Key takeaways
A classic study found a 1% treadmill incline matched the oxygen cost of flat road running at 10.5 to 18 km/h, though a 2026 study suggests it still slightly underestimates outdoor effort
Treadmill running mechanics are largely similar to running outside, with a flatter foot at landing and slightly less push-off
Treadmills are great for quality sessions in bad weather, but marathon runners should do most long runs outside
Running 5 km at 5:00 per kilometre on a treadmill doesn't always feel like the same run outside. Sometimes it feels easier, sometimes harder, and your watch may not even update your VO2max afterwards. Here is what the research says about treadmill vs outdoor running, and how to use both.
Treadmill vs outdoor running: the short answer
On a flat treadmill you don't push against air, and the belt helps carry your foot back. A 1% incline makes up for most of that at normal training speeds. Running mechanics are mostly similar, but indoor heat, belt calibration and the lack of hills, turns and wind mean the two are not identical.
| Factor | Treadmill | Outdoors |
|---|---|---|
| Air resistance | None | Grows with speed and headwind |
| Energy cost at same pace | Slightly lower at 0% | Slightly higher |
| Surface | Even, cushioned belt | Varied, firmer, with turns |
| Hills | Uphill only on most machines | Up and down |
| Heat | No cooling airflow unless you add a fan | Airflow, but weather varies |
| Pace accuracy | Depends on calibration | Depends on GPS quality |
| Watch VO2max | Often not updated | Updated on most watches |
Treadmill vs outdoor running speed and pace
The well-known "set it to 1%" advice comes from Jones and Doust (1996). Nine trained male runners ran at six speeds from 2.92 to 5.0 m/s (about 10.5 to 18 km/h, or 5:42 to 3:20 per km) on a treadmill at 0, 1, 2 and 3% and on a flat road. Across that range, the oxygen cost of running at 1% incline best matched the road.
Keep two caveats in mind. The study did not test slower jogging paces or faster sprinting speeds, and it was small. A 2026 study of 12 endurance runners by Shahidi and colleagues found that running outside still cost more energy than running at 1%, with the gap growing as effort approached the second ventilatory threshold. So treat treadmill pace as slightly flattering, especially for tempo and threshold work.
Treadmill displays can also be off. If your indoor paces look much faster than your outdoor ones at the same heart rate, calibration may be part of the reason. To set targets, estimate paces with the VO2max running calculator, then check your effort against heart rate.
Treadmill vs outdoor running biomechanics
The most thorough answer comes from a 2020 systematic review and meta-analysis by Van Hooren and colleagues (33 studies, 494 runners). Most measures of stride, joint angles, forces and muscle activity were largely comparable. The differences on a treadmill were small:
- The foot landed about 10 degrees flatter (a smaller foot-ground angle at landing).
- The knee was about 2 degrees more bent at landing, with a smaller knee range of motion during stance.
- The body bobbed up and down about 1.5 cm less, ground contact was about 5 ms longer and peak push-off force was a little lower.
In practice, treadmill training transfers well to running outside, but switching all your running indoors or outdoors overnight changes the load on your legs slightly. Make big changes gradually.
Heat, humidity and heart rate indoors
Outside, moving through air cools you. On a treadmill you stay in one place, often in a warm gym, so sweat and heart rate climb faster at the same pace. Use a fan, open a window and drink, or your heart-rate zones will drift upward during long sessions.
Heat matters outside too. Ely and colleagues analysed results from seven marathons and found that finish times slowed progressively as heat stress rose, with slower runners affected more. On a hot day, a cool treadmill can be the better place for a hard session. See heat training and VO2max for how heat changes your numbers.
Why your watch VO2max may not update on a treadmill
Most watches estimate VO2max by comparing your heart rate with your speed, and many only trust GPS speed. Garmin uses outdoor GPS runs on most models and Apple Watch uses Outdoor Run, Outdoor Walk and Hiking workouts, so treadmill runs often don't count (details in Garmin VO2max explained). Even when a watch does use indoor runs, it relies on a wrist accelerometer for pace, which is less accurate. If most of your running is indoors, a flat VO2max reading may simply mean missing data. Compare brands on the watches page and read how accurate watch VO2max is.
Treadmill vs outdoor running: which is better?
- Choose the treadmill for precise interval pacing, safe running in dark, icy or very hot weather, controlled hill repeats (see the treadmill hill session in VO2max workouts) and rehab after injury.
- Choose outdoors for race-specific running, downhills, turns, uneven ground, better watch data and, for many people, more enjoyment.
Both build fitness. The best option is the one you'll do consistently, a key point in how to run faster.
Treadmill vs outdoor running for marathon training
A marathon is run on roads, often with hills, camber, wind and drink stations. If winter pushes you indoors, keep the quality sessions on the treadmill (with 1% incline and a fan) and do most long runs outside, even if they're slower in bad weather. Include some downhill running before hilly races, since most treadmills can't simulate it. Practise drinking and fuelling at marathon pace in both settings, and keep easy runs easy, in line with 80/20 training. Short strides work better outside, and the Norwegian double threshold method is often run on treadmills because pace is so easy to control. Set your zones with the heart rate zone calculator and find more in VO2max for running.
Safety: use the safety clip, start the belt slowly and step off if you feel dizzy. If you have a heart condition or high blood pressure, get medical advice before hard sessions indoors or out.
Frequently asked questions
Is running on a treadmill easier than running outside?
Slightly, at the same speed. There is no air resistance and the belt is perfectly even, so a flat treadmill costs a little less energy. Setting a 1% incline closes most of the gap at typical training paces.
Is treadmill pace the same as outdoor pace?
Not quite. At 0% incline the same pace usually feels a bit easier indoors, and treadmill speed displays can be off. Use effort and heart rate as well as pace, and expect outdoor pace to be a little slower for the same effort.
Is treadmill or outdoor running better?
Neither is better overall. Outdoor running is more specific to road races and gives your watch better data, while a treadmill offers control, safety and easy hills. Most runners do best with a mix.
Can you train for a marathon on a treadmill?
Yes, many runners do much of their training indoors in winter. Try to do most long runs and some downhill running outside, and practise drinking and fuelling at race pace, so your legs and gut are ready for the road.
Sources
- Jones AM, Doust JH. A 1% treadmill grade most accurately reflects the energetic cost of outdoor running. J Sports Sci, 1996.
- Van Hooren B, et al. Is motorized treadmill running biomechanically comparable to overground running? A systematic review and meta-analysis of cross-over studies. Sports Med, 2020.
- Shahidi, et al. Overground running incurs a higher energetic cost than treadmill running at a 1% grade: a comparison of running economy, oxygen cost of transport, and energy cost in endurance athletes. PLOS One, 2026.
- Ely MR, et al. Impact of weather on marathon-running performance. Med Sci Sports Exerc, 2007.