Do I need 1% incline on the treadmill to match running outside?

Miller, Van Hooren and colleagues (University of South Australia, Maastricht, Montana, Macquarie) pooled crossover studies that compared running on a treadmill with running overground in healthy adults, for physiology (VO2, heart rate, lactate), perception (RPE, preferred speed) and performance (time trials and sprints).

Thirty-four studies qualified; 12 used a 1% grade and 3 used more than 1%. At submaximal speeds on a motorised treadmill, VO2 was similar to overground at 0% (MD −0.55 ± 0.93 mL/kg/min) and at 1% grade (0.37 ± 1.12), while lactate was lower (−1.26 ± 0.71 and −0.52 ± 0.50 mmol/L).

Heart rate and RPE on the treadmill were higher at faster submaximal speeds and lower at slower ones. Near maximal effort, VO2 and lactate tended to be lower on the treadmill while heart rate (0 ± 1 bpm) and RPE (−0.4 ± 2.0 units on the 6-20 scale) were similar; at maximum, VO2 (0.78 ± 1.55) and heart rate (−1 ± 2 bpm) were similar.

Endurance performance was poorer on a motorised treadmill (SMD −0.50 ± 0.36); sprint performance varied widely and did not differ significantly (−1.4 ± 5.8 km/h). The authors conclude that some, but not all, variables differ and that the differences may depend on running speed.

Our reading: for oxygen cost the 1% grade is a convention, not a necessity — at group level both 0% and 1% sat within about half a mL/kg/min of overground running, with confidence intervals spanning zero, which agrees with the base's single study that found 1% mattering only at faster speeds (REF-733, B, n = 9).

What does not transfer is the feel and the heart rate at a given pace, and the time-trial result.