Beyond confused about calorie burn during intense exercise

[JessieMM]

Okay, so I have noticed that, during a 45 minute, really intense spinning class, my Fitbit is saying I only burned 149 calories.  In fact, it never says that I burn more than 180 calories during one of these classes.  I am averaging over 50% of that time in fat burn/cardio/peak.  So when it says that I burned 149 calories last night in this class, is that IN ADDITION TO what I would burn if I hadn't done the class?  

[Heybales]

No, that would be gross. How much burned in this chunk of time.

Are you relying on auto-workout to start, and end?

Or manually starting the workout?

Have you checked the 24 hr graphs for HR to confirm it's not being lost for chunks of time as it goes higher?

Some people have accuracy issues, some have non-reading HR issues.

50% in elevated state doesn't actually sound like enough for a spin class, that lowest HR zone is pretty low, usually catches people doing a purposeful paced walk.

Usually a Spin class is more effort than a decent walk and HR should be higher.

What was actual avgHR for the entire 45 min?

maxHR?

[JessieMM]

For that particular class, I was only below zones for 11 minutes.  I had 15 min in fat burn, 11 min in cardio, and 8 min in peak.  Average HR was 137.  

[Heybales]

149 does sound very low then.

I have a Polar funded research study formula to calculate using body stats.

You can also try it yourself here, just enter info down to duration and hit calculate.

http://www.braydenwm.com/calburn.htm

Should give a clue if that was decent estimate or not.

[JessieMM]

Wow!  That gives me 471.92 calories burned, which sounds a little more like it.  It still makes me wonder, though, why my tracker would only say 149 calories.  So weird.

[Heybales]

Some potential reasons.

HR-based calculation is going to use the calculated HRmax 220-age.

Going to use what it thinks your restingHR is.

Going to to estimate a VO2max based on amount of exercise you do in some chunk of time, maybe 2 weeks or longer. (this is why it gets better estimates after you've used it for a bit).

It's making some assumptions on that VO2max based on BMI - bad BMI bad fitness, to other side of range good BMI good fitness level.

Obviously that isn't always true as plenty of unfit thin people.

So how long have you been using the Fitbit?

Where is your BMI in the scheme of things?

Ever had experience with really hard efforts and some sort of high maxHR in the past?

If the 220-age is calculating a HRmax much higher than reality, then it would appear your 137 avg could have been very low effort.

And if unfit right now it could have been a very hard workout for you, reaching close to a HRmax that is lower than calculated - nothing to do with fitness there, just genetics.

The restingHR figure attempts to provide the info a bit and hence the reason in the calculation.

What is your first thing before waking restingHR?

Compared to what Fitbit calls your restingHR (which seems to include daytime figures too)?

[JessieMM]

I have been using the Fitbit since mid-December.  BMI is currently sitting at 29.  But I feel like BMI is kind of a garbage measurement.  I just started spinning regularly to help rehab a knee injury.  Before that all I did for physical activity was walking, and a daily advanced yoga practice (I'm a certified teacher, though I don't currently teach).I wouldn't say I am unfit by any means, but it is very likely that my cardio capabilities are not where they should be due to never doing any kind of intense cardio before I started spinning.  220-age would be 176 for me, and in that particular class from the other day my heart rate topped out at 174.  Pre-wake-up resting heart rate in the morning is 64 according to my device.  Fitbit calls my RHR 68.

[Heybales]

So Fitbit has gotten to know you - that's not issue.

Ya, BMI was never intended for individual but population stats - but sadly it's used at personal level such as formula for HR calorie burn.

You likely compare well to the NASA folks a huge research study used as basis that came up with the data in the first place for VO2max.

Since you were hitting that high a maxHR during a workout, then your HRmax is most assuredly even higher.

Side point - I'd correct that Fitbit setting, call 174 about 90% of HRmax, and make the setting 193 actually.

Now in this case, it actually means you should have gotten a better calorie burn, 137 of 176 is 78% which is in the aerobic range.

After that tweak it'll be 71%, but that value is only part of calorie burn calculations, restingHR is in there too with other things.

And I don't see any reason why you wouldn't have gotten higher calorie burn too.

Using a public Garmin formula I get 364, and another public formula 365, both of which use some form of exercise level weekly - I picked 1-3 hrs.

So I'd see if your next workout hits the same low value, maybe it was just an anomaly.

That 471 estimate was also tad high since that formula had no VO2max figure to use. With no VO2max figure it does use BMI as part of it.

Try it again using 30 for that, one formula estimates that with weekly activity.

[JessieMM]

Same thing happened yesterday - Supposedly only 149 calories burned.  But I check my calories burned count on my device before and after the class and there was roughly a 350 calorie difference between starting calories burned and ending calories burned.  But for some reason the app only says I burned 149 calories.  I don't know what is going on, but I guess I'll just pay attention to my actual calorie count on my device from now on.

Would it have anything to do with setting a calorie deficit for weight loss on my dashboard?  i.e. maybe Fitbit is automatically subtracting actual calories burned for some reason?

[Heybales]

Good catch.

The selection for deficit plan changes the eating goal, it doesn't mess with calories burned, especially not a workout.

So do you manually start and end the workout session?

When you gave your time in zones previously, I was assuming that was taken from looking at the workout, because the time all added up.

Now I'm wondering where you are seeing that calorie burn of 149 when the daily count went up by 350?

Also, you can create an Activity Record after the fact, it's doesn't add calories or anything, it merely takes snapshot of the chunk of time you give and provides the stats for it.

So this is not the same as a Workout Record where you pick an exercise, Activity Record you provide the name of it, call it temp or Spin bike class, something. Give the start & end time of the class. That's it.

It'll show you the Fitbit stats for that chunk of time, calories, steps, distance (those 2 are meaningless on bike obviously), HR info, ect.

Then compare that to the Activity Record created at the time (that's all that is too), which you know is 149 calories anyway.

Can you screen shot the 149 calorie burn to see how it's shown?

Thanks

[JessieMM]

I am not manually starting the workout, only because I don't have the option for spinning.  I am doing the class, and then creating an activity record afterwards.  Here is a screenshot of the particular class in question.  

[Heybales · Accepted solution]

Perfect pic.

It's using step/distance based calorie burn at the time of the workout because it wasn't aware there was a workout without enough steps.

Which ain't much for the few steps it likely saw.

I'd say if this is device you can get Spin to list of workouts added do it - or pick Other - you just need something that's going to cause HR-based calorie burn to kick in.

The type of workout selected doesn't change any formula.

It'll create an Activity Record, and you can rename it afterwards as Spin, perhaps add what type of Spin if they do that.

I always found that interesting to compare later as fitness improved with similar type classes - like sprint focus, or hill focus, or even a month of recovery the instructor did.

[JessieMM]

Thank you for all your input!  It makes more sense now.  I'll try to log it differently and see if that helps.  😊

[Heybales]

Even if you picked running as the workout at the time, you can rename it whatever.

Just have to pick something to start when the workout starts.

HR-based calorie burn isn't enabled after the fact.

I am curious though like on that prior chunk of workout time with a tad over 3K steps seen - what distance did it think that was?

I know on road bike the vibrations vary by the road surface, so number of steps could be huge or small, but the distance was always terrible.

It must have thought I was doing a little kid run barely putting one foot before the other. I never mathed out what the stride length must have been for so many steps and so little distance.