What heart rate is considered low aerobic?

Probably a typing error, but the the lactate treshold (dealing with blood and steady state concentration of lactate) is often associated to the second ventilatory treshold (dealing, well, with respiration/VO2) and FTP (dealing with steady state output, ie an output that you could sustain for a long time). These three notions are separate and they are often considered the same or similar, but technically they are different. For training purposes, it is OK to assimilate them.

More information here:

Anaerobic threshold and other concurrent thresholds: 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3438148/

VT1, VT2 and VO2 Max

https://www.tymewear.com/blogs/tyme-wear-articles/thresholds-the-key-to-maximum-training-benefit

DaKo said:

But what heart rate is considered low aerobic?

Generally around 75% of FTP, 70% of HRR, 80% of HR Max according to

https://winklercycling.com/2020/10/13/training-zones-hr-maximum-reserve-threshold/

However, you will find alternative views of these % for VT1

The thing is that the watch is NOT using your heart rate zones to determine what is low aerobic vs high aerobic vs anaerobic. It only uses your VO2 Max, your resting HR as references, as your HR/HRV and pace or power during the workout.

It will be a bit or trial and error to avoid completely high aerobic.

Some tools like alphaHRV are being developing to helpt detect VT1 based on HRV Detrended Fractal Analysis (DFA). You can install the app on the watch and run a ramp test to determine your personal VT1.

That said, using the traditional % should be good enough for training most of us mortals.

It depends if you are talking about lactate threshold as opposed to the lactate turn point. I said VT1 because that is synonymous with the lactate threshold of 2mmol/L. But the only way to accurately tell would be to perform a lactate threshold test which I didn't do. Something about getting pricked for blood at regular intervals just didn't sound appealing at the time. Of course I am not a doctor, I simply rely on those that are to explain these things to me. Thanks for the alphaHRV reference. I see it is in the app store and will check it out. I use a Polar H10 connected to my watch, Just one more data point.

Lol, now that I had a chance to sleep on this, I can see where there might be some confusion. Just like there are two ventilatory thresholds (VT1 and VT2), there are also two lactate thresholds (LT1 and LT2). VT1 and LT1 are synonymous and represent the 2mmol/L (millimoles per litre) blood lactate level. VT2 and LT2 are also synonymous and represent the 4mmol/L level and is also sometimes referred to as the lactate turn point.

But this has gone way beyond the original question even though it is relevant. When I did my testing, I hit VT1 at around 136 bpm. But as Etupes25 pointed out, there is a bit of trial and error in determining where you have to be to get a certain training benefit as everyone is different (age, max hr, etc.). For me, if I go above 70-75% I start to see high aerobic and anaerobic benefit start to creep up. I hope this helps.

9573836 said:

Just like there are two ventilatory thresholds (VT1 and VT2), there are also two lactate thresholds (LT1 and LT2). VT1 and LT1 are synonymous and represent the 2mmol/L (millimoles per litre) blood lactate level. VT2 and LT2 are also synonymous and represent the 4mmol/L level and is also sometimes referred to as the lactate turn point.

Exactly.  BTW, it is less about the concentration value than about how it changes. At VT1, lactate is produced but is rapidly processed and therefore the concentration increases slowly with with further effort. VO2 accelerates also. At VT2, the rate of post processing falls significantly below the rate of creation, and the resulting concentration increases much faster with increased effort while lactic acid accumulates in the muscles (this is what creates the burn feeling and is some sort of protection mechanism). VO2 accelerates more and the ratio of VCO2 vs VO2(RER) changes also converging to slightly above 1 for VO2 Max.

 https://pubmed.ncbi.nlm.nih.gov/3416856/