Treadmill Miles: Does the 1% Incline Rule Hold, and What Transfers to the Road?

As the clocks change and the evenings darken, many triathletes in the northern hemisphere move part of their running indoors. Treadmills offer controlled pace, flat ground, no traffic and no ice. They also come with a piece of advice almost everyone has heard: set the incline to 1% to make it "like running outside."

Where does that rule come from, and does it hold? This article looks at the original study, what larger reviews show about treadmill and road running, and how to make treadmill miles count for triathlon.

Where the 1% rule comes from

The 1% figure traces back to a single study by Andrew Jones and Jonathan Doust at the University of Brighton, published in the Journal of Sports Sciences in 1996.

The idea behind it is simple. When you run outdoors, you push air out of the way, and that costs energy. On a treadmill you stay in one place, so there's no air resistance from your own movement. A small incline adds back some of the missing effort.

Jones and Doust tested nine trained male runners who were used to treadmill running. Each runner ran for six minutes at six different speeds from 2.92 m/s to 5.0 m/s. That's about 5:42 per km down to 3:20 per km. They repeated this on a treadmill at several inclines and once outdoors on a flat road, while the researchers measured oxygen uptake.

The findings:

• At the two slowest speeds, running on the road wasn't significantly different from running on the treadmill at 0% or 1%.
• At faster speeds, road running cost more energy than 0% on the treadmill, but less than 2% or 3%.
• Across the whole range, a 1% incline most accurately reflected the energy cost of outdoor running over these roughly five-minute bouts.

What the study can and can't tell you

This is a small, well-designed study, but its limits matter:

• Nine trained men. No women, no recreational runners, no triathletes.
• Short bouts. Each run lasted six minutes. Nobody measured a long run.
• A specific speed range. The slowest speed tested was about 5:40/km. Many age-group triathletes run their easy miles slower than that.
• Air resistance grows with speed. That's why the incline matters more at faster speeds. At easy paces, the missing air resistance is small.

Inference: for easy running slower than about 5:40/km, the incline probably makes little difference to the energy cost. For tempo and interval work, 1% is a sensible default. It isn't a precise conversion factor, and it doesn't account for wind, hills or the many other things that make outdoor running variable.

What larger reviews show

Since 1996, many more studies have compared treadmill and overground running. Two systematic reviews from the same research group pulled them together.

Effort and physiology

Miller and colleagues reviewed 34 crossover studies, published in Sports Medicine in 2019. They found:

• Oxygen uptake was similar at submaximal speeds on a motorised treadmill and overground.
• Blood lactate was lower on the treadmill at submaximal speeds, both at 0% and at 1% incline.
• Heart rate and perceived exertion depended on speed. They were higher on the treadmill at faster submaximal speeds, and lower at slower speeds.
• At maximal efforts, oxygen uptake and heart rate were similar.
• Endurance performance was worse on a treadmill, with a moderate effect size.

The authors concluded that some, but not all, measures differ between treadmill and overground running, and that the differences depend on running speed.

Mechanics

Van Hooren and colleagues reviewed 33 studies with 494 participants, published in Sports Medicine in 2020. They concluded that running mechanics are "largely comparable" between motorised treadmills and overground running. Most measures didn't differ. The differences they did find were:

• A flatter foot at landing on the treadmill, with an angle about 10 degrees smaller.
• Slightly lower push-off force.
• Slightly longer ground contact, by about 5 milliseconds.
• Less vertical movement of the body's centre of mass.

The evidence for these differences was rated low or very low quality. The authors cautioned that differences in joint angles deserve attention when extrapolating treadmill findings to running outdoors.

