Anybody who has spent any time using a piece of cardio equipment in the gym with a heart rate monitor will have noted the disparity between the calories calculated figure. The calories calculated by the treadmill (or elliptical / rowing machine / exercise bike etc) will often vary markedly from the figure presented by your heart rate monitor. Over an hour’s session, this difference can be as marked as 150+ calories, representing a 15-20% differential. So which is more accurate, the treadmill verses the heart rate monitor?
The conventional wisdom dictates that heart rate monitors are always more accurate, and ‘you shouldn’t trust the machine’. However, often the machines can be more accurate than the watches, so this is one piece of conventional wisdom that needs a rethink by serious gym goers.
First off, we need to make some base assumptions:
1) The data that you have inputted into your watch is accurate and is constantly updated. This data typically includes height, weight and sex.
2) The data that you feed into the cardio machine, typically including weight and sometimes sex, is accurate.
Heart rate monitors and treadmills calculate their calories using very different methodologies. For a full lowdown on how heart rate monitors measure calories, click here.
How our bodies actually burn calories
During hard bouts of aerobic exercise, the calorie burning cost is determined by two simple factors.
1) The intensity that you train at
2) Your bodyweight
Heart rate monitors ask for a lot of additional information, including the likes of your age, gender and height – this is surplus information which the heart rate monitor requires to make up for the fact that it doesn’t know what activity you’re doing. This information is of vital importance to make the watch as accurate as it can be, but ultimately it is limited by its data set.
So the simple two factor formula means there are two truisms of exercise:
1) The harder your train, the more calories you burn per minute
2) The heavier you weight, the more calories you burn per minute
The energy cost for running on a treadmill is a lot more fixed then many people realise. This energy cost is calculated in METs, which are a measure of the aerobic intensity that you’re working at. The MET expenditure is fixed enough that we can make the below general point with a relatively high degree of accuracy.
You and I can both go for a 3.0 mph walk up a hill with a 5 degree incline. Regardless of how we may differ on age, gender, height, or any other ‘critical factor’, we will both be working at the same MET intensity. In this example, it would approximately 5.4 METs.
From here, it is easy to see that every different speed setting, incline gradient or other treadmill factor will have a relatively set MET. Each of these settings on the treadmill will deliver its own workload (intensity that you’re training at, or part 1 of our two fold formula). Therefore if we can add in the individual’s weight, or part 2 of the formula, then it is no great challenge to work out the calorie cost of the exercise.
How machines can be more accurate
From here, we can see how popular pieces of cardio equipment, like treadmills can be more accurate than a heart rate monitor.
Modern pieces of equipment, made by leading brands like Technogym and Life fitness contain computer chips which enable them to be controlled by you. The settings are often very precise, as you can typically change the speed in increments as small as 0.1KPH and gradients by 0.5%. This level of precision, allied with the ability to simply time how long the trainee spends in each.
The gym machine manufacturers utilise some long standing and heavily validated formulas to work out the METs for each activity that you can perform on their devices. Treadmills are especially easy because the user is limited as to what they can do on it. Therefore, the only additional required information is your bodyweight.
Treadmill calorie accuracy
The calorie calculated figure is never going to be an absolute, but at speeds below 6.5 MPH, the figure is as accurate as is reasonable to assume without far more complex calculating. As you ramp the speed up above 6.5 MPH, the calorie reading is more prone to over reading, delivering a circa 15-25% high reading. There are more factors at play with running, with a greater difference between the outdoors and indoors, stride length etc – all of which build in some significant margin of error.
Elliptical or Cross Trainer calorie reading accuracy
Cross trainers or elliptical machines don’t mimic a traditional, natural movement. They force your body to follow the resistance curves of the machine itself, and with all machines been created differently, there is no set formula.
Therefore, for an elliptical machine to be accurate it requires the machine manufacturer to invest in creating a bespoke algorithm for their machine. This is a costly process, and as such many don’t undertake it. The only brand known to have tried is LifeFitness, although they have only utilised this research on a select few models.
Therefore, most elliptical machines will have highly inaccurate figures, for the simple reason that its often in a manufacturers interest to inflate their calories burned figure. Most users go to the gym with the aim of losing weight, and they naturally link the machines which burn the most calories to this goal. Therefore the most popular machines are those that present a really healthy calories burned figure, and with no real way for the user to substantiate them, then the manufacturer gets away with it. Over time, the popularity of their machines equals repeat orders from gym owners, providing a commercial reason to present misleading data.
When a heart rate monitor is more accurate than cardio machines and other methods of measurement
Heart rate monitors estimate the calories burned via a very different mechanism. HRMs are only privy to your heart rate, as well as some other factors which aren’t actually required to measure calories, like your height.
We have already determined that in highly structured, natural environments, cardio equipment has a good degree of accuracy. Where the machines fall down is when the exercise becomes less structured or natural, such as cross training.
Therefore if you exercise on more unstructured machines, or if you partake in gym classes or even weights circuits, then a heart rate monitor will typically be more accurate. Also, many of these sessions have no traditional way of measuring calories at all, so the heart rate monitors ‘best guess’ is a lot better then nothing.
You can improve the accuracy of heart rate monitors by helping them to replicate the environment where they were developed and tested. See our 5 tips to improve the accuracy of your heart rate monitor article in order to work on getting that number as accurate as possible.