A 1% treadmill grade reasonably matches the energetic cost of outdoor running at moderate-to-fast paces, but treadmill and overground running aren't biomechanically or perceptually identical even then
Interpretation
Jones and Doust's original study found a 1% grade best matched outdoor energy cost across a 2.92-5.0 m/s speed range, though the two slowest speeds tested showed no real difference between 0% and 1% grade either way. Two later systematic reviews (Van Hooren et al., 33 studies; Miller et al., 34 studies) confirm the broad comparability but find real remaining differences: a flatter foot-strike angle and marginally longer ground contact time on the treadmill, lower blood lactate at matched grades, and consistently worse endurance performance on the treadmill (SMD -0.50) — both reviews rate overall evidence quality as low to very low.
Practical implication
The 1% rule is a reasonable default for matching energy cost on the treadmill, especially at moderate-to-fast paces, but don't treat treadmill and outdoor sessions as perfectly interchangeable for pacing, perceived effort, or direct performance comparison — the differences are measured and real, just mostly small.
Sources
3 sources, independently verified.
Across a 2.92-5.0 m/s speed range, a 1% treadmill grade best matched the measured energetic cost of outdoor road running over roughly 5-minute bouts — the original source of the now-common '1% rule'. At the two slowest speeds tested, road running wasn't measurably different from either 0% or 1% grade, so the rule holds most cleanly at moderate-to-faster paces.
Most biomechanical outcomes didn't differ meaningfully between treadmill and overground running, though the treadmill produced a flatter foot-ground angle at footstrike (mean difference -9.8°), slightly lower propulsive force, and marginally longer contact time (+5ms). Overall GRADE-rated evidence quality was low to very low; the review's own characterisation is 'largely comparable,' not identical.
Submaximal VO2 was similar between treadmill and overground running, but blood lactate was lower on the treadmill at both 0% (-1.26 mmol/L) and 1% grade (-0.52 mmol/L), and endurance performance was consistently worse on the treadmill (SMD -0.50) — with the size of each difference depending on speed. Evidence that the 1% grade equalises energy cost but not every physiological or performance measure.