FatMaxxer

According to recent research (see below) the FatMaxxer Android app may help you to exercise at the optimum effort level for fat burning, measured using just a Polar H10 heart rate strap.

FatMaxxer provides real time Detrended Fluctuation Analysis of your inter-heart beat intervals to give real time feedback of alpha1 (⍺1) in real time. There are promising signs that running or cycling at ⍺1 = 0.75 corresponds to the first ventilatory threshold "VT1" or roughly FatMax which would otherwise be measured in a lab on a treadmill using gas exchange techniques (https://www.frontiersin.org/articles/10.3389/fphys.2020.596567/full).

See review:
http://www.muscleoxygentraining.com/2021/06/fatmaxxer-new-app-for-real-time-dfa-a1.html

FatMaxxer will only work with the Polar H10 (and possibly H9).