Why the treadmill can feel harder

Many triathletes find that a pace they hold comfortably outside feels harder indoors, especially for longer runs. The Miller review's finding that endurance performance was worse on treadmills fits that experience. A few likely reasons, mostly reasoning rather than tested explanations:

• Heat. Indoors, without the breeze created by your own movement, your body has less convective cooling. You sweat more and heat up faster. A fan helps.
• Fixed pace. Outdoors, you naturally vary your pace slightly. A treadmill doesn't allow that, which can feel harder mentally and physically.
• Calibration. Treadmill belts aren't always accurate. The displayed pace may be faster or slower than reality, especially on older or gym machines.
• Familiarity. Studies typically use runners who are already used to treadmills. If you rarely use one, your first few sessions may feel awkward.
• Boredom. Long, steady treadmill runs are mentally hard for many people, which affects perceived effort.

Where treadmills are genuinely useful

For triathletes, treadmills have specific advantages beyond bad weather.

Controlled intervals

Holding an exact pace for threshold intervals is easier on a treadmill than on a crowded road. If you're using critical speed to set your sessions, a treadmill lets you run exactly at, say, 95% of your CS for each repetition. See TriForward's "Critical Speed for Runners" for how to set those paces.

Indoor bricks

If your bike trainer is near a treadmill, you can practise the transition from cycling to running in a controlled way, with a short, predictable changeover. Running off the bike changes how your legs feel for the first few minutes. TriForward's "The Bike-to-Run 'Brick' Problem" covers the evidence on running after cycling.

Hill work without hills

If you live somewhere flat and your race has hills, a treadmill lets you practise climbing at controlled gradients. It can't replicate downhill running well, because most treadmills don't decline, and downhill running places very different demands on the quadriceps.

Heat preparation

Running indoors in a warm room is sometimes used as part of heat acclimation before a hot race. That's a deliberate use of the heat build-up, and it needs care. TriForward's "Heat Acclimation for Race Day" covers how many sessions are needed and how to do it safely.

How to set up a treadmill session

These suggestions are practical guidance based on the evidence above, not a tested protocol:

1. Use 1% for steady and faster running. For easy running, 0–1% is fine.
2. Use a fan, pointed at your upper body, especially for runs longer than 30 minutes.
3. Check calibration roughly. Compare your heart rate and perceived effort at a familiar pace with the same pace outdoors. If the treadmill feels very different, the belt may be off. A foot pod calibrated outdoors can also give you a second reading.
4. Judge effort by more than pace. Use heart rate and how you feel as well. They'll tell you if the treadmill's numbers are misleading.
5. Keep long runs mostly outdoors, if you can. The evidence on treadmill endurance performance, the heat build-up and the mental load all suggest long, steady runs are where treadmills are least comparable.
6. Use the safety clip, particularly for intervals and bricks.

Converting a road session to the treadmill

Here's how the evidence translates into a practical example. The numbers are illustrative, not from a study.

Your planned road session is 4 × 8 minutes at about 4:40/km with 2 minutes of easy jogging between repetitions, after a 15-minute warm-up.

• Warm-up: 15 minutes starting at an easy pace, at 0–1%. At easy speeds the incline makes little difference.
• Repetitions: 4:40/km is about 3.6 m/s, inside the speed range Jones and Doust tested. Set 1% for the repetitions.
• Recoveries: drop the speed to an easy jog. You can leave the incline at 1% to keep things simple.
• Check effort: if the first repetition feels much harder than the same session outside, and your heart rate is clearly higher than usual, the belt may be fast or the room may be too warm. Adjust by feel rather than forcing the displayed pace.
• Cool-down: 10 minutes easy.

The point isn't to match the road session to the second. It's to hit the intended intensity. If heart rate, breathing and perceived effort line up with what you'd expect outside, the session has done its job.

A simple indoor brick

A treadmill next to an indoor bike trainer makes bricks easy to fit into a dark winter evening. One practical structure, again illustrative:

1. Bike: 60–75 minutes on the trainer, including some time at your target race intensity in the final 20 minutes.
2. Transition: change shoes quickly, clip on the treadmill safety cord, and start the belt at an easy jog, not your goal pace.
3. Run: build to your target run pace over the first three to five minutes, then hold it for 15–20 minutes.
4. Cool down with a few minutes of walking.

