Is It Bad to Run on a Flat Treadmill?

Someone recently asked if running on a flat treadmill (0% grade) was bad. When I questioned a bit further, she confessed to reading an online article that put running on a flat treadmill in the top 7 list of things fitness experts wish you'd stop doing.

Being a fitness expert, I was lumped into this group who supposedly thought running on a flat treadmill was bad, worthless, did not 'burn' as many calories, and was even harmful.

Well, I have actually heard comments like this before. The main concern has classically been that running on a flat treadmill is not worth it, because it does not expend as much energy as running outside (i.e. overground).

Aerobic and Caloric Demand

To a degree, this concern is true. Now, I would not conclude that the flat treadmill is not worth it or bad, but research does support some differences between the two.²

1. At lower running speeds (6.5-7.5 mph), overground running has the same aerobic demand as running on a treadmill at 0% grade (flat) or 1% grade.
2. However, the treadmill is actually more demanding than overground running at these lower speeds at 2% and 3% grades.
3. At higher running speeds, (9.3-10.3 mph), overground running has the same aerobic demand as running on a treadmill with 1% or 2% grades.
4. Calorically, overground running at any speed from 6.5 to 11 mph is about the same as running on a treadmill at 1% grade.

Injury

There is some other evidence that the muscle activation of the lower legs and running kinematics differ between treadmill and overground running in both adults and children,^1,3,4 but I am not aware of any research indicating that flat treadmill running would cause additional pain or injury in someone without specific injuries or other contraindications.

Conclusion

I see no reason for you to stop running on a flat treadmill, unless you have a specific reason not to. Thus, running on a treadmill is not 'bad' per se, but does differ a bit from running overground.

If you do want your treadmill run to equate to running outside over the ground, both aerobcially and calorically, then put the treadmill at a 1% grade.

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References

1. Baur, H., Hirschmüller, A., Müller, S., Gollhofer, A., & Mayer, F. (2007). Muscular activity in treadmill and overground running. Isokinetics and Exercise Science, 15(3), 165-171.
2. Jones, A. M., & Doust, J. H. (1996). A 1% treadmill grade most accurately reflects the energetic cost of outdoor running. Journal of Sports Sciences, 14(4), 321-327.
3. Rozumalski, A., Novacheck, T. F., Griffith, C., Walt, K., & Schwartz, M. H. (2015). Treadmill vs. overground running gait during childhood: A qualitative and quantitative analysis. Gait and Posture.
4. Van Caekenberghe, I., Segers, V., Aerts, P., Willems, P., & De Clercq, D. (2013). Joint kinematics and kinetics of overground accelerated running versus running on an accelerated treadmill. Journal of The Royal Society Interface, 10(84), 20130222.