Treadmill vs Outdoor Running: What Actually Changes and How to Use Both

Almost every Manhattan building gym has a treadmill, and almost every runner who uses one has been told it “doesn’t count” the way a run in Central Park does. The research says otherwise, mostly. At the same speed, a treadmill costs you the same oxygen as the road, your stride is close to your outdoor stride, and your shin bones take less strain. What changes is smaller and more specific: the belt feels harder at tempo pace, your sense of speed on it is badly miscalibrated, and you lose the terrain, the air and the mood lift of being outside. This guide covers what actually differs, what the 1 percent incline rule is worth, when the treadmill is the smarter choice in this city, and how to use both without getting hurt.

The short version

• Treadmill miles count. Across 34 crossover studies, oxygen cost at a matched speed differed by less than about 1.3 mL/kg/min between treadmill and overground at every intensity. Your heart and lungs cannot tell much difference.
• But the belt feels harder at hard paces. The same review found endurance performance was worse on the treadmill (a moderate effect) while blood lactate at a given speed was lower. Train by effort or heart rate on the belt, not by matching your outdoor pace to the second.
• Your speed sense is off by about 30 percent. Runners blinded to the display set the belt to 2.73 m/s when their real outdoor pace was 3.80 to 3.99 m/s. Trust the console.
• Your stride is “largely comparable.” On the belt you land with a flatter foot (about 10 degrees) and a slightly more bent knee; peak impact force and loading rate were not significantly different when pooled.
• The shin bone gets a break. In the one in-vivo measurement, tibial strain was 48 to 285 percent higher running on the ground than on a treadmill. Useful during shin-splint recovery; a reason not to live on the belt if you want strong bones.
• The 1 percent rule is a convention, not a law. It comes from nine men in 1996, and at slower speeds a flat belt matched the road just as well. Newer work found 1 percent over-corrects for fast, trained runners.
• Outdoors wins on mood and enjoyment, by a modest margin and on thin evidence. People rate outdoor sessions more enjoyable and energizing; the 2019 review calls the evidence low quality.
• Pick the treadmill in July heat, on dark winter evenings and on wildfire-smoke days. Ordinary New York air is not a reason to stay in: at typical city pollution you would need 16 hours of walking a day before harm outweighed benefit.

What actually changes on a treadmill

Oxygen cost: the same

The largest review on this question pooled 34 crossover studies in which the same people ran on a treadmill and overground. Submaximal oxygen uptake at a flat belt differed from overground by a mean of -0.55 mL/kg/min; at 1 percent grade by +0.37; near maximal by -1.25; at maximum by +0.78, with 95 percent confidence intervals that crossed zero in every case (Miller, Van Hooren and colleagues, Sports Medicine, 2019). Maximal heart rate differed by 1 beat per minute. If your training plan says “40 minutes easy,” the belt delivers that.

Performance and feel: not quite the same

The same review found two things that explain why runners distrust the belt. Endurance performance was worse on the treadmill (standardized mean difference -0.50), and blood lactate at a given submaximal speed was lower (by 1.26 mmol/L on a flat belt). The authors’ summary: “Some, but not all, variables differ” between the two. Heart rate was higher on the treadmill at faster submaximal speeds and lower at slower ones. The practical reading: a 20-minute tempo at your outdoor pace is harder to hold on a belt, even though the physiology says it should not be. Run tempos by heart rate or by effort on the treadmill and let the pace be what it is.

One reason is air. A moving runner pushes through air and a treadmill runner does not; the extra oxygen cost rises with the cube of speed (the equation is Pugh’s, confirmed in Kipp, Kram and Hoogkamer, Frontiers in Physiology, 2019). At an easy pace it is only 1 or 2 percent of your total; at 5 m/s, a 5:22 mile, our arithmetic from that equation puts it around 5 percent. Another reason is economy: in 13 competitive distance runners, running on a track cost 215 mL of oxygen per kg per km against 237 on a 1 percent treadmill, about 9 percent less (Mooses and colleagues, Biology of Sport, 2015). Do not panic when your treadmill pace at a given heart rate is slower than outside. That is the belt, not your fitness.

