Treadmill Running: Do you need to run at 1%

Dear Gentle Reader,

Many runners have been told by a well meaning, and often woefully uninformed, friend or Facebook group that you have to set the treadmill to 1% to “mimic running outdoors.” It is one of the most repeated treadmill tips in running, and it sounds scientific enough that people treat it like a rule.

Here’s the TLDR (in case you don't want to dig into the science, which I'll speak to below); mix up the incline. Spend plenty of time running flat, or flat-ish, and also flux in some gentle hills. For most easy runs and long runs, 0 to 1% is my baseline range. From there, add hills with intention, think long and sustained climbs around 2 to 3%, and sprinkle in some shorter, slightly spicier climbs in the 3 to 4% range.

One note before we get into the details: this is written with a fairly typical road race course in mind, not something aggressively hilly where you would absolutely want more specific hill prep.

Treadmill running with varied incline settings

Where the 1% idea came from

The 1% recommendation largely traces back to a classic lab study that compared short treadmill bouts with outdoor running and concluded that around 1% could match the energetic cost of outdoor running under those specific conditions (Jones & Doust, 1996).

BOTTOM LINE: The study lacks context and was applied very broadly. It was NOT saying 1% is automatically best for every runner, every pace, every treadmill, and every workout.

Not a universal rule

The biggest issue with the 1% rule is that it tries to solve one variable, wind resistance, with one setting, and real running is never that tidy. When researchers zoom out and look across many studies, the story gets messier. A large systematic review and meta analysis comparing treadmill and overground running found that physiological and perceptual outcomes do not match perfectly across conditions, and that speed can change the direction of those differences. Their practical takeaway is that you cannot “balance” everything with one setting, because treadmill vs outdoor differences are not driven by one factor alone (Miller et al., 2019).

We also have newer work that directly challenges the idea that 1% reliably “equates” treadmill and outdoor running. In a 2024 study, runners showed higher cardiorespiratory responses on a specific treadmill compared to outdoor running, and setting the treadmill to 1% actually increased the difference rather than reducing it (Nolè et al., 2024). That is the key point, even if 1% works in one lab setup, it does not behave like a universal correction across devices and environments.

And yes, wind resistance is real, but it is also highly speed dependent. The energetic cost of overcoming air resistance rises as speed increases, which means the “gap” you are trying to close with incline is not the same at all paces (Pugh, 1971). Add in the fact that indoor running often includes less airflow and more heat buildup than outdoors, which can bump heart rate and effort even when pace is the same (Miller et al., 2019). So if someone is trying to use 1% as a conversion tool, they are basically ignoring a pile of other variables that can matter just as much as wind.

Translation in real life; the 1% rule is not “debunked” in the sense that it was made up, but it is oversold as a blanket prescription. We have other factors to consider, and the one I care most about from a coaching perspective is loading patterns, not just energy cost.

Now that part that we actually need to care about: LOADING PATTERNS and INJURY RISK

Running is thousands of repetitions. When you hold one grade for a long time, you keep stressing the same tissues in the same way, step after step.

Two things matter here.

1) Treadmill running can increase Achilles loading compared to overground. One controlled lab study found treadmill running produced greater Achilles tendon loading than overground at matched speeds, which matters if you are stacking a lot of treadmill volume (Willy et al., 2016).

2) Incline changes what does the work. Uphill running generally shifts demand toward propulsion and concentric work. Downhill running increases eccentric work and braking demands, especially through the quads, and it is a major reason descents can leave you sore when you are not prepared (Bontemps et al., 2020).

So if you do a long run at a steady incline, you are not just “simulating outside.” You are choosing a more repetitive, slightly different loading pattern for a long time, often with more demand through the calf and Achilles complex.

“But the treadmill has no downhills.” Do we need to fix that?

The treadmill usually removes most downhill eccentric loading, unless you have decline, and downhill running is its own very specific stressor (Bontemps et al., 2020). Do we need to address it? Usually, not aggressively. If you get outside even once a week, and you are strength training even a little, you are covering a lot of that missing stimulus.

When it REALLY matters:

• You are training for a course with real downhills

• You have been almost exclusively treadmill running for months

• You are prone to quad soreness or knee sensitivity when you finally get outdoors

Simple ways to cover the gap:

• Get outside when you can, even an easy run, so your legs remember real terrain

• Build eccentric strength with basics like slow step downs, split squats, and controlled calf lowers

• If you do have decline, use it as a small dose tool, not as your default

Bottom line

The treadmill can absolutely support great training. I just do not want runners treating 1% as a law. The smarter approach is to keep 0 to 1% as your baseline, keep plenty of true flat running in the mix, and then layer hills strategically so you get variety in loading and a better training stimulus overall (Miller et al., 2019; Willy et al., 2016).

If you tell me what the workout is, easy run, long run, marathon pace, intervals, and roughly what effort you are running, I can plug in an incline plan that matches it exactly.

References

Bontemps, B., Vercruyssen, F., Gruet, M., & Louis, J. (2020). Downhill running: What are the effects and how can we adapt? A narrative review. Sports Medicine, 50(12), 2083–2110. https://doi.org/10.1007/s40279-020-01355-z 

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. https://doi.org/10.1080/02640419608727717 

Miller, J. R., Van Hooren, B., Bishop, C., Buckley, J. D., Willy, R. W., & Fuller, J. T. (2019). A systematic review and meta analysis of crossover studies comparing physiological, perceptual and performance measures between treadmill and overground running. Sports Medicine, 49(5), 763–782. https://doi.org/10.1007/s40279-019-01087-9 

Willy, R. W., Halsey, L., Hayek, A., Johnson, H., & Willson, J. D. (2016). Patellofemoral joint and Achilles tendon loads during overground and treadmill running. Journal of Orthopaedic & Sports Physical Therapy, 46(8), 664–672. https://doi.org/10.2519/jospt.2016.6494