Heart rate monitors are now becoming more advanced than just telling one their BPM. As technology advances, heart rate monitor companies are using certain formulas and physiological occurrences to predict heart rate zones/intensities.
Without using the company name, a monitor is now using heart rate variability to estimate one's prescribed exercise zones as well as predicting one's energy source during workout (fat calories versus carb calories).
My question comes down to this: Is heart rate variability an accurate predictor of metabolic rate at 65 percent of one's MHR (this is stated by the company as their indicator point)? Can one's metabolic/intensity zones change based on a heart's physiological changes due to external stimuli? Or is this just a more fancy model of heart rate monitors with little scientific evidence to prove its functions?
The literature I could find about the most popular brand of heart rate monitors claims that their line estimates your VO2max from your age, height, weight, sex, physical activity level during the past six months and heart rate variability (HRV) at rest.
Presumably, they then take that estimated VO2max and estimate your caloric expenditure since there's a correlation between heart rate and VO2. You then take that amount of oxygen and estimate the calories burned (a liter of O2 yields approximately five kcals).
First, let's take care of a little semantics problem. Unfortunately, numerous web sites promoting these products offered this nebulous and unscientific "explanation" of "heart rate variability:"... a quality measure of heart's work. The lower the resting heart rate the higher the heart rate variability, and thus the better the quality of heart's functions.”
This erroneously implies that heart rate variability is simply the difference between resting heart rate and exercise heart rate. Heart rate variability refers to the regulation of the sinoatrial node of the heart by the autonomic nervous system.
When you’re at rest, your ECG goes through periodic variations in R-R intervals. (You may want to refer to your ACSM "black book" for a refresher.) This rhythmic phenomenon is called respiratory sinus arrhythmia (RSA) and occurs naturally through the influence of breathing on the flow of sympathetic and vagus impulses to the sinoatrial node.
Part of the problem with using heart rate variability to estimate VO2 max and then estimate caloric expenditure is that studies have shown a link between negative emotions - such as acute stress, anxiety and hostility - and reduced HRV. Even public speaking and making tough decisions have been linked to lower HRV.
(Aging decreases HRV and regular physical activity raises it, but presumably that's taken into account in the formula.)
We know that heart rate increases as exercise intensity increases. We also know that caloric expenditure increases as exercise intensity increases. The transitive property implies that heart rate may give us some indication of caloric expenditure.
But just because we know that there's a correlation between heart rate and caloric expenditure in a given person doesn't mean that there's a formula that will accurately apply to everyone (fitness level, efficiency, or even experience on the equipment) or even to the same person under different conditions (e.g., stress, nutritional status, hydration, ambient temperature, etc.)
It's a similar problem with estimating how many fat cals versus carb cals you burn. Studies have shown that what you've eaten in recent meals effects your rate of fat oxidation.
There're just too many variables and not enough data to accurately measure fat calories versus carb calories burned.
Besides, physiological studies have shown that it's the total calories burned that makes the difference, not the ratio of fat/carb cals in a given workout that's most important.
Of course, the caloric expenditure reading isn't completely worthless just because it's inherently inaccurate. You can use it to gauge the relative intensity of a workout or group of workouts for a given client and track progress over time.