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Running on a treadmill costs less energy than running outdoors if the treadmill is level. However, you can make your treadmill workout equivalent to an outdoor workout by adjusting the incline of the treadmill. According to Jones and Doust, (Jones, A. M., and Doust, J.H., A 1% treadmill grade most accurately represents the energetic cost of outdoor running, Journal of Sports Science, 1996, 14, 321-327a 1% grade is what you should use. % grade is rise over run. Most treadmills report incline in %grade. Be aware that your treadmill has to be leveled appropriately for this to work. You can check your treadmill's level with a carpenter's level. If you really want to be accurate, you can rig a protractor and a carpenter's level to measure rise and run and compare that to your treadmill's display. Also, you should check the accuracy of your treadmill's speed indicator. Put a chalk mark on the treadmill and count revolutions. With revolutions, and knowledge of the belt length, you can calculate treadmill speed.
The study used 9 male distance runners with an average VO2 max of 65 ml/kg/min. They were pretty fit. They ran for 6 minutes at each velocity and at % grades of 0, 1, 2 and 3%. Then they ran outdoors while expired air was collected in a Douglas bag. You have to appreciate the lengths to which researchers will go to gather data. The researcher had to bicycle along with the runner and hold the large weather balloon type bag, while keeping the runner on target speed. I hope he was wearing a helmet. Science is not always easy! They measured oxygen consumption and heart rate in all conditions. Outdoor running was only tested during calm wind conditions. The heart rate data for outdoor running and 1% grade treadmill are below. Note how similar they are.
MPH | 1% | Outdoors |
6.5 | 119 | 119 |
7.4 | 128 | 128 |
8.4 | 140 | 140 |
9.3 | 153 | 153 |
10.2 | 164 | 165 |
11.2 | 173 | 174 |
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