Indoor vs. Outdoor Bike Performance

The following is a question from a coach. He and the athlete he coaches are somewhat frustrated by the difference inpower numbers they produce relative to heart rate when riding indoors as opposed to out on the road. This is a common complaint this time of year, one that I hear a lot.

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QUESTION FROM THE COACH: I'm hoping to find an answer or an explanation to an athlete's FTP [functional threshold power] testing results.

Here's the 411.... the cyclist is a a masters racer... 44 years old, has been cycling for the last 6 years. Whenever he conducts a FTP test outside he is posting better 20 minute numbers vs a test on a trainer.  I looked at HR and that piece of the data is the same.  We have tried a multitude of things to improve his "trainer" FTP (to match his outside FTP), such as fans, using the trainer outside, music.... but there still seems to be a difference.  His prior coach actually had two FTP values for him depending on if he was doing a trainer workout or an outside workout.

Here are the numbers from a past test. I had him do a test on a trainer resulting in 258W and an average HR of 174.  He was frustrated at his low number so 10 minutes later he went outside and put down a 278W with 175 HR. 

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MY ANSWER: This is a common question every winter. A lot of riders experience this. But I’ve also heard from some who have exactly the opposite problem—better numbers indoors than out. So it remains a mystery. There are a couple of possibilities that may explain it, but they are rather lame I’ll have to admit. (I'm assuming the same power meter and bike were used on both tests.

One is that when riding on the road there are brief episodes of “recovery” due to slight hills, cornering and changes in wind direction. These little changes don’t occur indoors on a trainer where power is usually much steadier. Some research shows that these little “mini-recoveries” allow the rider to go harder overall. In fact, I’m told that when going for an hour record on the track they sometimes train to be “unsteady” in power output order to take advantage of this little oddity.

The next is that when riding on the road the bike actually rocks from side to side slightly while the body remains relatively steady over it. On the trainer it’s the opposite – the bike is locked in place so the rider’s body moves slightly side to side while pedaling. This requires a somewhat different technique and muscle recruitment and so may elevate HR and effort “artificially.”

It sounds like you have already eliminated the third one – that heat build-up elevates HR when riding indoors more so than when on the road. Lots of fans help here.

I’d also be interested in seeing if the rider paced the two TTs the same way. In other words, did he negative or positive split the two trials in the same manner? Someone may be more adapted to riding on the road than the trainer and be much more unsure of how to pace themselves indoors. Or the other way around.

Could one of these explain a 7% difference? Probably not. Could some combination of them account for that? Possibly.

Bottom line is that I really don’t know.

Comments

[Marcus]

Another thing different indoors is that deceleration is a much greater force than outdoors. If you stop pedaling on level pavement you'll coast for quite a while, while indoors the rear wheel will spin down rather quickly even on high-end trainers with large flywheels. I think that, for the same effort, you re-accelerate the flywheel for every pedalstroke so to maintain the same wattage indoors requires more effort than outdoors. I know some say a watt is a watt, but the fast spin-down of an indoor trainer has to account for something. Just my 2 cents!

[Russell]

Just to add my personal experience on this:

I am not a frequent indoor rider, but I do so occasionally when the weather is really bad. I have been training with power for more than a year.

I've found my numbers to be near-identical on the road and indoors. I do FTP testing indoors and out. I have found my HR and Power numbers to be consistent in both.

I have, however, found my RPE to much higher indoors. To me it 'feels' like the variation in effort you mentioned seems to be the culprit. I've also noticed the biomechanic effect of the stationary trainer, and now do almost all my indoor training and testing on rollers.

[Joe Friel]

Marcus--Very good observation.

[Paul Waite]

I'd just like to say that I had come to much the same conclusions as above independently. I have always noticed that a given average power via my power meter is harder indoors than out on the road.

I surmised that it was simply the 'steady-state' nature of the indoor setup both in terms of bike ergonomics (the side-to-side movements etc) and the more or less rock-steady effort. The body/mind simply don't like an unchanging effort like this, and we seem to be programmed for more variation whether physically or mentally or a combination of both.

Whatever the case I'm happy it's this way around. I know my indoor numbers and am improving those, and I know that I will be powering along generating more wattage on the road, where it counts!

