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, though heart rate and perceived effort can run a little differently. 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 but grows steeply with speed, scaling with its cube, so the true correction is under 1% for easy running and more than 1% for sprinting (Pugh 1970). 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. Use it for what it does well, and keep enough outdoor running for race specificity.