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For my n=1 experience, I did a 20 min test on the trainer for the first time ever a couple weeks ago, with an avg power of 263 watts. I was outside on my balcony and it was 28 deg F, so cooling was not an issue. One thing that I noticed was that I was forced to pedal at a lower cadence than I wanted to (around 94 rpm). I would prefer to be and I naturally settle in around 100 rpm for a hard effort, but if I shifted to an easier gear, it put me around 104 rpm which was too high for me. This definitely led to what I felt like was a sub-optimal test. I have not done a 20 min test outdoors since September, but my 20 min power then was 328 watts, and I'm positive I could do at least 295-300 watts outdoors around the time that I did the test on the trainer.

Another thing that I am curious about is if using a rear wheel based power meter such as a PowerTap affects measurement on the trainer. I have a PT and just bought a Quarq (I'm going to sell the PT). The other day, I did a 2.5 hr trainer ride with both the PT and Quarq on my bike, and the Quarq read around 10% higher on average (181 vs. 165 watts). To me, this seems a little high...I was expecting the Quarq to be maybe 5% high due to drivetrain losses. What if the PT measurement is somehow affected by the rear wheel being pressed up against the trainer cylinder? Later this week, I'll ride outdoors and see what the % difference is between the Quarq and PT...if the % difference is different for outdoors vs. indoors on the trainer, then that might explain something...


Well, the engineer in me says this rider was somehow able to use 21kJ more energy outside than inside for the same perceived (and measured - HR more or less the same) effort.

I would put this 21kJ down to many small things all adding up:

1. He is gaining efficiency by using a higher torque and thus cadence and for whatever reason this is suiting his physiology - his body is able to use more energy this way rather than the higher cadence method.

2. Outdoors there may be more spikes and recoveries in the data. The other day I was doing a HOP workout (53x12 all out spikes every 3 mins for 30 sec) and beat my previous FTP by 10W. Somehow it was easier doing it this way than a steady build, which was my previous FTP test. I spiked more and rested more, I think my body likes this rather than 60 mins of turning the screws. I go with the micro-recovery theory.

3. Also, he was MOTIVATED to prove the indoor number wrong

4. It's outdoors! The cooling effect is much stronger. Heat dissipation is actually the single most ineffcient thing in our bodies. The body is actually putting out more energy than the energy that is recorded at the back wheel. The delta is lost due to heat build-up, stabilizing the core, working our lungs etc..

Question: Did he do both FTP efforts back to back? It's not uncommon for the second effort to be better because you have already warmed up, primed the lactate buffers etc. Thus the RPE is less for moreater output...or the output is greater for the same RPE


In my experience, riding indoors with rollers has a lower PE (and higher resultant power) than riding indoors with a trainer.

It may be because riding rollers more closely matches the balance and muscle use of riding outside. (the rear skewer is not fixed).


Looking at this another way, does it really matter? Some people are able to post the same numbers indoors (or up hill, or on flats, etc) and others post different numbers. From a training standpoint can one say that they are equivalent? If you ride 20w lower in doors but once you go outside you still ride +20w then maybe you have two numbers instead of one.

Gerry Gucher

Hi, I am a fun cyclist doing occasional hobby triathlons and ride around 3000km/year. I also have quite a bit different FTP, carefully determined by the exact same protocol(monod-scherrer 5min/20min with 15min rest in between) using the same Powertap.
Outdoor flat - 229W, Outdoor uphill (6%) 240W, Indoor 219W (using single fan, open window).
I would think there is no single reason but an accumulation of little differences, i.e. a single fan can never cool as much as outdoor headwind, rolling momentum, psychology, slight difference in cadence etc. etc.
I even have a 7% different FTP between flat and uphill (uphill being higher). I can never hold the same top power cadence uphill (~80) vs. flat (~98).
For me it's pretty clear, depending on personal abilities a watt is not just a watt. I simply use different power zones indoor/outdoor/flat/hill when I do a systematic power based training.


My ftp numbers were higher on a kurt kinetic rock and roll vs the static road pro. It seems to have something to do with the bike moving and all the force not going through the body hence fatigue and rpe being impacted. Interestingly the ftp on the r&r is identical to my road number whereas the static machine was 10% lower with rpe higher. I guess these things can be quite atlete specific.

Randall Maddox

I did not have time to read every comment, but I imagine the difference has to do with INERTIA. Studied physics 43 years ago but I remember 'that if a body is in motion, it has a tendency to stay in motion.' (or something like that?). You have inertia working for you on the road and do not a fixed device. Well maybe the wheel has little? It is turning.

It also may have to do with the difference in what you are overcoming. Air drag on the road and wheel drag on the trainer?

I feel stronger about the inertia idea. We might consult someone who got past Physics 101? I am an Economics major, as you probably remember. So I would be better determining the TIME/Value of indoor training.