Your sense of speed: badly off

Twenty-one runners ran overground at 3.99 m/s, then were asked to set a treadmill, display hidden, to the speed that felt the same. They chose 2.73 m/s, about 30 percent slower, and when sent back outside ran 3.80 m/s (Gait and Posture, 2012). The authors blame the missing visual flow: nothing moves past you on a belt, so every speed feels fast. Set the console to your real pace and expect it to feel quick for the first few minutes.

Your stride: close, with a flatter landing

A second meta-analysis from the same group pooled the biomechanics. Ground contact time was 5 ms longer on the belt; stride frequency and stride length were not significantly different. The foot landed about 9.8 degrees flatter and the knee 2.3 degrees more bent at contact; peak propulsive force was lower by 0.04 body weights. Peak vertical ground reaction force (-0.05 body weights) and average vertical loading rate were not significantly different when pooled, and the authors rate most of the evidence low to very low quality (Van Hooren and colleagues, Sports Medicine, 2020). So the “treadmill ruins your form” claim is overstated, and so is the “treadmill is easy on the joints” claim: the impact numbers are close.

The one place the belt is clearly gentler is bone. Three runners with strain gauges on their tibias ran at 11 km/h on both surfaces; overground produced 48 to 285 percent higher compression and tension strains than the treadmill (Milgrom and colleagues, British Journal of Sports Medicine, 2003). It is three people, so hold it loosely, but it cuts both ways: lower strain is useful when you are returning from shin splints or a stress reaction, and less strain also means less of the stimulus that keeps bone strong. If every run you do is on a belt, keep the strength work in your week; our strength training for runners guide covers what that looks like.

The 1 percent incline rule

The rule that a treadmill should be set to 1 percent to match the road comes from one 1996 study of nine trained men running at six speeds from 2.92 to 5.0 m/s (about 6.5 to 11.2 mph). At the two slowest speeds, both 0 and 1 percent matched the road. The 1 percent grade only clearly mattered from 3.75 m/s (about 8.4 mph, a 7:09 mile) up, and at 5.0 m/s the road sat between 1 and 2 percent (Jones and Doust, Journal of Sports Sciences, 1996). A 2024 study of 14 highly trained men found the opposite problem: at 14 km/h, a 1 percent treadmill cost 12.6 percent more oxygen than the track (Current Issues in Sport Science, 2024).

What to do with that: if you run slower than about 8:00 per mile, the incline setting is optional. If you are doing marathon-pace or faster work and want the belt to approximate the road, 1 percent is a reasonable convention. Either way, the honest fix is the one above: train by effort and heart rate on the belt and stop trying to make the numbers match.

When the treadmill is the smarter choice in New York

• Heat. The standard event guidance flags a wet bulb globe temperature of 73 to 82 F as high risk and above 82 F as cancel-or-withdraw territory (ACSM flag system, 2007, as summarized). A July afternoon on the Greenway can easily reach that band. Move the quality session to the building gym and keep the easy miles for 6 a.m.
• Dark. Seventy-eight percent of US pedestrian deaths in 2022 happened in the dark, against 19 percent in daylight (NHTSA, Traffic Safety Facts, 2022 data). Between November and February, the 6 p.m. run is a treadmill run or a well-lit park loop with reflective gear, not a crosstown street.
• Smoke days. Ordinary city air is not the problem. At the global urban average of 22 micrograms per cubic meter of PM2.5, the point at which pollution harm outweighs exercise benefit is 16 hours of walking per day (Tainio and colleagues, Preventive Medicine, 2016), and New York’s annual PM2.5 fell 36 percent between 2009 and 2024 (NYC Health, Community Air Survey). The exceptions are wildfire-smoke days like June 2023. Day to day, pick the park interior or the river over avenue sidewalks, because traffic density drives local pollution.
• Shin pain and comebacks. For the bone-strain reason above, the belt is the safer surface for the first weeks back from a tibial stress reaction, with your doctor’s or physical therapist’s sign-off.
• Precise intervals. Nothing outdoors holds 7.5 mph for exactly three minutes. For structured speed work, the belt removes the guesswork, with the caveat that it will feel harder than the same pace outside.

When outside is the smarter choice

Three reasons, in order of how well they are supported.

