Does the charge 3 calculate fat burning zone differently than charge 4?

[Ninetwentyfive]

I used to have a charge 4 until the wristband broke and ordered another charge 4, but realized too late to dispute it on eBay that I was sent a charge 3 as replacement instead.

well, people were telling me they pretty much are the same so I didn't worry too much. But I'm noticing that it seems way too easy to get into the “fat burning zone” compared to before. My resting heart rate hasn't changed and before I had to get up to 127 BPM for it to say I was in the fat burning zone. Now it's saying I'm in the fat burning zone when my BPM is as low as 95 (!). 

I know there's a difference between Active minutes and active zone minutes between the charge three and the charge four, but I didn't think that would affect the division between the particular zones… does it?

[JohnnyRow · Accepted solution]

Yes, along with the switch from Active Minutes to Zone Minutes and the recommendations goals for them, also came a switch to how the zones were calculated, generally making the zones harder to reach in the newer devices, or easier in the Charge 3.

For one thing in old formula (Charge 3) Resting Heart Rate makes no difference. Zones are simply based on % of max HR:

fat burn 40-60% max HR

cardio 60-84%

peak 85+ % max heart rate

so someone terribly out of shape might have gotten into fat burn zone with little additional effort.

The newer zones are also based on heart rate reserve - difference between RHR and max heart rate, but also I think are made harder to reach, leading to lowered suggested goals.

Also, unlike zone minutes, active minutes have to be maintained for at least 10 consecutive minutes.

Hope that all didn't just confuse you more.  Many people preferred the older system and hated the change-over as it made zone minutes harder for them to get.  But the newer system (Charge 4) is more in line with the updated guidance from CDC and American Heart Association, but I wouldn't worry about it; you can still get all the benefit from the Charge 3.

You can read more details on the zones here: https://help.fitbit.com/articles/en_US/Help_article/1565.htm?Highlight=heart%20rate%20zones  and scroll down to the section "What are heart rate zones" and it gives details for  both categories.

[Ninetwentyfive]

Thanks so much! I guess I am the opposite of most people, haha, because the charge three being ‘easier‘ is causing me trouble! going from the charge four to the charge three I knew something was up, and when i finally realized that the device was calculating me as being in the fat burning zone a mere 10 BPM above my resting heart rate I was frustrated because I then knew why I wasn't burning any fat. The whole reason I got the Fitbit in the first place was so that I could work hard enough to burn fat but not hard enough to make my medical condition worse (it's a tough one to explain, because in most cases you can't work ’too hard’). I just wish I would've noticed something was off before I gained 20 of my pounds back! Anyhow, your explanation was perfect—and the detail you gave helped me a lot, thanks again!