Charge 5 active zone minutes are innacurate

[RMT24]

I did an intense spin class this morning but forgot to register the exercise on my Fitbit (Charge 5) it said in total I earned 2 active minutes!! I think it’s based my active zones on when my resting heart beat is highest in the month- I think we need more flexibility for women with these zones.

Moderator Edit: Clarified subject

[Julia_G]

Hi, @RMT24 , your Active Zone Minutes are not calculated based on your resting heart rate.  They are calculated based on a standard formula calculating your theoretical maximum heart rate of 220 minus your age.  So, for example, a 35 year old woman would have a theoretical maximum heart rate of 220-35=185.

From that information your heart rate zones are calculated.  Peak is 85-100 percent of your maximum heart rate, Cardio is 70-84 percent of your maximum, and Fat burn is 50-69 percent of your maximum heart rate.

While you are working out your Charge 5 awards Active Zone Minutes (AZM) depending on which Zone you are in.  Below Fat burn zone no active minutes are awarded.  In Fat burn zone you get one active minute per full minute you are that zone.  In Cardio or Peak you will get 2 AZMs for every minute you are in that zone.

The calculation is the same for men and women.

If I understand correctly what you say in your post, you forgot to start the exercise app on your Fitbit before you began your spin class, and this may help explain what you saw.  Throughout the day and night your Fitbit samples your heart rate (as long as heart rate is turned on).  However, day to day the sample is actually taken at only one minute intervals, and then averaged over 5 minutes to give you your readings.  However, during exercise HR is sampled every 10 seconds and then averaged over 1 minute.  From that you will see that you will get much more detailed (and accurate`) HR readings when you start your exercise app.  If you exercised in very short intense bits then without exercise app started your heart rate will be much more “smoothed” (I hope that makes sense!) so even though your exercise will be recorded, you may not be awarded AZM that accurately reflect your short bursts of intense activity.

The moral of the story is, for the kind of exercise you are doing, for best results start the workout using your exercise app on the Fitbit.

For more information you may want to have a look at this help article which explains the details of Active Zone Minutes.

I hope this helps.  Welcome to the forums!

[N8teGee]

Hi @RMT24.

The following (below) is from a help article explaining how Fitbit calculates heart rate zones. Have you tried setting a custom hear rate zone? 

Custom zones

• From the Today tab in the Fitbit app, tap your profile picture.
• Tap Fitbit settings Activity & Wellness Heart Settings.
• Tap Heart Rate Zones.
• Tap the switch next to Custom Zone and enter your custom zone.

[N8teGee]

@Julia_G I don't think the heart rate sampling is quite as bad as that. The last I read is that it's sampled every 5 seconds during passive wearing and then every 1 second during manually started exercises, although your point regarding accuracy still stands and an exercise mode should be used to ensure the most accurate reading. How the Fitbit app itself displays this data seems to be a different matter though. 

[Julia_G]

@N8teGee  You may very well be right.  It is a long time (like years) since I read the details of the HR sampling and it is quite possible that it has either been improved or that I am remembering wrong.  I believe that info is no longer available.  However, as you say, the basic point is the same - you will get much more accurate info using the Exercise app and that not using the exercise app could very well explain the lack of AZM.

[LZeeW]

Hi @RMT24.  Both @Julia_G and @N8teGee make valid points.  I have a different perspective on your issue.  First:

• Heart rate sampling frequency is different from display frequency in different graphs
• Your Charge 6 is always sampling at the same frequency
• Fitbits must sample more often than once every second in order to calculate your HRV in your Health Metrics
• If your heart rate is 150 bpm, then it is beating 2.5 times per second

That being said, I think you are aren't getting an accurate heart rate measurement because of a type of interference called optical noise.  The heart rate signal is reflected and the muscle tension in your forearm keeps you from getting an accurate heart rate.  There are other posts about this problem during spinning, outdoor biking, and using a rowing machine.  It happens with other brands, too.

Some suggestions.  Fitbit recommends moving your Fitbit three finger widths away from the bony prominence on your wrist during exercise.  Try to ease up on the grips.  Sweat will also interfere with heart rate accuracy, but I don't know what you can do in your spin class to help with that.

[N8teGee]

@LZeeW Yes that's a good explanation, and thinking about it more makes perfect sense. So it would seem what most are actually (unknowingly) referring to when they speak about polling rate is actually the data recording rate and not the rate the sensor is measuring at. Obviously the 1 second (during activity) vs normal 5 sconds heart rate measuring is more a memory storage compromise.

[RMT24]

Thanks Laurie, I will try this and try not to forget setting up the exercise.