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"Calories Calculated with Heart Rate
Measuring energy expenditure based on heart rate has come a long way over the years along with technological improvements, and more are still popping up as we speak. There are several generations of algorithms that are derived from scientific metabolic testing, so trust that the companies aren’t just throwing numbers at you. Most of the HR based calculations are within 10-20% accurate.
That’s not as close as within 5% with power, but not everyone wants to or can invest in that technology.
Most all HR devices use the basic user input metrics including gender, height, weight, and activity level combined with the heart data recorded. So the main thing with HR devices is to make sure you input as much accurate information as possible. In some devices actually inputting your tested VO2max over the device-estimated value can improve the accuracy from 20% to more like 12%. Some of the more advanced methods evolving now evaluate the time between heartbeats, called beat to beat, to estimate MET (Metabolic Equivalent), which finally is used to determine actual work expenditure. And some devices also have a “learn” function that with continued use tracks your changes in fitness and adjusts the energy algorithm. So sharing a device with a family member or friend once you may not notice different results, but frequently, then you probably would.Calories Calculated with Time and Distance
Here is where the measure of energy expenditure really is tough to blame on the device. When there is no data reported from your body, the device is left to calculate energy expenditure based on the raw metrics including time, distance (if available), age, weight, and activity level (if device has this setting). That’s why this method can range from being 20-60% off.
So, really you can’t depend on calorie estimates from a device with just these metrics. For example, a ride that may actually be 600 kcals total (500 kcals to summit a climb but a minimal 100 kcals to soft-pedal back down hill) would actually report as potentially a 960 kcal activity if the 60% inaccuracy is true. That’s substantial enough in a single workout to throw off the common -500 kcal/day deficit that often person aim for to lose 1 lb/week safely.
There are some general guidelines that exist for these calculations, but as you can see by the chart they don’t take into consideration your effort level to achieve the speed. This is a large factor because quickly you can conceptualize the difference of effort that is required to pedal at these rates; into a headwind or crosswind versus a tailwind, up a steep climb versus on the flats or descending, or even on a technical mountain bike trial versus a leisurely regional gravel path."