How heart rate monitors calculate calories & 5 tips to improve accuracy

Heart rate monitors serve many purposes in the gym, including tracking training zones, measuring workout intensity and calculating calories. Anecdotal evidence, from first hand observations in the gym, right the way through to online research, indicate that its the calorie monitoring aspect of HRMs that persuades many to use them. However often there are some pretty big discrepancies between the reading on your heart rate monitor and the reading on pieces of cardiovascular equipment. This causes confusion, and on the back if several email requests to cover this, topic, below is the lowdown on how heart rate monitors calculate calorie expenditure. We are also following this article up with a detailed look at the calorie figures compiled by popular peices of cardio equipment verses those given by heart rate monitors. This article focusses on how heart rate monitors calculate calories

Read: the ultimate guide to heart rate monitors in 2013

The first thing to note is that (the overwhelming majority of) heart rate monitors don’t know what activity your performing. Therefore their calorie calculation is based exclusively on your heart rate, which the device then runs through its algorithm, monitored against key data inputs, in order to churn out a calorie figure. The more data inputs that the HRM has, the more accurate it will be (presuming the calorie algorithms are all created equal, which of course they aren’t). Common data inputs that most good heart rate monitors ask for, include:

Your age: this allows the device to calculate your maximum theoretical heart rate.

Your gender: male and female bodies burn calories at different rates

Your weight: this is a core input from which the HRM calculates expenditure. A rough formula which is well accepted states that you can expect to burn 1 calorie per kg of body weight per 1km you cover. It’s likely that its tweaks on this formula that cardio equipment, like treadmills, use to provide their readings.

Your height: used to calculate your BMI reading

Heart rate monitors use a chest strap or wrist pulse reader in order to access your heart rate during your workout. This is a live data stream, refreshed multiple times per minute (rate dependent on brand and quality bracket, seek 60 updates per minute), which feeds the monitor. Each heart rate monitor manufacturer has their own bespoke algorithm from which they calculate calories. Polar and Suunto are considered to be the more accurate brands here, with Garmin known to over read the calories for many.

How accurate are heart rate monitors?

Generally speaking, the calories expended figure is a broad ‘best guess’ from the monitor. Realistically heart rate monitors aren’t collating enough physiological data relative to you, in order to work in absolutes. They also lack data on the specific activity that your performing, and naturally this does play a significant role in calorie expenditure. The figure given is not useless though, yet a heart rate monitors calorie reading can be less reliable then many cardio equipments calorie calcultaor. Ultimately absolute calories burned doesn’t matter, this figure is best used for goal setting, trend analysis and for improving future workouts. There are a few things you can do to improve the calorie accuracy of heart rate watches though.

Quick tips to make your heart rate watch more accurate

Tip 1: Update your weight and other stats regularly: as you improve your fitness and hit your goals, your basic stats change. Don’t set and forget, make it a habit to update the core data, especially your weight, weekly.

Tip 2: Accurately determine your max heart rate: heart rate watches use the basic 220 minus your age calculation to set your max HR. This figure is of huge importance because the watch calculates your training zones based on your max theoretical heart rate. A fully trained personal trainer can set up an interval test which can determine this figure more accurately for you. The ~£30 cost is worth probably worth it if you value the calories burned figure. The fact your reading this probably means you do.

Tip 3: Avoid strapless watches: I know that heart rate monitor straps are hugely annoying. They’re uncomfortable, bulky and can rub against your skin, yet they’re vital for your heart rate watch to gather a constant stream of accurate heart rate data.

Tip 4: Clean your strap regularly: ensure the surface of the chest strap is cleaned after every session. As you sweat, you release minerals and salts, which over time can ‘clog’ up your straps sensor. This decreases the accuracy of the reading.

Tip 5: Change the batteries / recharge frequently: batteries get weaker as they age which can lead to a decreased ability for the watch and strap to ‘talk’. This naturally can lead to missed data points and reduced accuracy.

So in conclusion, heart rate monitors calculate calories by running collected data (your heart rate) against input ted data (your stats) in order to recognise your training zone and the likely calorie cost of been in that zone. The reading is not an absolute and shouldn’t be treated as such, yet its a useful guide to your workout.

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