Why Your Indoor Watts Are Lower: Heat, Fans and the Smart Trainer

It's a familiar winter frustration. You set your trainer to the power you hold comfortably on a summer road ride, and within twenty minutes your heart rate is climbing, sweat is pooling on the floor and the effort feels much harder than it should. Some triathletes conclude that they've lost fitness, others that their smart trainer reads high. Usually neither is the main problem.

This article covers what studies comparing indoor and outdoor cycling actually show, why heat and still air are the most likely causes of the gap, how accurate smart trainers are, and how to set up indoor sessions so your numbers mean something. It stays on trainer-specific causes. For heat on race day, see TriForward's "The Watts the Heat Takes" and "Humid, Not Just Hot".

The indoor gap: what riders report

Many triathletes say their indoor threshold is lower than their outdoor threshold, but the size of the gap people report is anecdotal and varies widely. The research gives a more nuanced picture. Indoor and outdoor power can match closely under good conditions, and can differ a lot under poor ones.

Three things change when you move from road to trainer:

• Cooling. Moving at 30 km/h creates a strong headwind that evaporates sweat. A trainer doesn't move.
• How you ride. Outdoors, power rises and falls with hills, wind and corners. Indoors it's usually steady, or controlled by the trainer in ERG mode.
• The measuring device. Your outdoor power meter and your trainer's built-in power may not agree.

The studies below look at each of these.

Studies comparing indoor and outdoor power

The same effort, very different watts

The study that most triathletes would recognise is a 2014 trial by Mieras, Heesch and Slivka in the Journal of Strength and Conditioning Research. Twelve recreationally trained male cyclists rode 40 km twice, once in the laboratory and once outdoors, in random order. They were asked to give the same perceived effort both times, rather than to ride as fast as possible.

• Power output was much higher outdoors: 208.1 W against 163.4 W in the lab.
• Heart rate was higher outdoors too: 152 against 143 beats per minute.
• Core temperature was similar, but skin temperature was cooler outdoors, creating a larger gap between core and skin.
• Wind speed was higher outdoors.
• Perceived exertion, body-weight change and attentional focus didn't differ.

Inference: indoor power was about 21% lower than outdoor power at the same perceived effort. The authors concluded that outdoor cycling lets riders work at a higher intensity, and suggested that riders may need to ride at a higher perceived effort indoors to match outdoor training. Note that the abstract doesn't say whether the same power meter was used in both settings, and these were recreational men riding by feel, not racing.

Time trials can match

When riders are asked to go as hard as they can, the gap can disappear. In 2001, Smith, Davison, Balmer and Bird published a study in the International Journal of Sports Medicine. Eight trained male cyclists completed three indoor 40 km time trials on an air-braked ergometer and three outdoor 40 km time trials on a local course, with power measured by an SRM power meter.

• Mean power didn't differ significantly: 303 W indoors and 312 W outdoors.
• Completion time was faster indoors, but that reflects the ergometer's calculated "speed", not real-world speed, so it doesn't mean much.
• Power was reproducible in both settings, with a coefficient of variation of about 2% between trials.

Inference: the indoor figure was about 3% lower than the outdoor one, but with eight riders the difference wasn't statistically significant, so it could be chance.

A larger 2019 study by Jeffries, Waldron, Patterson and Galna in the International Journal of Sports Physiology and Performance tested 20 male competitive cyclists in two 20-minute time trials, one on an outdoor racing circuit and one on an electromagnetically braked laboratory ergometer, a week apart. Both were ridden on the same bike with the same portable power meter.

• Mean power was almost the same: 280 W indoors and 284 W outdoors, not significantly different.
• The two were strongly correlated, so riders who were strong outdoors were strong indoors.
• Power was far more variable outdoors. The within-person variation was 69 W outdoors against 33 W indoors.

That last point matters for interval sessions. Outdoors, you constantly surge and ease. Indoors, especially in ERG mode, you hold a steady pressure with no freewheeling. The same average power can feel different when you never get a moment's relief.

Why the studies disagree

Put together, these studies suggest that a well-motivated, maximal effort in a reasonably cool lab can match outdoor power. The big gap appears in less controlled conditions, as with Mieras. None of the three studies was set in a typical home pain cave, and none included women. The most likely reason for the difference between them is cooling, which the next section covers.

