Indoor cycling power can match outdoor power, but still air and heat often pull it down; airflow restores most of the gap
Interpretation
Direct indoor-outdoor comparisons disagree: 8 trained men (Smith 2001) and 20 competitive men (Jeffries 2019) produced similar time-trial power indoors and outdoors (303 vs 312 W; 280 vs 284 W), while 12 recreational men riding by feel produced 163.4 vs 208.1 W (Mieras 2014). The airflow mechanism is consistent across lab studies: still air cut 700 kJ time-trial power from 247-250 W to 232 W in 12 men at 32 °C, with no further gain above 16 km/h (Bright 2023); unexpected wind raised 15 km power from 246 to 256 W in 10 men (Teunissen 2013); still air raised heat storage and body temperature over 2 h (Saunders 2005). Trainer error was about -1.5% in one 2016 rig test (Hoon). Gaps: all small, male or sex-unreported samples; no study of home setups or women.
Practical implication
Use a strong fan aimed at the upper body, keep the room cool, set indoor zones from an indoor test and compare indoor numbers with indoor numbers. Treat a lower indoor number as a setting difference, not lost fitness. Based on small male lab studies, not tested in home setups or in women.
Sources
7 sources, independently verified.
Two 40 km rides at the same perceived effort, one in the laboratory and one outdoors. Power was 208.1 W outdoors vs 163.4 W in the lab and heart rate 152 vs 143 bpm; core temperature similar, skin cooler and wind speed higher outdoors; RPE, body-weight change and attentional focus did not differ. Small, all-male, ridden by feel rather than maximally; abstract doesn't state whether the same power meter was used.
Three indoor 40 km time trials on an air-braked ergometer and three outdoor 40 km time trials, power by SRM. Mean power 303 W indoors vs 312 W outdoors, not significantly different (P = 0.34); coefficient of variation about 2% between trials in both settings. Very small, all-male sample.
Randomised 20-minute time trials outdoors on a racing circuit and indoors on an electromagnetically braked ergometer, same bike and power meter. Mean power 280 W indoors vs 284 W outdoors, not different (P = .256), r = .94; within-person power variability far higher outdoors (SD 69 vs 33 W). All-male.
700 kJ self-paced time trial at 32 °C, 40% RH in still air or with 16, 30 or 44 km/h airflow. Mean power 232 W in still air vs 247, 250 and 248 W with airflow; airflow conditions did not differ. Evaporative capacity 160 vs 298-324 W/m²; peak core temperature 39.4 vs 39.0, 38.8 and 38.8 °C; RPE higher in still air. Hot lab conditions, all-male.
15 km self-paced cycling time trials in warm or hot conditions. With unexpected wind during km 3-12, skin temperature fell by 1.47 °C, RPE, thermal sensation and comfort improved, power averaged 256 vs 246 W and riders finished 67 s faster than without wind. Small, all-male.
Two hours of cycling at 33 °C and 59% RH with air velocity of 0.2, about 10, 33 or 50 km/h. In still air, heat storage, body temperature and RPE were higher than in all other conditions; no difference between 33 and 50 km/h; higher fluid intake did not change heat storage or body temperature once airflow matched road speed.
Rig-driven incremental test from 50 to 400 W. KICKR mean error -1.5% (range -3.1 to 0.0%) vs -1.6% for the SRM; typical error 1.5% vs 0.7%; two KICKR units changed significantly between tests. One trainer model, 2016. Cited by repository URL because the listed DOI did not resolve.