How is Cardio Load calculated?

[tesb]

I am utterly baffled by the cardio load 'feature' on my FitBit. I get a cardio load score for a light walk where my heart rate hardly goes into the light exercise zone, but nothing at all for a pretty vigorous 25-minute Spin session of constant high heart rate.  Can anyone explain how that can be the case?

As a result i can have had a good cardio workout but still have the app pestering me to increase my cardio load. No sense whatsoever. 

Moderator edit: clarified subject.

[ManuFitbit]

Hi and welcome back to the community @tesb

Thank you for sharing with me those details about the Cardio Load issue. 

Spinning can be tricky for wrist-based sensors. When you’re holding the handlebars, the tension in your wrist can sometimes make it hard for the sensor to 'see' your actual heart rate. If your device doesn't catch those high-intensity peaks, it won't trigger the Cardio Load points, whereas during a walk, your arm's natural movement makes it much easier to get a perfect reading.

Please, the next time move the band a bit higher up your forearm and snug it up just a notch. Also, manually starting the 'Spinning' mode on your wrist tells the sensor to work extra hard to catch every beat.

[tesb]

Nope - the app records the heart rate pretty well, with zones, zone minutes and peaks captured fine (albeit with a few annoying random dips) - but even when my heart rate is above 150 for 8-12 minutes several days in a row, the workouts still score 0 cardio load and the app tells me my cardio fitness is declining and I need to work harder. 

I tried using the manual exercise setting, but since my tracker has no button, I found there was no way to stop it again as it appears any hint of perspiration and the whole thing stops responding. Infuriating! 

[LZeeW]

@tesb   What model Fitbit do you use now?  I looked at your public profile and saw that you have posts in several different model forums.

[tesb]

I'm using an Inspire 2 at the moment, but as mentioned above, the data is being recorded by the device - it's when it's transferred to the app that it can't do sums and inexplicably ranks a light stroll as having more cardiac impact than an intense 25 minute spin session. 

[ManuFitbit]

Thanks for sharing those additional details @tesb.

We'll try to pinpoint where the issue is coming from. In the meantime, please try following these steps within the app:

• Log out from the Fitbit app.
• Force quit the Fitbit app and reboot your phone.
• Open the Fitbit app and log back into your account.
• Check if the issue is still happening.

[YachatsMike]

I researched TRIMP and found the original Bannister formula.  TRIMP = Duration (minutes) x %HRR x .64 x e^(Y x %HRR). Y =  1.92 for men, 1.67 for women .

When I track a 40 minute treadmill session on my Fitbit, it shows a Cardio Load of 67, when I use the formula I get a TRIMP score of 66.  Tracks pretty close.

The second element is the Cardio Load Target.  That's more of a black box.  I don't know how they do that.

---

I did a little more research on how to set targets for performance.  I learned about ACWR - Acute:Chronic Workload Ratio.  Its a ratio of the average workout intensity (TRIMP score) of the last week  (Acute score) divided by the average workout intensity of the last four weeks (Chronic score).  The ratio is interpreted as follows:

• 0.8 – 1.3 (Optimal): Known as the "Sweet Spot," where fitness gains are maximized while injury risk is minimized.
• < 0.8 (Undertraining): Indicates the athlete may be losing fitness, which can ironically increase injury risk when they return to high intensity.
• > 1.5 (Danger Zone): Represents a rapid "spike" in training that significantly increases the risk of soft-tissue injuries.

This sounds kind of like what I see for daily Cardio Load target setting. 

[YachatsMike]

Discovered this paper on line.  It really explains a lot about how Cardio Load was designed and how it works.  I have a much better understanding on how to use the tool to maintain my fitness and when I should not rely on it because the output might be unreliable.

https://arxiv.org/html/2508.11613v1#:~:text=Cardio%20Load%2C%20introduced%20by%20Google,of%20activit...

Moderator Edit: Formatting.

[tesb]

 

Useful info there, but still doesn't explain why an intense 25 minute elliptical session generates a CL score, as does a slow, lazy afternoon stroll, but not an intense 25 minute spin session. 

[YachatsMike]

I spin a lot.  I used to get real low Cardio Load  scores until the latest App update.  (Maybe it was my latest Charge 6 update, I can't remember which)  Now my Spin numbers are way up and correlate to my independent TRIMP calculations.  Are your devices up to date?