Treadmill running
Evidence: moderate
A treadmill is a valid substitute for outdoor running, with broadly similar mechanics and oxygen cost. The famous “set it to 1%” rule is a rough, speed-dependent correction for the missing air resistance, not a universal law: it matters at tempo pace and faster and is needless for easy running. The main real downsides are heat build-up and race-specificity.
Treadmill and overground running are biomechanically much alike, with similar stride timing, length and peak vertical loading and only minor differences in footstrike kinematics and propulsion (Van Hooren et al. 2020). Submaximal oxygen cost is broadly preserved — VO₂ is similar to overground at matched speeds — though blood lactate runs lower and heart rate and perceived effort show speed-dependent differences, elevated at faster speeds and reduced at slower ones (Miller et al. 2019). Endurance performance is meaningfully poorer on a motorised treadmill than overground, even when the physiological measures look alike, so use it for fitness building but not as a race-effort substitute (Miller et al. 2019). As a way to train fitness, the treadmill is a genuine stand-in.
The 1% rule, honestly
Indoors there is no air to push through, so at the same speed a treadmill is slightly easier than running outdoors. The well-known fix, a 1% gradient, comes from a single study of nine trained runners and made treadmill cost match outdoor cost across roughly 2.92 to 5.0 m/s (about 3:48 to 5:43 per km); at slower paces no incline was needed (Jones & Doust 1996). The rule is therefore speed-dependent, not absolute. Air resistance is a small share of the energy cost at distance pace — around 8% at 5 km pace — but grows steeply with speed because the cost scales with its cube, reaching roughly 16% at sprint pace (Pugh 1970), which means the gradient correction needed to match outdoor cost is likewise speed-dependent, increasing from negligible at easy pace to well above 1% at sprint speed. Treat 1% as a sensible default for tempo-and-faster sessions and ignore it for easy runs.
Uses and limits
The treadmill earns its place for control: precise pacing, programmed incline work, safety in the dark or on ice, and steady conditions for a key session. Two limits are worth knowing. First, standing still relative to the air means far less convective cooling, so heat builds faster than outdoors at the same effort; a fan and ventilation matter, and the heat link is the thermoregulation one. Second, the setting can shift submaximal economy: high-level runners were measurably more economical on a track than on a treadmill at the same speed, even though their VO₂max was unchanged (Mooses et al. 2015), and the belt and fixed environment make it less specific to an outdoor race. Individual gait is a further caveat: a wearable study found a single lab or treadmill session represented a given runner’s real-world gait only loosely, so treadmill-based form analysis of one runner should be read with caution (Davis et al. 2024). Use it for what it does well, and keep enough outdoor running for race specificity.