Enjoyment, which is what keeps you running. A review of 11 trials (833 adults) found a single outdoor bout was associated with greater revitalization, energy and enjoyment and lower tension than the same bout indoors (Thompson Coon and colleagues, Environmental Science and Technology, 2011). A larger 2019 review of 28 trials was more sober: it rated the evidence low quality and found enjoyment higher outdoors in three of four acute trials, with no consistent difference in perceived exertion (Lahart and colleagues, International Journal of Environmental Research and Public Health, 2019). People like outdoor runs more. That is not a small thing when the goal is to still be running in March.

Terrain and race specificity. The belt is flat, straight and predictable. Central Park is not: the full loop is 6.1 miles, the Reservoir track is a 1.58-mile soft-surface loop, and the upper loop carries the steepest hills on the drives (Central Park Conservancy, Runners’ Guide). Hudson River Park runs about four miles along the west side (Hudson River Park Trust), flat and car-free, which makes it the natural place for tempo work. If you are racing, some of your long runs have to be on ground that turns, climbs and descends, because downhill running is a different event: at a 9-degree descent, impact force peaks rose 54 percent and braking force peaks 73 percent, and the muscle damage that follows impairs running mechanics for up to 48 hours and leaves quad strength down for several days (Lu and colleagues, Frontiers in Bioengineering and Biotechnology, 2025). Most treadmills, and nearly all building-gym treadmills, do not decline. If your race has downhills, your legs need to meet them before race day. Our NYC Marathon guide covers how to get in.

Vitamin D is not one of the reasons. Above 40 degrees latitude, which includes New York, winter sunlight is too weak for skin to make vitamin D in some months regardless of skin type (Kift and Webb, Nutrients, 2024). Run outside because you like it, not for the December sun.

Using the incline

Incline is the treadmill’s best feature and the one most people ignore. Uphill running raises energy cost roughly in a straight line with grade, shifts work from the ankle toward the hip, and raises calf and quad muscle activity (soleus activity up about 14 percent and the vastus group up about 23 percent in the Lu review above). You get hill stimulus without the eccentric damage of the descent, because you never run down. For hill repeats, a building treadmill at a moderate grade, 4 to 6 percent for 60 to 90 seconds with a walking recovery on the flat, is how we substitute for Harlem Hill (our coaching practice, not a research finding). Add incline gradually if your Achilles or calves are touchy.

For walkers, incline is the lever that turns a walk into a workout. Using the standard ACSM metabolic equations (reproduced by IDEA Fitness), a flat 3 mph walk costs about 11.5 mL/kg/min of oxygen; the same 3 mph at a 12 percent grade costs about 28.9; a 5 mph jog costs about 30.3. The viral “12-3-30” workout, 12 percent, 3 mph, 30 minutes, is roughly a 12:00-per-mile jog’s worth of work at walking impact. Two caveats: the walking equation was built from level and moderate-grade walking, so treat 12 percent as an estimate, and it assumes you are not holding the rails.

Treadmill safety, especially over 50

The treadmill’s real hazard is not the joints. It is the fall. US emergency departments recorded an estimated 451,250 treadmill-related injuries between 2000 and 2022, roughly 20,000 a year, with adults 50 and older making up about a third of the lower-extremity cases (Forrester, Lower Extremity Review, 2024, from CPSC surveillance data), and adult treadmill head injuries rose more than tenfold between 1997 and 2014 (Catapano and colleagues, Brain Injury, 2018). Most building gyms are unstaffed. The routine that prevents most of it, from Consumer Reports and manufacturer guidance (Consumer Reports, 2015; NordicTrack):

• Clip the safety key to your clothing every time. It is the only thing that stops the belt once you are no longer upright.
• Straddle the belt, start it at 0.5 to 1 mph, then step on. Never step onto a moving belt.
• Eyes forward, not at your phone. Looking down is how people drift to the edge.
• Change speed in small steps. Let the belt stop completely before you step off.
• At home with small children: unplug it and remove the key. Children between 1 and 6 sustain more treadmill injuries than any other age group (Consumer Reports, citing CPSC), and in one burns-unit series most were hand burns from touching a belt someone else was using (Kim and colleagues, Medical Journal of Australia, 2009).

