@RoyVP Fitbit HR zones can be modified only by changing maximum HR. There's not much else that can be done.
If you need to adjust them to get zone minutes (I assume so) then focus on the border HR between Cardio and Fat Burn zone:
You don't need to worry about finding Peak because both Cardio and Peak multiply zone minutes x2.
Treat zones as intensity zones. Since it may be problematic to find real maximum HR so you may need to rely on your own judgment of effort. The Fat Burn will contain easy effort, steady, something you can sustain for over an hour. Cardio and up will contain any effort that can be sustained for an hour and less. When walking you may feel like burn in legs etc. Of course, you may want to scale that down to your abilities. Make the zones working for you. When you decide what should be the threshold HR between Fat Burn and Cardio adjust Max HR so the number will get set to what you want. I'm not sure whether you can do it any other way. You didn't say anything about your abilities but mentioned your age so probably doing max HR test is not an option (and even that doesn't guarantee zones will be correct). You can't set each zone individually so finding the "sweetspot" between Fat Burn and Cardio based on your own judgment will give you best result.
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Best Answer@RoyVP that is true. It's much better to use a lactate threshold to define HR zones but then one has to do an LT test to find out own LT. With Fitbit that wouldn't work because the zones are built around the age formula (old and proved invalid) and resting HR and the system won't allow full customization so you need to work with what you've given. In general, what is the purpose of zones? Zones define heart rate thresholds which indicate certain changes happening with your body and with metabolic processes. For example, LTHR indicates HR above which the fatigue kicks in very quickly and there is a more rapid drop in the athlete's stamina. It doesn't mean that below one threshold you burn fat and above you suddenly stop (which is a common misconception among Fitbit users worried that going Cardio they stop burning fat 🙂 ). It's a lot more complex and very individual. See this chart. It's a finish of an easy indoor bike ride with a sprint to the finish:
Stamina (orange) starts dropping BEFORE HR reaches LTHR but as soon as the higher output power is generated during the ride. Eventually, HR will reach the threshold and when kept the same power output it will hover around that area within the threshold zone. It takes time for the heart to accelerate. The fatigue kicks in slightly sooner (it's kind of a shock for muscles and the cardiovascular system to switch so suddenly from an easy ride to a sprint). I show you that, so you can see that HR zones are not only non-linear functions of age but the effect is "blended" between zones. So even if you don't get accurate thresholds for zones that may still work alright for the purpose.
Fitbit zones are a very simplified model that doesn't really reflect changes in the metabolic processes. I set it using the Max HR field but I just adjust that parameter so my Peak starts at my LTHR (I know my LTHR). This way I know, Peak starts where LTHR is (Threshold for the majority of HR zone models). For most people, Fitbit zones will be misaligned anyway due to the formula used. The meaning of zones isn't clear either. It's not based on any known model (like ie. Coggan's). It's just Fitbit's made-up thing. Hence my advice is to set it to something that works for you and not worry too much about it. You decide what each zone means to you based on your effort. There's very little you can change (as you said, ideally, zones shouldn't be a linear function of age) but you can try to get it as close to reality as possible and looks correct for you.
The ideal solution (sports watches, software like TrainingPeaks, etc.) would allow you to assign different zones for different types of activity. For example, cycling LTHR is usually lower than running LTHR which would shift all zones for cycling down. My cycling LTHR is 168BPM (I set this one on Fitbit when I cycle as it's seen in the picture I posted previously) while for running it is 176BPM - a big difference for zones. The reason is that each sport uses the body differently. I'm too lazy to change it every time but, like now, I do a lot more cycling due to a sore tendon, so I changed my Peak threshold so it better reflects my efforts on the bike. When I'm back to running, I will change it to 176BPM. It means changing the max HR value from 190bpm (which gives 168bpm in Peak) to 200bpm (which will shift my Peak to 176bpm). This is how I make it work for me.
Rated Perceived Exertion (RPE) and HR can be a help here but not many people are even able to judge their effort and assign RPE to it (usually, we tend to overestimate our efforts).
I can tell you a lot about HR zones but most people don't really have to even think about it. HR zones start making sense when you train with certain training effects in mind. Then you probably already know how your body responds to changes in heart rate. Fitbit zones are just there to make people move harder than they'd do otherwise. These are not training zones. People care about zones mostly due to AZM, and not for the training effect that zones may have. Hence, I kind of understand why Fitbit keeps this system so simple (and despite I think that not being able to customize all the zones is just a bad design decision, how many people really would know what to set?).
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