How do I turn on "exercise mode"?

[423536271827]

Is there a "turn on threshold" of any stat for exercise mode? If the turn-on threshold is heart rate, how much load in percent does my heart have to be working at?

(Remeber: Load=HRcurrent/HRmax.)

[JohnnyRow]

Can't answer that without defining what you mean by 'turning on exercise mode'.  I am assuming you mean getting credited an exercise session.

The best/surest way is to go to the Exercise App, select your exerciser type, and Start.

The other way is to do an activity that is one of the exercise types that Fitbit can auto-recognize and continue it for the minimum amount of time, 15 minutes by default.

Neither of these has anything to do with heart rate.

[423536271827]

How does the Alta Hr know you're exercising if it isn't heart rate? @JohnnyRow 

[JohnnyRow]

Because you wrist is doing a continuous regular motion that Fitbit recognizes as one of the activities.

If you want to test this, sit or stand still, and swing your arms front and back regularly as if walking at about the same rate as if walking.  Keep this up regularly for at least 15 minutes; I would expect you would see this auto-recognized as a walk, though your heart rate would presumably go up only very little.

  Fitbit trackers without heart rate measurement, such as the Inspire, Alta, and older models, auto-recognized walks and some activities.

P.S. But if you are talking about Active Minutes or Active Zone Minutes, those do depend on heart rate, showing how it is possible to get (Active) Zone Minutes without an exercise session, and vice versa.

[423536271827]

@JohnnyRow For active minutes, what is the turn-on threshold for heart load in percent?

(Load=HRcurrent/HRmax)

[JohnnyRow]

See What are Active Zone Minutes or active minutes on my Fitbit device? 

Active Minutes are based on Mets based on calorie burn a s described below.

If you want to know heart rates for METS, you would have to experiment for yourself; what is your BMR calorie burn per minutes and what heart rate get you to triple that calorie burn.

As you can read in the link above, (Active) Zone Minutes in newer devices are based on heart rate zones which are based on heart rate reserve.  See How do I track heart rate with my Fitbit device?  and scroll down to "What are heart rate Zones?'

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[423536271827]

So the "turn on threshold" is 50%?