How is Cardio load calculated?

[patilu]

In response to a prompt, I indicated I wanted to improve my cardio load. The first few cardio load scores in the '40s based on treadmill workouts seemed reasonable. Then yesterday I got a score of 1 for a 30 minute  intermittent interval spinning workout where my average heart rate was 132 which is well into the vigorous range for me and more challenging than my typical spinning workout. According to Fitbit, My cardio fitness score is 39 to 43 which is considered excellent for my age. I don't understand the why discrepancy between the treadmill scores and the spinning score.

Moderator edit: updated subject for clarity

[phatbitzr]

@alexthecatThanks, I read that link already.  I guess the confusion is you are using the terms Cardio Load and Target Load getting used interchangeably.

[weekendwarrier]

Cardio load is a new well meaning feature and clearly hasn't been properly tested. It doesn't pick up spinning (5 points on an hour spin), but does on actual cycling and treadmill. It's clear it needs arm movements or moving. Waste of time at the moment and I hope google pick up the issue and sort it or ditch it.

[Missifit]

I agree, it only frustrates me to watch the numbers at the moment.  It's not helpful at all because it's only randomly accurate.  I'm also bummed because it seems like since I got the new feature in a recent update, my location is wonky too.  It indicates that GPS is connected during the workout, but, for example, it logged a cross country ski as 0 miles.  It shows where I skied on the map in the phone app afterward, but still 0 miles & 0 mph average.  And you guessed it...0 cardio load.  I might start another help thread about this extra issue, or I might just wait to see if they fix the bugs and it goes away.  These things have happened in the past and waiting it out for a few weeks worked.

[Farmy]

I see the same issue with cardio load and spinning using charge 6.  Just did 96 minutes, 127 avg HR, 89 zone minutes... 14 cardio load.  Have seen similar results in previous sessions.

Other activities I've done seem to track fine: Weightlifting, CrossFit and Outdoor Running

[amackayj]

My Sense2 GPS is also playing up and I'm not sure the HR measurements are totally accurate (or I'm getting really, really fit!).

Let's give the data time to settle and the devices time to calibrate before taking anything too seriously.

[amackayj]

This checks out. Look at the time in zone values to see whether they correlate with cardio load

Screenshots are from 2 rides on the same bike in similar conditions on different days

 

 

 

 

 

 

[patilu]

Yep. Today I got 77 active minutes for 42 min spin with average heart rate 74 beats above my resting heart rate and got a cardio load of only 3 for the workout. Later in the day I shoveled snow for an hour, with a lower average heart rate, about the same number of active zone minutes as the spinning workout, and got a cardio load of 56 for that activity. I briefly (30 seconds-1 minute) stopped about six times during the hour that I was shoveling snow, but the spinning activity was continuous. 

Thanks for your contribution.  Hopefully, Fitbit will recognize and address this issue.

[SunsetRunner]

I have the same issue, with a static bike. I was on for 40mins with my heart rate in the vigorous zone for 30mins (and a daily readiness at 59) and apparently produced a Cardiac Load of 5. Then a Cardiac Load of 17 for some light weights afterwards where my heart rate was pretty steady. 

Useless feature unless they can sort it out for gym equipment.

[patilu]

Agreed

[jbatchellor]

I do Peloton bike workouts where my heart rate I KNOW is 140 and higher and I get 0 cardio load.  If I run, I get the right amount.  Frustrating.

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It's just like not getting steps when you're pushing a cart when grocery shopping.  

[RiekoC]

Hello @amackayj , @jbatchellor , @SunsetRunner , @patilu , @Farmy 

I saw a Moderator post from yesterday (07 Jan 2025) stating that an update to the Fitbit app will start rolling out soon (this week?) and includes general issue fixes for the Cardio Load and Target Cardio Load. The updates tend to rollout in waves. Keep checking your app store for the update and install it when it's available to you.

Hopefully, the update will fix the Cardio Load inconsistencies that we've been experiencing.

[amackayj]

Thanks! Is this an update to the phone app (android) or watch (Sense 2 firmware)

[YachatsMike]

I think your analysis has merit.  I've seen the same thing but taken a different response.  When I saw low cardio points for obviously vigorous exercises, I quit paying attention to the cardio points.  I measure my exercises by Active Zone Minutes because I think that is the most relevant measure.  It is directly measured (heart rate and time) and the standards developed by HHS. 

[jbatchellor]

Agreed. I actually turned the cardio load off so I can't even see it now.

[phatbitzr]

@YachatsMikeI think Active Zone Minutes is just a general health guideline, as exercising at the peak of your fat burning zone ears you your standard active minute, but it will think you are working twice as much just by pushing just 1 BPM harder.

Cardio Load is based on a modified Banister TRIMP model, which is weighed in favor higher heart rate, in proportion to blood lactate accumulation at those intensity.  So there is a difference in cardio load rate between just reaching cardio zone, top range of cardio zone, base of peak zone and peak of peak zone.  The unknown is the sampling frequency of how this rate is calculated, as the standard Banister TRIMP model is based on the duration of the exercise, which would average out the HR.  Fitbit's cardio load graph has a 1 minute resolution, which means it averages the HR at 1 minute interval or less.  1 minute would still be too long, as it averages out much of the peak intensities from HIIIT intervals.  It would be easy and more accurate to calculate the TRIMP number as the HR arrives and accumulate the total cardio load based on that.

[RiekoC]

Hello @amackayj 

As I understand it, this will be an update to the Fitbit app, most likely for both Android and iOS users.

[NancyJames]

But as people keep pointing out, the cardio load as calculated by fitbit is a load of rubbish and based on how much you move your arm, not your heart rate. I got 7 points from scrubbing my toilet, same as it thinks crochet burns lots of calories because you move your hand a lot. 

I'm also not convinced that it takes readiness score into account. I had a readiness score of 100, my target was 4-14. I got over 200 so now it's really cross with me. The next day my readiness was 69, the goal was 4-25, I got 233. Readiness 69 again the next day goal 4-36. Today my readiness is 57 (got a cold), same 4-36 and it hasn't moved even a mm down the overtraining bubble.

It's recommendations seem to have no logic to them at all. When I had a readiness score of 46, the recommendation was 42 - 58

[YachatsMike]

FWIW:  Your heart rate graph looks kind of noisy.  When I had that problem, I started cleaning the back of my watch and my wrist with water and making sure my watch was a full two fingers above my wrist. I made it part of my exercise ritual.   It made my noise problem go away.

[patilu]

How are you seeing my heart rate graph?

[YachatsMike]

Sorry.  Meant response for a different user.  Thought I was responding to amackayj's screen images.