Previous Treadmill Incline Guidelines May Be Wrong

I have written before that running on a treadmill indoors is easier than running outside and that research shows that setting your treadmill at 1 percent incline simulates outdoor running (Jones and Doust, 1996). This is especially so if you run at speeds of 12-17 km per hour. Under 12 km an hour, no need to set the incline up as the difference is small.

However a new study (Shahidi et al, 2026) found that the energy cost of running outside is still about 4 percent higher than running inside on a treadmill at 1 percent incline.

The researchers had 12 trained runners (21.3±‍2.4 years, 176±‍6.8 cm, 67.8±‍5.9 kg) first tested for their VO2 max. They subsequently ran 3 minutes at speeds between 8 to 15 km/hr both indoors on a treadmill and outdoors on a 400m track. Gas analysis was averaged over the final 60-90 seconds of each stage to determine steady-state VO2. 

The results (pictured above) show that running economy was significantly higher while running outside on a 400m track compared to running indoors on a treadmill. Especially as you run harder and closer to your limit (70-100 percent of the ventilatory threshold or VT2). Running outdoors requires much more energy than running on a treadmill.

At slower recovery paces, the physical effort between the 2 feels amost identical. Once your intensity hits the high effort zones, the treadmill becomes easier than the track.

Regardless  of how fast or slow the runners ran, running outside always used more total energy per kilometer than running on a treadmill.

The findings suggest that the commonly used 1 or 1.5 percent treadmill grade correction does not fully replicate the energetic demands of running outside. 

So, you will burn more total calories running 5 km outdoors than running 5 km on a treadmill if you go at the exact same pace. Your heart and lungs have to work slightly harder every single second you are outside to cover the same distance. Since the energy cost is higher across all speeds, an outdoor longer run will always leave you more physically spent than an identical treadmill long run.

Note that this is a small sample size study with only 12 national level trained subjects. Will the results apply to the rest of the population? Also they were made to run with the apparatus pictured above to measure exchange of gases. The runners may have to breathe harder outdoors.

In Singapore, the temperature and humidity affects us enormously when exercising outdoors, air conditioning while running indoors mitigates that.

One can run by heart rate and/ or time running on a treadmill and perhaps not worry about pace since the treadmills may not be properly calibrated as well.

May I also suggest running more outdoors if you are training for a road race/ marathon rather than solely indoors on a treadmill since the treadmill is softer and will not mimic the road conditions that you race on. Specificity is key. The gym environment is temperature controlled unlike outdoors. So if the race location is hotter and more humid then you will be less able to handle it.

References

Jones AM and Doust JU (1996). A 1% Treadmill Grade Most Accurately Reflects The Energetic Cost Of Outdoor Running. J Sports Sci. 14(4): 321-327.

Shahidi SH, Can R, Paca FM et al (2026). Overground Running Incurs A Higher Energetic Cost Than Treadmill Running At A 1% Grade: A Comparison Of Running Economy, Oxygen Cost Of Transport, And The Energy Cost In Endurance Athletes. PLoS One. 21(8): e0355988. DOI: 10.1371/journal.pone.0355988