Treadmill vs outdoor running: what actually differs

The advice to raise the treadmill slightly comes from a 1996 experiment. Nine trained runners ran six-minute bouts at six speeds: five times on a treadmill set between 0% and 3%, and once along a level road. Expired air was collected in the last two minutes of each bout to measure oxygen uptake. Two large reviews have since covered physiology and running form. Here is what they found, and where their conclusions stop.

Treadmill vs outdoor running: the energy cost

Outdoors, part of your energy goes into pushing air aside. On a belt you are not moving through the air in the room, so that cost disappears. Pugh (1970) measured the oxygen a runner used on a treadmill against a headwind: the extra cost rose with the square of wind speed. He estimated that at 21.5 km/h (13.4 mph, 2:47/km or 4:29/mi, 5000 m race pace) air resistance takes about 8% of total energy cost, and about 16% in a 10-second 100 m sprint.

At training speeds the share is much smaller. Resistance drops with speed, and on an easy run there is little of it left.

Which incline makes a treadmill match the road

That experiment by Jones and Doust is where the 1% incline rule comes from: between 10.5 and 18 km/h (6.5 and 11.2 mph), that grade brought treadmill oxygen cost into line with road running. Speed by speed, the picture varies.

Up to 12 km/h (7.5 mph), a flat belt already matched the road, so on easy runs you can leave the incline at zero. On tempo work it brings the treadmill closer to outdoor effort. The calculator linked above turns any belt speed and incline into the equivalent flat pace.

Running form is nearly the same, except at footstrike

Van Hooren and colleagues (2020) pooled 33 studies in which the same people, 494 in total, ran both on a treadmill and overground. Most measures did not differ. On the treadmill the foot lands flatter (the foot-to-ground angle at contact was 9.8° smaller on average), the centre of mass bobs up and down less (by 1.5 cm, about 0.6 in), and ground contact lasts slightly longer (by 5 ms).

The authors rated this evidence as low quality. For a runner the takeaway is plain: regular treadmill running does not wreck your form. A flat belt also has no corners, uneven ground or descents.

Heart rate and effort depend on speed

Miller and colleagues (2019) gathered 34 studies comparing oxygen uptake, lactate, heart rate and perceived effort. At moderate speeds treadmill oxygen uptake matched overground at both 0% and 1% grade, while lactate was lower. Heart rate and perceived effort were less tidy: higher on the treadmill at faster speeds, lower at slower ones.

So the pace you hold at a given heart rate may not carry over between belt and road. If you run easy zone 2 sessions, go by heart rate rather than an outdoor pace. The same review found one more thing: in endurance tests, performance on a motorised treadmill was poorer on average than overground.

Gym heat and console numbers

Indoors there is no oncoming airflow cooling you the way it does outside, so a fan in front of the treadmill helps. The console speed is computed by the machine, and nobody at the gym has to check it. You can: measure the belt’s full loop, stick a strip of tape on it and, standing to the side, count revolutions in one minute. Speed in km/h equals belt length in metres × revolutions per minute × 0.06. A 3 m belt turning 60 times a minute moves at 10.8 km/h (6.7 mph).

Without GPS, a watch estimates treadmill distance from arm swing or a foot pod, so its pace can disagree with the console. When they disagree, trust a belt you have checked by counting.

What this research does not tell you

The incline rule comes from nine trained men and bouts of about five minutes. The experiment did not test beginners, women or hour-long runs. The form review rests on low-quality evidence, and the physiology review states outright that the differences depend on the speed at which they are measured.

Treadmill or outdoor running: which is better for you

• Training for a road race. Do long runs and goal-pace sessions outdoors, with the air, corners and surface you will meet on race day. Work out goal pace from time and distance in the pace calculator.
• Ice, darkness or heat. A treadmill stands in for the road on energy cost if you raise the belt on faster efforts, as in the table above.
• Intervals at an exact speed. The belt holds the speed for you; outdoors you chase pace on your watch.
• Easy runs. The energy difference is close to nil, so pick whichever is convenient and watch your heart rate.