Airflow: the missing headwind

A lab test of fan speed

The clearest evidence comes from a 2023 study by Bright, Clark, Jay and Périard in Medicine and Science in Sports and Exercise. Twelve male cyclists completed a self-paced time trial in which they had to complete 700 kJ of work as fast as possible, at 32 °C and 40% relative humidity. They did it four times: in still air, and with air blown at them at 16, 30 and 44 km/h.

• Mean power was lower in still air: 232 W, against 247 W at 16 km/h, 250 W at 30 km/h and 248 W at 44 km/h.
• The three airflow conditions didn't differ. Going from 16 to 44 km/h added nothing measurable to performance.
• Peak core temperature was highest in still air, at 39.4 °C, against 39.0 °C at 16 km/h and 38.8 °C at both higher speeds.
• The air's capacity to evaporate sweat roughly halved in still air, at 160 against 298 to 324 W per square metre with airflow.
• Perceived exertion was higher in still air than at the fastest airflow, even though riders were producing less power.

Inference: still air cost these riders about 6 to 7% of their power compared with any of the airflow conditions. The study was run in hot conditions, warmer than most home training rooms in winter, but a small room with a running rider and no window open can warm up quickly.

Cooling, comfort and pacing

A 2013 study by Teunissen, de Haan, de Koning and Daanen in the European Journal of Applied Physiology looked at how wind affects how riders feel and pace. Ten male participants completed 15 km self-paced time trials in warm, humid conditions. In one trial, wind was switched on unexpectedly between kilometres 3 and 12.

• Skin temperature fell by about 1.5 °C during the wind.
• Perceived exertion, thermal sensation and thermal comfort all improved.
• Power output rose, averaging 256 W against 246 W without wind.
• They finished 67 seconds faster on average.

Riders didn't need to be told to push harder. When they felt cooler, they chose a higher power.

Heat storage over two hours

An older study by Saunders, Dugas, Tucker, Lambert and Noakes, published in Acta Physiologica Scandinavica in 2005, looked at longer, steady rides. Nine cyclists (sex not stated in the abstract) rode for two hours on five occasions at 33 °C and 59% relative humidity. Air velocity ranged from virtually still (0.2 km/h) to about 10, 33 and 50 km/h.

• In still air, heat storage, body temperature and perceived exertion were higher than in all the other conditions.
• There was no difference between 33 and 50 km/h.
• Drinking more didn't change heat storage or body temperature once airflow matched road speed.

The authors argued that lab findings in windless conditions can't be extrapolated to outdoor riding, where air velocity is rarely lower than the rider's speed. The reverse also applies: outdoor numbers can't be expected indoors without replacing that airflow.

Is your smart trainer reading low?

Measurement error is the second explanation people reach for. Most smart trainers estimate power from the resistance unit rather than measuring it at the crank or pedal.

A 2016 study by Hoon and colleagues in the Journal of Science and Cycling tested five Wahoo KICKR trainers and one SRM power meter on a motor-driven calibration rig, from 50 to 400 W.

• The KICKRs averaged -1.5% error, ranging from -3.1% to 0%, against -1.6% for the SRM.
• Reproducibility was poorer: typical error was 1.5% for the KICKRs against 0.7% for the SRM, and two units changed significantly between tests.

This was one trainer model, tested on a rig rather than by a rider, ten years ago. Newer trainers and other brands weren't tested. But it suggests trainer error is generally a few per cent, smaller than the gaps heat and still air can create. Inference: if your outdoor power meter and your trainer disagree by a few per cent, compare indoor numbers only with other indoor numbers, or use the same pedal or crank power meter in both places.

Other reasons indoor riding feels harder

The evidence for these is thinner, but they're worth knowing:

• No coasting. Jeffries found outdoor power was about twice as variable. Indoors, especially in ERG mode, there's no freewheeling down a hill, and the steady pressure can feel harder over time.
• A fixed bike. On a trainer the bike doesn't sway, which changes how you move. Some riders find this harder, particularly when standing.
• Motivation and boredom. In Mieras's study perceived effort and attentional focus didn't differ, so this may matter less than people think.
• Position. Riding in an aero position for an hour on a trainer can feel harder than outdoors, because you rarely sit up.

How to set up indoor sessions

The following combines the studies above. It's inference for triathletes, not a tested protocol:

1. Use a powerful fan aimed at your face and torso. In the lab, 16 km/h of airflow restored performance as well as faster airflow. A strong floor fan or two is more likely to get close to that than a small desk fan.
2. Keep the room as cool as you comfortably can. Open a window, or train in the coolest room available.
3. Compare like with like. Set indoor zones from an indoor test, and outdoor zones from an outdoor test, if the two differ.
4. Expect the first indoor sessions to feel harder. Use heart rate and perceived effort alongside power for the first few weeks.
5. Use one power source if you can. If your bike has a power meter, using it on the trainer makes indoor and outdoor numbers directly comparable.
6. Be careful about adding effort to match outdoor power. Mieras suggested riding at a higher perceived effort indoors, but if heat is the cause, that may simply mean more heat strain.