Lots of comment on why, but very little on what to do about it. Here's my thought...
At 259W the zones are:
Z1 0 142.45
Z2 145 194.25
Z3 197 233.1
Z4 236 271.95
Z5 275 310.8
Z6 313 388.5
Z7 388.5 +

At 278W the zones are:
Z1 0 152.9
Z2 156 208.5
Z3 211 250.2
Z4 253 291.9
Z5 295 333.6
Z6 336 417
Z7 417 +

If the intent is ensure the athlete is training in the proper zone.. indoor or outdoor, have him train in the overlapping portion of two sets of zones... specifically:
Z1 0 142
Z2 155 194
Z3 211 233
Z4 252 272
Z5 294 311
Z6 336 389
Z7 417 +
Wouldn't this ensure he gets the aerobic or anaerobic benefit of his power-based training regardless of whether he's training indoor or outdoor? I get there's gaps between the zones, but that's easily manageable in actual training, and wouldn't this be better than trying to remember a different set of zones depending on whether your training was occuring indoor or outdoor?
(and yes, it's winter again so the discussion on this topic is popping up again!)


I have been curious about the discrepencies of training indoors and outdoors. Yesterday I did a little test. After a 45 minute warm-up I rode up a 7% grade at Sub-LT power and it felt comfortable. I then rode on a flat at the same power and it felt much more challenging. Then I compared data from yesterday to indoor rides of similar power and found that in all situations, the torque was about the same for the wattage. As a matter of fact, most measurements were similar. The only difference was RPE. I have always thought I am more suited to climbing, I find it easier than pushing on the flats, but now I am coming to the conclusion that the biggest variable, perhaps the only significant variable, is psychological. You hate TT, they will be challenging, hate hills and they grow steeper and longer. Hate indoor training, it will show.


There are a couple of things I have found about the trainer that cause issues for me. First of all is that it is unnatural. The bike does not natural track in a straight line. The bike torques ever so slightly underneath the rider as they crank on the pedals. More efficient cyclists have less of an issue with this as they have learned to be more smooth. This could account for some higher power numbers. For lower power numbers there is the mental issue, I output less power than I have seen athletes whose power numbers are lower in headwinds, on false flats and on the trainer simply cause they feel like they should be moving faster. It defeats you mentally to hammer and move nowhere.


For what it's worth, I'm curious as to the type of Power meter used. Is it possible that if the sensors in a crank (for instance) are affected by change in temperature from indoors and outdoors and the rider doesn't calibrate properly, this would have an effect on the reading?

Tieme Roosjen

I have a question about FTP, which I can't find in your books or when using Google., so I ask you here (English is not my native language) :

I’m a triathlete (76kg, racing sprint, short and some mid-distance races), using a correctly calibrated SRM on my TT bike (my roadbike has a powertap).
I calculated my actual FTP from a flat sprint distance triathlon with a 19km bike in 28m22s (2nd fastest bikesplit) on a bikecourse with a lot of cornering. It resulted in a 288 Watt average (298watts Normalized) using mostly the aero position. I finished the bike fit enough to have a great and fast 5k run

At the moment I’m focussing one of my weekly bikesessions to improve my FTP further (which I didn’t before, mostly basic endurance and tempo sessions with some hill work). Yesterday I did my first session (2x 12’ interval @ 275W average each in aeroposition, trying to stay between 270 and 295W). Of course I noticed when I leave the aerobars I could easily push more watts… so I assume when I do a FTP test with my hands on the basebar, it would result in higher FTP value…

So, my question is, which one should I use as the reference? An aerobar-FTP or a basebar FTP? (I’ll perform a new FTP test this month. This time on with no hard cornering in it)

Joe Friel

Tieme Roosjen--Very good question. I'd suggest doing your FTP test in the position you will be racing in. That then also means that FTP-based workouts (intervals, tempo, etc) should be done in that same race position.


Hi Joe

I'm a lightweight and therefore a good climber at least compared to the flat :-).
So I can go with the group in the mountains at a comfortable HR (166) but despite the slipstream I'm in trouble to hang in the group in the flat. Leading a group is impossible and it happend that I didn't managed to rejoin after a couple seconds in the lead, because I was completly in the red. I also suffer a lot more in the flat from a feeling perspective.

It's now approaching winter and I started my favorite season of the year. Winter training on rollers, yeah.

My FTP Outdoor (Mountains) is around 266 Watts. Indoors I manage only around 225. Massive lower than outside, around 15%

First of all, I thought about wrong data. But now I guess its correct and that this is the reason for getting dropped in the flat. Did you experience such hugh differences in indoor, outdoor (flat/mountain) FTP.

In my point of view this has also to do with the profil of a rider/muscles how the different FTP are. Climber/Allrounder/TT Specialist/Sprinter has different FTP on the different terrains.

What do you think about it?

Joe Friel

Silvangarderen--A rider's FTP will almost always be higher on a climb than on flat terrain. This is especially so for smaller climbers who don't have high "absolute" power. Power normalized for weight, however, is generally quite high for smaller riders.

Patrik Boström

I have the experience that my power decreases just as most have. The question is, do I get the same training effect at a lower power so long as I keep the pulse at the right place?

Joe Friel

Patrik Boström--You're _probably_ seeing lower numbers indoors because you are less economical. That's not good. But sometimes necessary.


