Tis the season for cardio machines to be full – inspired by new year's resolutions or guilt from holiday splurges. With no time to dissect the motivational or emotional aspects of these efforts, I turn attention to the virtue of honesty. Are cardio machines lying to you when estimating energy expenditure (calories) from a workout?
I will focus on treadmills and ellipticals.
Prediction Equations: How They Work
All cardio machines depend on equations to 'predict' how many calories are expended (not burned) during a workout. These equations must first estimate how much oxygen you consumed during your workout. Clearly, it does not know how much oxygen you breathe, so it makes its best guess based on the length and intensity of the workout. If the equation can estimate how much (volume) of oxygen you consumed, we can use a simple (a bit too simple) conversion:
1 Liter of oxygen consumed during exercise = 5 calories expended
Treadmills have a bit of an advantage over ellipticals, because many of the equations that have been formulated over years of research have been done using walking or running on a treadmill. These equations usually estimate using the speed of the treadmill, and what incline or grade you have chosen.
For example, the following two equations* are widely-used by the American College of Sports Medicine for oxygen consumption during walking and running, respectively.
WALKING: Oxygen Consumption = 0.1 (speed) + 1.8 (speed) (incline %) + 3.5
RUNNING: Oxygen Consumption = 0.2 (speed) + 0.9 (speed) (incline %) + 3.5
Susan, a 150 pound (68 kilogram) woman, gets on her favorite treadmill. She walks for 30 minutes at a speed of 2.0 mph, at a incline grade of 2%. The walking equation would estimate that Susan consumed 22 liters of oxygen during her workout. If 5 calories are estimated to be expended every liter of oxygen consumed...
The treadmill calorie counter would read:
Such equations are commonly agreed to be a suitable 'estimation' of energy expenditure (in calories) for walking and running on the treadmill.3
The elliptical is a different story. The treadmill equations do not work, and less research has been done. The few studies that have been done show that elliptical machines OVER-estimate how many calories are expended – especially at higher resistance and intensity levels.
- Over-estimated at higher intensities by 18% | Precor EFX 556i2
- Over-estimated 21% to 28% at various strides and resistance levels | Precor EFX 534i5
- Over-estimated up tot 18 calories per minute at higher resistance levels | Precore EFX 576i, Precor AMT 100i4
The good news is that equations are being developed1, but it is not known if the ellipical you are on is using the new equations. Feel free to calculate yourself!
Oxygen Consumption = 3.5 + 0.15 (cadence) + 1.22 (resistance) - 0.11 (weight)
Take Home Message
- Calorie estimation is just a guess of heat energy produced during the workout, estimated from oxygen consumption, and is not that accurate.
- Cardio machines depend on estimation equations to 'predict' how many calories you expend during a workout.
- Treadmills appear do better than elliptical machines, but are not perfect.
Also, remember that everyone is different – with different body types, metabolisms, body compositions, and training levels. Thus, these equations, even if you put in your body weight into the machine's calculation, are not going to be 100% accurate. Do not expect them to be accurate, and be okay with never confirming their accuracy.
You might also be interested in reading about the myth that elliptical handles will 'cheat your calorie burn'.
Instead, join the others who use total minutes and level of effort as their main outcomes – and not calories.
If you are stuck on seeing calories, you might consider a validated physical activity monitor (e.g. SenseWear Pro Armband) that does not care what machine you are on. Just remember that these monitor are estimating caloric expenditure in a very similar way to the cardio machines.
In the above equations:
- 'Speed' was in meters/min (1 mph = 26.8 meters/min)
- '% grade' was calcluated as a decimal (e.g. 2% = .02)
- Calculates 'Oxygen Consumption' as milliters of oxygen / kg of body weight / minute (ml/kg/min)
- Dalleck, L. C., & Kravitz, L. (2007). Development of a metabolic equation for elliptical crosstrainer exercise. Perceptual and Motor Skills, 104(3), 725-732.
- Graybosch, G., Verducci, F., Kern, M., & Lee, C. M. (2011). Accuracy of estimated energy expenditure from the caloric display of an elliptical trainer. Medicine and Science in Sports and Exercise, 43(5), 474-475.
- Hall, C., Figueroa, A., Fernhall, B., & Kanaley, J. A. (2004). Energy expenditure of walking and running: Comparison with prediction equations. Medicine and Science in Sports and Exercise, 36, 2128-2134.
- Louie, L., & Lam, Y. T. (2011). A Comparison on Selected Physiological Variables between Two Models of Elliptical Cross Trainers. Asian Journal of Physical Education and Recreation, 17(1), 91-103.
- Mier, C. M., & Feito, Y. (2006). Metabolic cost of stride rate, resistance, and combined use of arms and legs on the elliptical trainer. Research Quarterly for Exercise and Sport, 77(4), 507-513.