You heart rate monitor lies to you. It doesn’t mean to. But our OCD society, wanting things tidy and compartmentalized, asks it to do more than it’s capable, which is little more than providing you with numbers to begin your training assessments. The most important indicators you need to follow are (still) performance and feel. Let’s look at why.
A heart rate monitor can only do one thing, count how many times your heart beats. Beyond this it uses calculations to estimate various things, like caloric burn and other factors that the device’s marketing departments think will make you more likely to buy one. The problem with these estimates is that they are very limited in scope, whereas humans are extremely complex.
This article I wrote explains how HR monitors attempt to help you train in zones. As you’ll see, it’s often wrong. Sometimes it’s right, too. The key takeaway is that you can’t really know, which isn’t particularly helpful when 10bpm is the difference between training to separate energy systems.
Caloric burn numbers are even more random. During anaerobic periods (the target area of almost every resistance, HIIT, interval, and even cardio workout) your heart can’t respond fast enough for it’s bpm numbers to matter. It’s beating as fast as it can because you’re out of oxygen for the given task (hence anaerobic). Once your set finishes it continues to beat in order to catch up but this doesn’t fit very well with “training in a heart rate zone” theme programmed into your device.
Furthermore, those anaerobic sets have a cause/effect with your body called hormonal cascading. Hormonal stimulation during anaerobic exercise changes your metabolic process in varying degrees, which effectively changes your caloric burn numbers while at rest, too, as your body recovers from a workout. The cliché “cardio burns calories while you workout and weight training burns calories all day” comes from this. It’s inaccurate as well, since most “cardio” includes anaerobic training as well as aerobic, but it gives you an idea of the processes at work.
In this scenario, your HR monitor tries to estimate anaerobic caloric burn using an algorithm. Problem is there are many different ways to load force and no way for the monitor to assess it. Simple examples are the amount of weight added to a movement, the percentage of maximal force used for explosion or a landing, which are virtually unnoticed by your heart (it’s going as fast as it can already) but has great impact on your hormonal response. This can lead to staggering differences between what happened during a workout and what you’re reading on your watch.
This is why I always look to performance, first. You don’t write down how much weight you used, how quick your sets were, how high you jumped, etc, to boost your ego. You do it to have a reference on how your training is going.
Other than adaptation periods, when you get worse before you get better (which should be short) workout performance should always be increasing. When it’s not your doing something wrong. It’s really that simple.
Just because you’re pushing a gadget outside of its comfort zone doesn’t mean it’s worthless. Read the article in the above link to see how those numbers can benefit you. But you also need bounce the numbers you’re getting off of your performance and how you feel. Otherwise you can end up adjusting things based on numbers instead rationale, which makes little sense when you know those numbers are probably wrong.