. Oh I've seen this.
I did an easy recovery run during a massive heat wave 1
summer and Strava thought I'd just run a half marathon PR.
My HR was just through the roof from the heat.
That's a perfect example. The main confounders are
(07:25):
environmental stress, so heat and humidity, dehydration,
getting sick, psychological stress from work or life, and
stimulants like caffeine. Sure, if you run in 90° heat,
your heart rate spikes because your body is trying to cool you
down, not just move you forward.So the RE score gets inflated.
And since the entire system is built on how much time you spend
(07:45):
in these different zones, the accuracy of your Max HR has to
be absolutely critical. It is paramount the whole system
stands or falls on having an accurate Max HR if you're just
using those generic age based formulas like 220 minus age.
So many people do. So many people do, but they can
be off by 10, even 20 beats per minute.
(08:07):
And if your Max HR is wrong, your zone 4 and zone 5 are
wrong, which basically makes thewhole RE score pretty much
meaningless. Now you mentioned already
prioritises intensity to fix theold suffer scores flaws, but
some people especially in the ultra endurance world, they
argue that might have been a bitof an overcorrection for what
they do. That's the other side of the
coin, yeah. By waiting high intensity so
(08:29):
heavily, the model can sometimesunderestimate the sheer
cumulative fatigue from these extremely long low intensity
efforts. Like 100 mile bike ride or a
long mountain. High exactly an effort where
you're mostly in zone 2. Your heart rate curve kind of
flattens out, but you are accumulating massive muscle
damage, tendon stress, glycogen depletion.
So RE is great for cardiovascular stress, but for
(08:51):
ultra distances it kind of misses the local muscular
damage. It does that non cardio stress
just isn't fully captured by theheart rate curve, which is why
those athletes often rely more on external metrics like power
or pace which feed into something like training stress
score or TSS. And we should probably just
state this clearly before we getinto devices RE and fitness and
(09:12):
freshness and all that. It's not free.
Correct, these are all subscriber only features on the
Strava platform. OK, so let's get practical.
How do you actually use this live on your device?
Especially for Garmin users since they're just so dominant
in the endurance space. Well, live relative efforts
pretty widely available. It's built into Wahoo devices,
(09:34):
it's on Santo through Sunto Plus.
And for Garmin, yeah, you just need a Connect IQ capable
device. And there's a really important
technical step for Garmin users,right, to make sure what you see
on your watch matches with, Strava says later.
Yes, this is key. You have to download the Scrava
Relative effort CIQ app. It's super popular, but more
(09:54):
importantly, you must go into your settings and make sure that
the heart rate zones in your Garmin Connect IQ profile file
exactly match the HR zones in your Straba profile.
If they're different, the numbers won't line up.
The calculation on your watch won't align with the final score
that gets generated after you upload.
They have to match. And what about indoor training?
You're on a smart trainer or a treadmill, No GPS.
(10:15):
For stationary activity, a heartrate monitor alone usually isn't
enough. You need some kind of external
work metric to go with it. So the speed sensor.
The speed sensor, a power metre for the bike or a foot pod for
running. The system needs to see that
your internal load, your HR, is actually tied to you doing some
work. Otherwise your heart rate could
just be drifting up because you're getting hot and the score
(10:37):
would be invalid. Interesting little detail here.
Even if you're not a paid subscriber, you can still see
the metric while you're working out.
That's true. Free users can see they already
score live on their Garmin via the Connect IQ app.
But it's it's ephemeral. It'll vanish once the workout is
done. It won't appear in your activity
details online or in the app. So to synthesise all this,
(10:59):
Strava's relative effort is a simplified, multi sport, pretty
actionable metric that makes sophisticated load management
accessible for the everyday athlete.
It is. It's an accessible gateway into
periodization, but for the listener who wants to take this
whole concept to the next level,we'd suggest monitoring
something called decoupling. Decoupling.
(11:20):
It sounds complicated. How does that tie back to RE?
It's basically the comparison between your internal cost and
your external work. So you take your internal load,
which is your RE, and you compare it against an external
metric like your pace or if you use a power metre, your training
stress score TSS. And what are you looking for?
You're looking for a disconnect.If you notice, your relative
(11:42):
effort is disproportionately high compared to your output, so
you're working a lot harder for the same speed or power.
That decoupling is an early warning sign.
A sign of what? It signals accumulating fatigue
or maybe significant heat strain, or just a drop in your
efficiency. So your body is basically
working harder than it should beto get the same result, which
means what you really need isn'tmore training.
(12:04):
It's recovery. It's recovery that really
transforms RE from just a score you can brag about into a high
level diagnostic tool for coaching yourself.
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