[neeraj]

What was the distance covered during the indoor vs outdoor tests? I would put my money on more distance covered during the outdoor test. Anyways, what i'mgetting at is that the trainer resistence might be set at too high. And also I would check the rider's pedal stroke. He may not have an efficient pedal stroke. When outdoors, momentum helps but when on a trainer, the extra resistence might hurt output.
Just my inexperienced $0.02 :)

[Douglas]

I know people who have problems with this (and I do occasionally) but I think it's mental a large part of the time. I got my first winter of riding on the trainer extensively last year due to a move northward, and I have no problem putting out the same power and HR numbers so long as I don't overheat. I've done FTP tests on the trainer in winter and on road in spring and the difference was exactly what you'd expect due to a progression in training (drawing from month-to-month increases in the winter tests).

That said, I have had a harder time adjusting to the trainer mentally this year, and the same effort feels harder than if I do it outside... for now. I expect this to change as I become accustomed to the mental state again.

Another explanation is similar to your muscle recruitment one - different people have different pedaling motions, so holding the bike stationary might affect them less. This is similar to why things like Rotor's Q-rings increase wattage for some people but not others - some people's neuromuscular pathways don't adapt to them as well as others.

[gary]

Another potential variable is cadence. For many riders, at a given power power output, higher cadence will be associated with higher average heart rate. This observation also has relevance when estimating aerobic decoupling if cadence changes significantly during a workout. Another factor affecting the power/HR relationship is body position (standing vs in the drops or aerobars). At an absolutely steady power output on my trainer, going from aerobars to sitting upright causes a 5-10 point increase in HR.

[Paul T]

While most bike trainers are stationary, the Kurt Kinect Rock and Roll Trainer allows for the trainer to go side to side. Do you think there would be less of a chance to have that artificial elevation of heart rate with this trainer?

[Pieter Vuylsteke]

I'm sometimes using my Powertap wheel on my Bushido trainer. The "pure numbers" of the Powertap are 16W less than the Bushido.

My FTP outdoors is 282W : determined using a Coggan-alike FTP-test protocol, with at the end full 20minutes all out effort, and then -5%.

I observed this morning on my Bushido that 260W at 85tpm is harder than 260W at 93tpm. I did a training of six times 10minutes at 270W (5m) and 260W (5m). The two last intervals I had a higher cadans, and rather significantly lower hartrates...

Also, at 270W my bpm tends to go higher all the time, at 260W it stabiles.

Remember, 270W on Bushido is 256W on Powertap, making this 282W for FTP quite unbelievable, indoors...

And of course, this gives wrong numbers for TSS and so on...
FWIW, of course...

[Gary Smith]

Joe, I would add one other factor: mental. I'm not talking about motivation -- although that can be a problem indoors -- but rather the ability outside to to focus on landmarks and different parts of the course. Most people do their threshold test on the same route, so they know if they can "just reach that stop sign they have a short downhill to recover" or if they can "just power up this last hill it's only .5 miles to the finish." You get the point, I'm sure. Indoors, there's usually nothing but your powermeter to focus on. Makes the time seem much longer and motivation (and hence power) tends to wane....

[Kirk]

Great thoughts! So Joe what are your experiences when a FTP testing is done indoor on rollers vs outside vs trainer? My simpleton thoughts re: lower FTP on a trainer vs on the road focus more around rolling resistance of the tire on the road vs on the roller of the trainer; it's been my impression that a significantly larger (and concave) tire surface area is at play (higher friction, heat production +...) when I'm on the trainer. So HRT drifts up as I over come these extrinsic factors at significantly lower FTP.

[Jerome]

One thing to take into consideration also, is that "out of the saddle" position is much more difficult to adopt on trainers, though its part of the changing movement process, and provide some micro recovery also..

[erik]

Test on a trainer resulting in 258W and an average HR of 174, 10 minutes later he went outside and put down a 278W with 175 HR......I have been doing this for many years now,and I have never been able to put in any hard work after my tests.So I wonder if this master cyclist realy put everything into his first test??

[Carl]

One more possibility, riding outdoors, you have a "ram" effect of the air you breathe in and this air is always fresh or new air in terms of oxygen content. Whereas, indoors, your kind of breathing air that's being recirculated and you don't have that "ram" or turbocharging effect.