The controlled setting is ideal for noticing how your legs feel in the first few minutes off the bike, and for practising settling into pace rather than charging out of T2. The heat build-up indoors is a real issue here: two consecutive indoor sessions can leave you hotter than a race in mild weather, so use fans for both.

Incline training for hilly courses

Many triathlon run courses have significant climbs. A treadmill lets you practise sustained uphill running at a fixed gradient, which is hard to find in flat areas. Inference rather than tested advice: short, controlled uphill blocks at 4–8% can build the strength and rhythm needed for climbing, while keeping pace and effort predictable.

Two cautions. First, climbing on a treadmill loads the calves and Achilles more than flat running, so build gradually. Second, most treadmills can't simulate descents. If your race has long downhills, you'll need some outdoor downhill running too, because downhill running is what tends to leave quads sore and damaged late in hilly races.

Keeping your legs road-ready

Because treadmill running is largely comparable mechanically, a winter block indoors is unlikely to change your running form much. But the small differences, such as a flatter foot at landing and slightly less push-off, together with the absence of camber, turns, wind and uneven surfaces, mean your body gets a slightly different stimulus.

Inference: keep at least some running outdoors each week if you can, and ease back into longer outdoor runs in spring rather than jumping straight to your previous long-run distance on the road.

Who benefits most from treadmill running

Treadmills aren't equally useful for everyone. Inference from the evidence and from the practical constraints most age-groupers face:

• Athletes who train before or after work in winter darkness get the most obvious benefit: safe, lit, predictable running.
• Athletes returning from injury can control pace, gradient and surface precisely, and stop at any moment. That's valuable when a physiotherapist has set specific limits.
• Athletes who struggle to run easy can lock the belt at an easy pace, which removes the temptation to speed up with a running partner or on a familiar route.
• Athletes preparing for a flat, fast race lose little by doing some quality sessions indoors, because the course itself is close to treadmill conditions.

Athletes preparing for a hilly, technical or hot race, or anyone who finds treadmill running mentally draining, will want to keep a larger share of their running outdoors.

What we don't know

• Whether 1% is right for women or for slower runners. The original study was nine trained men, and its slowest speed was faster than many age-groupers' easy pace.
• Long-duration equivalence. Most comparisons used short bouts. How a 90-minute treadmill run compares to the same run outside hasn't been studied in detail.
• Treadmill-specific injury risk. There's no good evidence that treadmills are safer or riskier for injury overall.
• Curved, self-powered and cushioned treadmills. The reviews focused on standard motorised treadmills.

Common mistakes

• Running every easy run at 1–2% "to be safe." At easy speeds, it adds effort without much reason.
• Trusting the display completely. Gym treadmills can be noticeably off.
• No fan. Overheating is one of the main reasons treadmill runs feel harder.
• Racing the treadmill. Treadmills are good for controlled sessions. Using them to chase pace PBs tells you little about road fitness.
• Doing all long runs indoors before a road race. Keep some outdoor long runs if you can.

References

• Jones AM, Doust JH. A 1% treadmill grade most accurately reflects the energetic cost of outdoor running. Journal of Sports Sciences, 1996;14(4):321–327. doi:10.1080/02640419608727717
• Miller JR, Van Hooren B, Bishop C, Buckley JD, Willy RW, Fuller JT. A systematic review and meta-analysis of crossover studies comparing physiological, perceptual and performance measures between treadmill and overground running. Sports Medicine, 2019;49(5):763–782. doi:10.1007/s40279-019-01087-9
• Van Hooren B, Fuller JT, Buckley JD, Miller JR, Sewell K, Rao G, Barton C, Bishop C, Willy RW. Is motorized treadmill running biomechanically comparable to overground running? A systematic review and meta-analysis of cross-over studies. Sports Medicine, 2020;50(4):785–813. doi:10.1007/s40279-019-01237-z