Miller et al. 2019, Physiological, perceptual and performance differences between treadmill and overground running

Systematic review and meta-analysis of crossover studies (Sports Medicine, 49(5):763–782). Submaximal VO₂ is similar at 0 % and 1 % grade on a treadmill versus overground, but blood lactate runs lower on the treadmill; heart rate and RPE show speed-dependent differences (elevated at faster speeds, reduced at slower ones). Endurance performance is significantly poorer on a motorised treadmill than overground, though sprint performance is unaffected. Moderate.