Beyond the 1% treadmill rule: 30 years of evidence on treadmill and overground running economy.

The energy cost of running (Erun) has traditionally been assessed in the laboratory as the mass-specific rate of oxygen uptake during treadmill running. Since 1996, a 1% treadmill incline has been used to best approximate overground Erun. Building upon previous reviews questioning the universal applicability of this correction, we re-examine from biomechanical, physiological, and technological perspectives whether a fixed 1% treadmill gradient is appropriate, and under what circumstances. Evidence accumulated over the past three decades suggests that the energetic equivalence between treadmill and overground running is influenced by running speed, environmental conditions, methodological factors, and inter-individual variability. Consequently, a fixed 1% incline may not consistently reflect overground Erun across different contexts or populations. We therefore advocate for more individualized and context-specific approaches to Erun assessment when extrapolation from laboratory to outdoor running is required.