Why Is My Power Lower on the Trainer? Indoor vs. Outdoor FTP for Triathletes
In a 2022 study, 14 elite cyclists did the same set of maximal efforts on their own Tacx NEO trainers and on a steady road climb, with two sessions in each setting and the same crank power meter on the bike throughout. Indoors, their critical power averaged 19 watts lower. The average hides a wide spread. One rider was 152 watts lower indoors over a one-minute effort.
If your power is lower on the trainer, you have plenty of company, and the size of the gap varies a lot from rider to rider. This article covers what the research shows about indoor vs. outdoor FTP, the main reasons trainer numbers come in lower and what to do about each, and two reasonable ways to set your zones: one FTP for both settings, or a separate number for each.
Indoor vs. Outdoor FTP: What the Research Shows
The studies that compare indoor and outdoor power don't agree on one number, and the reasons they differ are useful.
Study | Riders | Protocol | Result |
Jeffries et al., IJSPP 2019 | 20 competitive male cyclists | Maximal 20-minute time trial; trainer with a fan vs. a flat outdoor circuit; same power meter | 280 W indoors vs. 284 W outdoors, not a significant difference |
Smith et al., IJSM 2001 | 8 trained cyclists | 40 km time trials, three indoors and three outdoors, SRM in both | 303 W vs. 312 W, not a significant difference |
Lipski et al., IJSPP 2022 | 14 elite male riders (UCI Continental and WorldTour) | Maximal efforts of 1 to 14 minutes on their own Tacx NEO vs. a 2–3% road climb; same crank meter | Indoor power 4.2–8.8% lower; critical power 330 W vs. 348 W; large individual differences |
Mieras et al., JSCR 2014 | 12 recreational men | 40 km at the same perceived effort indoors and out (not a maximal test) | About 30% more power outdoors at the same perceived effort |
Put together, the gap in maximal efforts runs from no significant difference to several percent, depending on the rider and the setup. The 30% figure from Mieras answers a different question: at the same perceived effort, riders chose to produce much more power outdoors. That matters for unstructured indoor rides done by feel, and it doesn't mean your FTP drops by 30% indoors. The Lipski authors concluded that for an individual rider, indoor and outdoor test results aren't interchangeable. No study supports a fixed conversion factor.
Athlete Takeaway: Treat your indoor-outdoor gap as something to measure for yourself, using the protocol at the end of this article.
Five Reasons Your Power Is Lower on the Trainer, and What to Do About Each
1. Heat and airflow
Outdoors, riding at 20 mph creates its own cooling breeze. Indoors, without enough airflow, heat builds and your body sends more blood to the skin, and the result is less power for the same effort. A 2023 study by Bright and colleagues in Medicine & Science in Sports & Exercise had 12 cyclists complete self-paced time trials in a 90°F (32°C) room. In still air they averaged 232 W. With airflow of 16 km/h (about 10 mph) or more, they averaged 247–250 W, and more airflow added nothing beyond that point.
Heart rate tells the same story. In a 2023 study from Georgia Southern University, 16 recreationally active adults rode for 40 minutes at a workload adjusted to hold heart rate at 70% of heart rate reserve, once with a fan and once without. With the fan, they sustained a 15% higher workload at the same heart rate. At a fixed workload, the effect shows up the other way around: in a 2008 study, heart rate drifted up 11% over 30 minutes of steady riding at 95°F (35°C), compared with 2% at 72°F (22°C).
Those studies used lab conditions, and a home fan's airflow at the rider is hard to know without measuring it. The practical fix is the same either way: move a lot of air across your body, keep the room as cool as you can, and drink on a schedule indoors just as you would outdoors.
2. Position
Triathletes often train indoors in the aero position and test outdoors on the hoods, or the reverse. Lower torso angles cost some power, though less than many athletes expect. A 2021 study by Faulkner and Jobling had 11 trained time-trial and triathlon riders ride in their own position and at several hip angles. At the most aggressive hip angle, power at a fixed blood lactate averaged about 16 W lower than in the riders' own position, a moderate effect, while the other angles stayed within about 3 W. Time-trial performance didn't differ significantly across positions. Test in the position you plan to train in, and compare aero numbers with aero numbers.
3. Two different power meters
If your trainer measures your indoor power and your pedals or crank measure your outdoor power, part of the gap may be the devices. Validation studies of trainers and power meters usually find errors in the range of 1–3%, and they vary by unit and cadence. In one study, five units of the same trainer model had a mean error of −1.5%, and two of the five differed significantly from the calibration rig. Another found a pedal-based meter reading 2–10 W below an SRM, depending on cadence. The fix is to use one power source for both settings, either by pairing your bike's power meter to control the trainer or by recording both and comparing.
4. How the trainer rides
A bike locked into a trainer doesn't sway side to side, and the flywheel's momentum differs from the momentum of a bike and rider on the road. Both change how the pedal stroke feels and which cadence you settle into. The Lipski group listed crank inertia and reduced bike movement among the possible reasons their riders produced less power indoors. One 2012 study of nine riders measured about 10% lower cycling efficiency on an older stationary ergometer than on the road, though that finding comes from older equipment and doesn't transfer directly to modern direct-drive trainers.
