Why Is My Power Lower Indoors Than Outdoors?

James. Hello and welcome to the WattPlan Podcast. I'm James. Somewhere between the school run, the day job, the odd podium and building WattPlan, I read a frankly silly amount of sports science. This week's question turns up every November. You do your usual session on the turbo, it feels awful, and the numbers are twenty watts down on the road. So why is my power lower indoors? Lyra, our AI research voice, has been through the studies, and they're all linked in the show notes.

Lyra. Mostly heat. Outdoors you ride through a constant breeze that strips heat off your skin, even at a gentle pace. Indoors no air moves unless you move it, so more of your effort goes into cooling and less into the pedals. There are two smaller reasons behind that, how still the bike is and the fact you never stop pedalling, but heat is the big one.

James. Heat, though? It's November. My garage is freezing.

Lyra. For about ten minutes, yes. Then your skin warms up, blood gets sent out to the skin to shed heat, you lose plasma as you sweat, and your heart rate climbs to move the same oxygen. Eventually your body declines to keep producing the power you're asking for. A cold room buys you a good start. It doesn't replace moving air.

James. Go on then, how big a gap are we talking?

Lyra. It depends hugely on the conditions, and the two studies we looked at sit at opposite ends. The first is a 2014 paper in the Journal of Strength and Conditioning Research. Twelve recreational male riders did a long trial in the lab and another outdoors, told to ride both at the same perceived effort. Outdoors they averaged 208 watts. In the lab, 163. [1]

James. Wait, so they just didn't try as hard inside?

Lyra. That's the interesting bit, they rated the two rides just as hard. Same effort in their heads, about a fifth less power at the pedals. And the clearest difference between the two settings was the air. Outdoors there was a steady breeze. In the lab there was none, and their skin ran warmer. The authors even suggested riders might need to push a harder perceived effort indoors to get the same training.

James. Okay, that's grim. A fifth.

Lyra. It's the worst case, though. The second study is from 2019, in the International Journal of Sports Physiology and Performance. Twenty competitive men rode a flat-out 20 minute time trial outdoors on a racing circuit and again indoors, on the same bike with the same power meter. Indoors averaged 280 watts, outdoors 284. No real difference. What changed was how jumpy the power was. Outdoors it swung around far more. [2]

James. So one study says a fifth and the other says nothing at all. Which do I believe?

Lyra. Both, for their conditions. One was riders pacing by feel in a still lab. The other was experienced racers doing a maximal test with matched kit. Put them together and the honest answer is that conditions decide your gap, heat most of all. If you sweat heavily or train in a warm room, expect more of it.

James. You said there were smaller reasons too.

Lyra. Two. Outdoors you shift about, stand on climbs and rock the bike, so you keep using slightly different muscle. On a fixed trainer you load the same fibres in one position for the whole session, which may tire them sooner. And an indoor hour is a full hour of pedalling. No junctions, no descents, no freewheeling, so it's denser for the same average.

James. Right, but surely the simplest explanation is that my trainer's just reading low?

Lyra. Everyone's first suspect, and usually innocent. A trainer that reads a touch low is still fine for training, because every session is measured against the same reference week after week. The real trap is switching devices. Test on a pedal power meter and train on the trainer's own reading, and a few percent between them is normal. If you've got a meter, connect it in the player, and its numbers are the ones WattPlan records instead of the trainer's.

James. Okay. So do I keep two FTPs then? One for inside, one for out?

Lyra. Some riders with years of data do, and it's more precise. But WattPlan works from one FTP and scales every workout from it. So if you're moving indoors for the winter, re-test in the room you'll train in. Your zones then fit the sessions you do most, and your outdoor rides feel slightly easy, which is a much better way to be wrong.

James. Hmm. I'm not sure I love setting a lower number on purpose. Doesn't that just mean I train softer?

Lyra. I'd push back on that. Think about the opposite. You test outdoors in September, train indoors all winter against that number, fail interval after interval, and decide you're going backwards. You're not, your zones are set for a different room. And the app won't catch it for you. From ordinary rides it only ever suggests raising your FTP, never lowering it, so a lower indoor number has to come from a test.

James. Ah, so the number can't drift down on its own. You have to go and get it.

Lyra. Exactly. And it isn't lost fitness either. Fitness doesn't drop between a Saturday outdoors and a Tuesday in the garage.

James. What about someone who's new to this and hasn't got a power meter at all? Does any of it matter?

Lyra. The heat matters just as much, you just meet it as feel rather than watts. If you're new to structured training, ride by feel and expect the same effort to arrive sooner indoors. That's normal, not a sign you're unfit. The fixes are the same for everyone. The biggest fan whose noise you can stand, aimed at your chest and face, the room as cool as you can bear, and more to drink than feels necessary.

James. And the racers? Because their big days are all outdoors.

Lyra. If you race, the second study is the reassuring one. Competitive riders, the same power meter, and a hard 20 minutes indoors matched outdoors. The difference was all in how much the power jumped about on the road, and the authors said indoor testing should be designed with outdoor demands in mind. So the indoor number is a fair number. It just doesn't rehearse the surges.

James. And for those of us who train with power and hate testing, can't I just knock five percent off and call it done?

Lyra. I wouldn't. Five percent is a guess, and between them those two studies put the gap anywhere from almost nothing to about a fifth, depending on the room. A test on your own setup, in your own room, is how you find your number rather than someone else's. Once it's set, compare indoor rides with indoor rides, not with a sunny Sunday.

James. Where does the evidence run out on this?

Lyra. Fairly quickly. Both studies were small groups of men, twelve and twenty. Neither tested a fan, so nobody can tell you how much of your gap a good one closes. And a lab isn't your spare room. What holds up is that moving air was the obvious difference in the first study, and with matched kit and a maximal effort the second found almost none.

James. Right, give me something to actually do this week.

Lyra. Three things. One, sort the fan before anything else. Two, if you're moving indoors, ride the 20-Minute FTP Test or the Ramp Test from the workout library in that room, and accept the new FTP when the player offers it, even if it's lower. Three, if you train with power from a pedal or crank meter, connect it as Power in the player so the numbers match the ones you tested on.

James. Fan first, test in the garage, same meter. And stop comparing my Tuesday nights with that August tailwind.

Lyra. Yes. Your watts haven't gone anywhere. Set your number where you actually ride, and the sessions will start fitting you again.

Lyra. The full article and every study we mentioned are on the WattPlan blog, linked in the show notes. And if you've enjoyed this, follow the WattPlan Podcast wherever you're listening, so you don't miss the next one.

James. And if you want a plan built around the time you've actually got, get WattPlan on the App Store or Google Play. Thanks for listening.

Voices produced with ElevenLabs. James's voice is a clone of his own, Lyra is an AI voice, and James reviews every episode.