Dear Joe, thank you for the very interesting topic and initiated discussion. It seems the discussion is probably going to take further waves in the near future, since power meters are getting more affordable and more people will meet these issues, especially if powermeters become more compatible with more bikes. In my example I race with a XC bike but train on a roadbike as well and on a trainer with my spare bike in bad weather.
I would first like to react to the very first comment and then add my own case and experiences, conclusions for thoughts, due to the significant difference of 20 BPM I noticed myself outdoor vs indoor during two efforts with equal wattage.
The very first comment from Marcus on a possible explanation of the roller-resistance indoors vs the "free roll-out" outdoors initially seemed like a plausible aspect, but at further thought there is always a resistance that creates wattage: either through acceleration, through the effort needed to overcome gravity on a climb, the wind resistance on the flat, rolling resistance of the tires/bearings, etc. or the resistance of the indoor trainer (magnet). So even when you ride on the flat, the wind's resistance at a speed where you create such force to keep up your maximum effort for the 20min (which should mean a lot of wind et al resistance) should be a constant force. In other words, there is no "free roll-out" where you would suddenly be in a vacuum with chance for recovery or decreased wattage. If you let off on the torque, it simply means a drop in your wattage, meaning no sustained effort, but these inbetweens could be traced easily afterwards by looking at your effort level graphically and any drops in that. Equally, you can let off on a climb or on an indoor trainer as well.
The fact that the trainer's magnet slows down the effort, is irrelevant when you choose not to exert the effort while it slows down: the test is not about constant speed, but about exerted effort. In addition, if due to some slowing down (esp on hillclimbs or on trainers) the resistance suddenly decreases and you want to get back on the same cadence and/or speed, it simply means that you are initially on a lower wattage, which you need to compensate immediately through a higher wattage than before, to get back on track. This change in effort however will be equal in the case of hillclimb, trainer or flat (with wind + other resistance), as the counter forces are constant.
One very small impact I would add to this, which is of course much less relevant in case of air: in water, water displacement means that keeping a momentum requires less effort on short distances than slowing down and suddenly getting back on the same speed, just try accelerating with a boat from standstill vs in movement. In other words, water "carries us" along for a little while. But in case of air, this is neglectible in compariosn to our body weight, especially considering the very small differences of letting off on the wattage for micro-recovery split-seconds.
As for my personal experience: incidentally I am about the same age as the subject at the time of writing and have started biking about the same time ago, so feel the relevance of getting to know my limits and developing myself in a proper manner. Since I am new to power meters (got one to try out for some weeks), I stumbled on this same issue and found your article, which is quite useful for understanding the differences and prepare myself for races. I am trying to find answers the same way for my case from the past 2 days.
I haven't done a full FTP yet, but had an indoor session on a roller with two 240W efforts of 10min with 5min recovery inbetween. I could not manage to perform the second one fully, true enough though it was preceded by some efforts for 40min, of which around 200W for 20min. Next day however I was using the same bike outside on a hill climb of 9% and found that doing 240W was easy as pie after a 15min warm-up. True enough, I only did one ride of ca 9min on that hill and no second one, but this one was in no comparison with the previous day's first 240W exertion, when even the first 10min was on the edge with serious strain.
The relevant part I consider to be the heartrate comparison of the two occasions: while on day1 I peaked around 181BPM for the first 10min (for accurate comparison 180BPM after 9min, while reaching 170BPM after 2min into the effort), on day2 on the hill climb outside my HR reached only 163BPM, up from 157BPM from the first minutes of the climb. This I consider to be a huge difference in HR for about the same effort exerted and according to my morning HRV measurement I was equally rested on both days, in fact on the first day I had a restday before.
As my individual anaerobic threshold is around 220W @ 167-168 BPM currently, these two divergent results make a tremendous difference in my self-evaluation. Of course there is a major distortion probably due to the earlier mentioned 40min prior efforts, still the perceived effort difference was tremendous.
Since I am a XCM mountainbiker and the powermeter supports me temporarily in my roadbike training sessions, I am unlikely to find a good objective measure of my abilities this way, as I would need to use a consistent measurement, which seems to diverge on roadbike inside, outside and most likely MTB outside as well.
From the comments on your writing I have gathered some useful insights, but do not see a single most important determinent of the differences. I would think it's probably a combination of many factors:
1. mostly the cooling aspect, though some riders are using fans or are outdoor and see no substantial difference;
2. perhaps the oxygen uptake, though I have understood that we never make full use of the oxygen from the air we breathe in, so for such a short test and my particular example at lower effort, this should not be a major determinent;
3. probably the physical movement on the bike which can follow your specific neuro-muscular pathways better than a steady trainer. I am inclined to see this as a major factor;
4. most likely the mental state of being outside and a more focussed mind: I noticed during earlier indoor tests of step-wise raising of the wattage by 30W every 3min that when they take blood to measure the lactate changes at each step, while I'm holding my head still (as blood is taken from my earlobe) during these few seconds my HR actually DEcreases slightly after this concentrated effort, even when my wattage remains stable.

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