A triathlon scenario

This example is illustrative, not drawn from a study. A 38-year-old triathlete sets her winter zones from an outdoor FTP of 230 W. On the trainer in a small spare room with the door closed, she struggles to hold 210 W for her threshold intervals, and her heart rate is 10 beats higher than outdoors at the same power.

A better approach, based on the evidence above:

• Week 1: add a large fan aimed at her upper body, open the window, and repeat the session. Her heart rate falls and she holds more power.
• Week 2: run an indoor ramp or 20-minute test with the fan on, and set her indoor zones from it.
• Through the winter: compare indoor sessions only with indoor sessions. When she goes back outside in spring, use an outdoor test to reset her outdoor zones.

If her indoor numbers are still lower, that isn't a sign of lost fitness. It's a difference between two settings.

Common mistakes

• Using outdoor zones indoors without checking. If indoor power is lower, every interval is effectively harder than planned.
• Training without a fan, or with a small fan pointed at your legs.
• Assuming the trainer is wrong. Check calibration, but heat is the more likely cause.
• Pushing harder to make up the difference in a hot room.
• Comparing two different power meters and concluding you've lost fitness.
• Testing outdoors in summer and indoors in winter and reading the difference as a fitness change. See TriForward's "Racing by Feel vs. Racing by Numbers" for why the number is only as useful as its context.

How this connects to winter training

Indoor sessions are a big part of most triathletes' winters. TriForward's "Can the Turbo Replace the Long Ride? Indoor Volume and Winter Durability" looks at whether shorter indoor rides can stand in for long outdoor ones, and "Do You Need Carbs on a 60-Minute Turbo Session?" covers fuelling. Indoor sessions in a warm room can also work as a mild heat stimulus, though TriForward's "Heat Acclimation for Race Day" explains why planned heat sessions are better than accidental ones.

What we don't know

• Women. Every study here included only men, or didn't report sex.
• Home setups. No study has tested typical home fans, rooms and smart trainers against outdoor riding.
• Modern smart trainers. Accuracy data for current models from different brands are limited.
• Why the studies disagree. Whether the gap in Mieras's study came mainly from heat, pacing by feel or measurement isn't clear.
• Long sessions. Most comparisons used efforts of 20 minutes to about an hour.

References

• Mieras ME, Heesch MWS, Slivka DR. Physiological and psychological responses to outdoor vs. laboratory cycling. Journal of Strength and Conditioning Research, 2014;28(8):2324–2329. doi:10.1519/JSC.0000000000000384
• Smith MF, Davison RCR, Balmer J, Bird SR. Reliability of mean power recorded during indoor and outdoor self-paced 40 km cycling time-trials. International Journal of Sports Medicine, 2001;22(4):270–274. doi:10.1055/s-2001-13813
• Jeffries O, Waldron M, Patterson SD, Galna B. An analysis of variability in power output during indoor and outdoor cycling time trials. International Journal of Sports Physiology and Performance, 2019;14(9):1273–1279. doi:10.1123/ijspp.2018-0539
• Bright FM, Clark B, Jay O, Périard JD. Influence of air velocity on self-paced exercise performance in hot conditions. Medicine and Science in Sports and Exercise, 2023;55(8):1382–1391. doi:10.1249/MSS.0000000000003168
• Teunissen LPJ, de Haan A, de Koning JJ, Daanen HAM. Effects of wind application on thermal perception and self-paced performance. European Journal of Applied Physiology, 2013;113(7):1705–1717. doi:10.1007/s00421-013-2596-9
• Saunders AG, Dugas JP, Tucker R, Lambert MI, Noakes TD. The effects of different air velocities on heat storage and body temperature in humans cycling in a hot, humid environment. Acta Physiologica Scandinavica, 2005;183(3):241–255. doi:10.1111/j.1365-201X.2004.01400.x
• Hoon MW, Michael SW, Patton RL, Chapman PG, Areta JL. A comparison of the accuracy and reliability of the Wahoo KICKR and SRM power meter. Journal of Science and Cycling, 2016;5(3):11–15. Full text