About the handrails. Use them to get on, and for the first minute if your balance is uncertain. After that, fingertips at most. Holding on changes the workout: oxygen cost was 4 to 8 percent lower with light rail support on a step treadmill (Christman and colleagues, Research in Nursing and Health, 2000) and roughly 15 to 17 percent lower during a graded treadmill test (Guiraldelli and colleagues, Revista Contexto e Saude, 2023), though on a flat belt at 3.5 and 5.5 mph one small study of 12 young adults found no difference (Dalton, JEPonline, 2013). On an incline, leaning on the rails erases much of the incline’s point. The calorie display assumes no hand support, and machine displays run high anyway: on ellipticals, the one machine we found a published calorimetry comparison for, the readout overshot by about 100 kcal per 30 minutes (Glave and colleagues, Exercise Medicine, 2018). Treat the treadmill number the same way.

What “brisk” means on a belt. Comfortable walking speed in a meta-analysis of normal values was about 1.43 m/s for men and 1.31 for women in their 50s (3.2 and 2.9 mph), falling to about 1.26 and 1.13 m/s in their 70s (2.8 and 2.5 mph) (Bohannon and Williams Andrews, Physiotherapy, 2011). So brisk starts around 3.3 to 3.8 mph for most people over 50, and for many, a modest incline at 3 mph is a better lever than more speed.

A simple way to split the week

This is how we structure it for clients; it is coaching practice, not a research protocol.

• Easy runs: outside when the weather and light allow, for the enjoyment and the terrain. Treadmill when the heat reaches the black-flag band or after dark in winter.
• Intervals and hill repeats: either; the belt gives precision and incline without the descent. Expect it to feel harder at the same pace, and run by effort.
• Tempo runs: outside on the Greenway if you can, because the belt’s endurance penalty shows up most here. If indoors, hold heart rate, not pace.
• Long runs: mostly outside if you are racing, for terrain, pacing feel and fueling practice. Coaches who use treadmill long runs suggest changing incline, speed or focus every 400 meters and raising the incline to 2 or 3 percent for stretches of it (Runner’s World Australia; coaching practice, not research).
• Progression: the surface does not change the rule that matters most. In 874 novice runners, those who increased weekly distance by more than 30 percent had more distance-related injuries than those under 10 percent (hazard ratio 1.59, not quite significant) (Nielsen and colleagues, JOSPT, 2014). We found no evidence that treadmill versus outdoor running changes injury rates either way.

How we use both

Our trainers work in clients’ building gyms and in the parks, often in the same week, so this is not theoretical for us. A typical runner’s week with Type A puts the strength session and the structured intervals in the building gym, where the treadmill, the rack and the trainer are in one room, and the easy and long runs outside, where the trainer can run the Reservoir or the Greenway alongside the client and watch form on real ground. If you are over 50 and new to the belt, the first session is the safety routine above and a walk with a little incline, not a run. If you want that built into your week, get matched with a trainer who comes to your building.

Frequently asked questions

Is running on a treadmill as good as running outside?

For fitness, yes: oxygen cost at a matched speed is the same within about 1 mL/kg/min across 34 studies, and your stride is largely comparable. For racing, not entirely: you miss terrain, descents, air resistance and the pacing feel, and endurance performance on the belt is measurably worse even though oxygen cost matches.

Should I set the treadmill to 1 percent?

Only if you run faster than about 8:00 per mile and want the belt to approximate the road. At slower speeds the 1996 study found a flat belt matched just as well, and a 2024 study found 1 percent over-corrected for fast runners. Running by effort makes the question go away.

Why does the treadmill feel harder than running outside?

Two reasons with evidence: nothing moves past you, so runners asked to match their outdoor pace by feel set the belt about 30 percent too slow; and endurance performance is measurably worse on the belt even though oxygen cost matches. A third is common sense rather than measured here: no moving air, so you run warmer. Set the real pace, give it a few minutes and judge by heart rate.

Is the treadmill easier on my knees?

Less than advertised. Peak impact force and loading rate were not significantly different from overground when pooled across studies. The clear difference is in shin bone strain, which was far lower on the belt in the one direct measurement.

Is running in New York City air bad for me?

No, on ordinary days. At typical city pollution levels the exercise benefit outweighs the pollution harm until about 16 hours of walking a day, and the city’s fine-particle levels have dropped by more than a third since 2009. Stay in on wildfire-smoke days, and favor the park interior and the river over avenue sidewalks.

Can I train for a marathon on a treadmill?

You can do most of the volume there, and coaches do in bad-weather weeks. Do at least some long runs and the hill work outside: downhill running loads the legs in a way a building treadmill cannot reproduce, and you need to practice pacing and fueling on real ground.