Just my opinion.
Carl

[Madeleine]

If episodic recovery is a primary cause of the higher wattage outdoors, would you expect to see a greater difference between normalized and average power for the outdoor piece compared to the indoor piece? Or do you think that 20min is too short an interval to see a statistically significant difference?

[mike]

If ambient temperature and cooling have been ruled out then I think you need to look at the gearing/cadence used in both tests and the resistance level of the trainer compared to the resistance overcome on the road, ie was the road flat and the trainer felt more like riding up a hill? Was the cadence maintained in both tests the same? How do we know FTP was really being measured in both cases? Sure the heart rate was the same in both tests but how long was this measured over ? It sounds like this wasn't a true 1 hour FTP test but a shorter test and these are not so reliable. Over much shorter periods non optimal gear selection and cadence (either over or under reving) can affect heart rate and distort the HR vs power relationship.

[Joe Friel]

Madeleine--Would expect to see a lower VI for indoor TT. For example, for a 20 min test the VI may be 1.0 indoors and 1.05 outdoors depending on terrain, wind, traffic, etc.

[David]

On point 2 is there another effect at work here?
There is an energy cost to moving the (heavier) body from side to side while the (lighter) bike stays vertical on an indoor trainer that is higher than the cost of moving the bike from side to side with a quiet body outdoors.
This leaves less energy available at the rear hub (or crank for SRM users)

[Shane Dunleavy]

I am the cyclist who performed the above mentioned tests and have enjoyed reading the comments. I'll post the actual numbers from the 2 tests below. The trainer test was done after a 30 min warm up on the road at 200W avg followed by a 5 min effort at 280W, then 5 min spin before the test - it was on a Kinetic Road Machine. The road test was done on the canyon road I live on at a grade of 3-5%. I was fairly cooked after the trainer and disappointed with the road number as well. When I repeated the test 3 wks later on the same road it was 10W higher at the same HR.

Trainer Test:
Duration: 19:48 (20:00)
Work: 307 kJ
TSS: 30.3 (intensity factor 0.958)
Norm Power: 259
VI: 1
Pw:HR: 0.85%
Pa:HR: -3.42%
Distance: 8.976 mi
Min Max Avg
Power: 199 383 258 watts
Heart Rate: 160 190 173 bpm
Cadence: 71 101 88 rpm
Speed: 24 35.2 27.2 mph
Pace 1:42 2:30 2:12 min/mi
Crank Torque: 196 332 247 lb-in

Road Test:
Duration: 19:43 (20:01)
Work: 328 kJ
TSS: 34.9 (intensity factor 1.03)
Norm Power: 278
VI: 1
Pw:HR: 2.49%
Pa:HR: 25.7%
Distance: 4.683 mi
Min Max Avg
Power: 101 437 278 watts
Heart Rate: 132 199 175 bpm
Cadence: 56 93 77 rpm
Speed: 9.9 21.4 14.3 mph
Pace 2:48 6:02 4:13 min/mi
Crank Torque: 115 520 309 lb-in

[Joe Friel]

Thanks, Shane. That should help people better consider what you did that day.

[Peter]

When I do an indoor test or workout session I will often get a higher HR, but not always. My observation is that when indoors, my mind wanders to "visioning" being in a race situation and my HR rises about 10 bpm immediately. Effort/watts seems the same. Changing my thoughts to something more mundane brings my HR down with same power output. Outside, I tend not to do this while training/testing.

[Neeraj Engineer]

Thanks Shane for sharing the details.

Joe, is the expectation that the FTP numbers match between the 2 workouts even fair? The outdoor workout is hill climb and the trainer workout is more like a TT. Wouldn't it be natural to have a disparity between the 2 for almost everybody?

thanks
-Neeraj

[Joe Friel]

Neeraj--It's possible altho 20w for a 3-5% grade seems a bit high.

[Gordon Woodhead]

I posted 300W up Alpe d'Huez for an hour, but can only manage 250W on a flat TT of the same time, and about 230W on the indoor trainer (using a Tacx Fortius, but not calibrated against a powertap or anything like that).
Interestingly, I note my watts indoors vary depending on my cadence too - about 10W lower on a higher cadence (102 vs 95rpm). This is over a 45 minute test.

[Gordon Woodhead]

Sorry, should add to my previous comment that the higher cadence was in a gear lower.