Training with a target heart rate can be a good way to monitor exertion and control your intensity intensity, however, it is important to know the limitations of heart rate training and not fall into the trap of getting lost in data that is not actionable.
Heart rate based training is only really useful for low-intensity steady state (LISS) cardio. Heart rate monitors often do not respond quickly to heart rate changes and may take some time to converge on a correct value. Also, quick sudden movements (such as lifting weights) may shift the monitor and skew the measurements. It can have some use during high intensity interval training (HIIT), but depends heavily on the quality and capabilities of your heart rate monitor. More than likely, it will just end up as a tool to review your training. Finally, many heart rate monitors are going to have some measurement error and may not produce useful data – for truly accurate monitoring, you’ll need a chest strap.
Aside from the tracker, things like the weather/temperature, stress, how much caffeine you had, smoking, and many other factors can affect your heart rate during training. It is important that you have a good idea what your heart rate normally is at different paces and how it changes due to common external factors. This means if you are new to cardio, heart rate training is not going to be all that useful to you.
The Fat Burning Zone
Many people look to their heart rate because they are trying to target “the fat burning zone” – while this does exist, it is frequently misinterpreted.
People mistake the fat-burning zone to mean the body burns a greater amount of fat during lower-intensity aerobic exercise compared to higher intensities. In reality, the body burns a greater percentage of fat at lower intensities than at higher intensities. This means that at lower intensities the body take 50 percent of the calories it uses from fat, while at higher intensities it may take 35 percent.
On the other hand, total calorie usage is much higher at at higher intensities, which will generally mean you burn more fat calories overall than you do at lower intensities in the same time period. The real benefit of staying at a lower intensity heart rate is that you can exercise for longer before tiring out and potentially burn more calories in total.
However, keep in mind that calorie usage numbers are often drastically overestimated and diet is what has the biggest effect on losing fat. Any and all body fat loss will still be entirely dependent on your sustained calorie deficit.