ERG mode adds one more factor. It holds your target power regardless of cadence, so as you fatigue and your cadence drops, the trainer raises the resistance to keep the watts constant, and each pedal stroke gets heavier. It also takes pacing decisions away from you. We haven't found published research comparing ERG mode with resistance or slope mode, so treat this as a description of how the mode works rather than a measured effect. Many athletes find a mix of ERG sessions and free-ride sessions in resistance or slope mode keeps their indoor riding closer to what they do outdoors.
5. Motivation and terrain
Outdoor riding is variable. In the 2019 Jeffries study, power fluctuated about twice as much outdoors as indoors during the same 20-minute effort, likely because terrain and wind keep changing the load. Indoors, a steady load can feel harder to sustain. Scenery on a screen doesn't seem to change power output much: two studies of virtual courses in untrained and recreational riders (2019 and 2026) found no difference in power compared with riding without them, though people enjoyed the rides more. Competition does change it. In a 2012 study, riders who believed they were racing an on-screen opponent finished a 2,000-meter effort about 2% faster. Test-to-test variability also matters. A 2022 study of 20-minute time trials on Zwift found a typical variation of about 3.7% between attempts, so a gap of a few percent between two single tests may simply be noise.
Athlete Takeaway: Before deciding your indoor power is lower, set up strong airflow, use one power meter for both settings, and match your test position. Then look at the gap again.
One FTP or Two? Two Reasonable Approaches to Indoor vs. Outdoor FTP
Coaches are split on this question, and the research doesn't settle it. TrainerRoad and CTS generally recommend one FTP with a well-set-up indoor environment. Hunter Allen, writing for TrainingPeaks, recommends separate numbers when they differ by 10 watts or more. Both approaches work. The right one depends on how and where you train.
When one FTP makes sense
A single FTP tends to work well for riders who train indoors regularly, have good cooling, use the same power meter in both settings and train in the same position. If your indoor and outdoor numbers land within a few percent of each other, that difference sits inside normal test-to-test variability, and one set of zones is simpler to live with. You can still adjust in the moment: if an indoor interval set feels much harder than its outdoor equivalent, heart rate will tell you whether to back off.
When two FTPs make sense
Separate numbers tend to work better for riders whose indoor power stays lower after cooling, equipment and position are sorted. They also suit athletes who do most of their winter work indoors and race outdoors, and those who ride indoors in aero but outdoors in a more upright position. For these riders, setting indoor zones from an outdoor FTP can make every indoor session harder than intended, which shows up as missed intervals and extra fatigue across a winter block.
Athlete Takeaway: Once cooling, power meter and position are matched, if your indoor and outdoor tests land within about 4% of each other, use one FTP. If the indoor number is lower than that across two matched comparisons, set separate indoor zones.
How Heart Rate Helps Either Approach
Power tells you the work you're doing. Heart rate tells you what that work is costing on a given day, and indoors that cost depends heavily on cooling, hydration and fatigue.
For the same watts, you may see a higher heart rate indoors, especially in longer sessions and when airflow is limited. In maximal tests, average heart rate has come out similar indoors and outdoors (Jeffries and Lipski both found this). What changes is the heart rate you see at a given wattage.
If you use one FTP, compare your heart rate at a familiar wattage indoors with your heart rate at that wattage outdoors. If it's running well above your outdoor number, check the fan, the room temperature and your fluid and carbohydrate intake before you decide anything about your fitness.
If you use two FTPs, look at your heart rate at threshold in each setting. If your indoor FTP is lower but your heart rate at that indoor FTP matches your outdoor threshold heart rate, the lower number reflects how much power you can express indoors, and it's the right base for your indoor zones. This is a coaching rule of thumb built on the studies above. It hasn't been validated as a testing method of its own.
Our guide to heart rate training zones covers how to set the zones themselves, and how accurate your watch's VO2max and threshold estimates are covers the error bars on the numbers your devices report.
Athlete Takeaway: Ride your power targets indoors, watch heart rate at those targets, and treat a big jump in heart rate as a signal to fix cooling and fueling first.
How to Measure Your Own Indoor-Outdoor Gap
A simple comparison will tell you more than any average from a study. Over one or two weeks with similar fatigue, do the same test or the same steady threshold session once indoors and once outdoors:
Use the same power source for both.
Ride in the same position for both.
Set up strong airflow and a cool room for the indoor session.
Pick a flat or steady outdoor route where you can hold a constant effort.
Record heart rate for both, and compare your power and your heart rate at a given power.
If the gap is more than about 4%, repeat the comparison once before you set separate indoor zones.
For more on reading these numbers without overanalyzing them, see our post on analyzing your training data. If you're spending most of the winter indoors, our post on how much indoor training is too much covers the balance, and our IRONMAN bike pacing guide explains how your outdoor FTP sets race-day targets.
Athlete Takeaway: Run one matched indoor and outdoor comparison early in your winter block, and let the result decide whether you train from one FTP or two.
The Bottom Line
Lower power on the trainer is common, and the size of the gap ranges from no significant difference to several percent depending on the rider and the setup. Cooling, position, equipment, the way the trainer rides and motivation all play a part, and most of them can be improved. Once they are, some riders do well with one FTP and others need a separate indoor number. Either way, let power lead your sessions and use heart rate to check what each one is costing you.
If you'd like help building a winter training plan that uses your indoor time well, set up a call with us now, and we will help you be ready for your best race day!
