Calculation of Active Zone Minutes

[Research_01]

Hi Fitbit Community,

I’m manually calculating Active Zone Minutes (AZM) from per-minute heart rate data and I’m curious about how Fitbit determines certain metrics. Specifically:

1. Does the Maximum Heart Rate (Max HR) used for calculating Heart Rate Reserve (HRR) and AZM change daily, or is it a fixed value based on the formula [Max HR= 208-(Age*0.7)]?

2. If it’s fixed, could you please clarify which formula Fitbit uses to calculate Max HR, HRR, and AZM?

Thank you!

[LZeeW]

@Research_01   Fitbit uses the 220 - age = MPHR formula, although there is considerable research that your formula is more predictive of a person’s MPHR.  That number changes on your birthday.

Here is a link to Fitbit’s Help article  with the relevant section expanded for you.  It gives an example of how heart rate zones are calculated.  The zone minute thresholds can change from day to day with a change in your RHR.

 I hope this helps.

[Research_01]

Thank you for the reply. I downloaded one of my research subject's heart rate data from Fitbit and manually calculated the Active Zone Minutes based on time spent in different heart rate zones(using minute Heart Rate data from Fitbit). However, the AZMs calculated from my formula don’t match the AZMs shown in the Fitbit app. Could someone help me understand why there’s a difference?

I used the following method for my calculation:

 MPHR formula = 220 - age 

Heart Rate Reserve= Maximum heart rate - resting heart rate

• 1 AZM for each minute in the fat burn zone 
• 2 AZMs for each minute in the cardio or peak zones 

Are there other factors or Fitbit-specific algorithms that could be causing this discrepancy? Any guidance would be appreciated.

[LZeeW]

@Research_01   You correctly calculated the HRR.  Calculating the zones is next step.  The Sense 2 uses moderate, vigorous, and peak intensity, not fat burn, cardio, and peak.  The lower and upper limits of each zone are calculated from the following equation: (Percent x HRR) + RHR = upper or lower limit, depending on what you are calculating.

In the case of moderate intensity, the lower threshold is 40% and the upper threshold is 59% .  Vigorous is 60-84%.  Peak is greater than 85%.

My real world numbers.  I am 69 years old and my RHR is 43 bpm (not making that up).  That makes my MPHR = 151 and my HRR = 108.  My moderate lower limit is (0.40 x 108) + 43 = 86.  The rest of the math says my moderate zone is 86-106 bpm, my vigorous zone is 107-133 bpm, and my peak is 134+ bpm.

Does this help?

[Research_01]

Yes, I was able to determine the ranges for fat burn, cardio, and peak zones. Thank you!

I calculated the AZM per day, but there still seems to be a 10 to 15-minute discrepancy.

Could you please confirm if the resting heart rate changes daily and if that could be affecting the AZM calculation?

Are there any other factors that could be contributing to this difference?

[LZeeW]

@Research_01   Yes, I can confirm that the zones can change daily if the RHR changes.  I have one thought about the discrepancy and it might depend on how you are analyzing the heart rate data.  Someone must be in a zone for a full minute to get that AZM.  If I run to catch the bus, but it took 1 minute and 55 seconds, I would get 1 AZM. If you looking at aggregate data of time spent in a zone for 24 hours, then that is different from how you earn AZMs in real time.

I also want to note that Fitbit can recalculate your RHR during the day.  I've seen it happen.