Indoor FTP Vs Outdoor FTP: Why They Feel (And Are) Different
We recently came across an article from TrainerRoad on the topic of indoor vs outdoor FTP (Functional Threshold Power). Anyone who trains both indoors and outdoors will be acutely aware that it often feels harder to produce power indoors vs out, at least unless you spend the majority of training hours on the turbo each week.
What caught our attention is that the article went on to state that although it feels harder to ride inside than out, your FTP is the SAME under both conditions.
Here’s a quote from the article:
“One of the most common questions we receive here at TrainerRoad is whether you need to adjust your FTP between indoor and outdoor workouts. The short answer is no, as FTP is rooted in physiology and not affected by external factors.”
We believe this statement is incorrect on multiple counts... Indoor FTP is almost always lower than outdoor FTP. And FTP is not rooted in physiology and it is affected by external factors!
Since there’s a lot of misunderstanding in this area, we felt compelled to do some explaining, and look at what the scientific evidence says about why FTP is lower indoors vs out and what you should do about it.
What is FTP?
Let’s first start with a quick reminder on what we mean by FTP…
FTP, or ‘Functional Threshold Power’ can be defined in several ways depending on how exactly it’s tested. The classic definition is the average power output that can be sustained for around 1-hour, or during a 40km time trial. Various abbreviated tests have also been developed, with the most common one being 95% of the average power sustained in a 20-minute time trial which follows a 5-min hard effort.
In all cases, FTP is a performance measure. In other words, it’s a test of the power you can produce on a given day, under given conditions. Contrary to the statement in the TrainerRoad article, it is impacted by a multitude of external factors, and will vary from day-to-day, depending on variables like nutrition, time of day, motivation, fatigue levels… and importantly, the bike you’re riding on!
If we read further in the TrainerRoad article, it becomes apparent that they have unfortunately fallen into a common trap of confusing FTP with laboratory measures of physiological thresholds, such as the maximal lactate steady state (MLSS), or the ventilatory threshold. It’s important to understand that these physiological thresholds are all different to FTP (see e.g. Inglis et al., 2019 for a nice scientific demonstration of this).
Nevertheless, even if we accept that TrainerRoad have got their terminology a bit muddled, and they are trying to tell us that physiological measures like MLSS don’t vary in response to external factors, then this is still incorrect!
Indeed, there’s evidence that numerous external factors, such as crank length (Sullivan, 2019) or prior nutrition (Pinna et al., 2014) can impact physiological measures of threshold power.
If we take this concept to its extreme and imagine comparing two different exercise modalities (e.g. cycling and rowing), it seems pretty obvious that a trained cyclist would have a higher threshold (relative to e.g. heart rate or oxygen consumption) than rowing, if they don’t train for rowing. It’s the same principle when you change to a different environment, like riding indoors or out, but the difference is more subtle.
How much higher is outdoor FTP vs indoor FTP?
Before we look at why indoor and outdoor FTPs are different, it makes sense to consider how big this gap might be.
Perhaps the difference between indoor and outdoor FTP is so small that it’s not worth worrying about?
Well, it seems the gap can be really quite large…
A study of recreational-level cyclists (Mieras et al., 2014) found that outdoor power was 30% higher on average during a 40km time trial. There was a lot of individual difference, but outdoor power was consistently higher for all participants, ranging from an 11% increase to a 69% increase. It’s worth noting that the discrepancy in power might have been exaggerated due to the fact that no fans were used in the indoor trial.
In professional cyclists, and under more real-world conditions (e.g. using fans indoors), the gap seems to be closer to 20-50W (Lipski et al., 2022; Bertucci et al., 2012), which is still a meaningful gap, but not as large as in the Mieras study.
It’s worth noting that not all studies observe a difference in power (e.g. Jeffries et al., 2019). But these studies have limitations, such as requiring athletes to ride unfamiliar bikes (which would tend to narrow the gap between outdoor and indoor riding due the biomechanics being unfamiliar in both conditions), and the outdoor test being done on a flat course with many corners, which is not optimal for scoring a power PB.
In practice we tend to find the difference in power among our athletes is in the region of 5-10% for cyclists who ride regularly both indoors and out, and a little larger for athletes who are less familiar with indoor riding.
Why is indoor power lower than outdoor power?
So, why is training indoors harder? There appear to be a number of notable factors…
1. Mental
Mental factors associated with riding indoors (e.g. boredom, lack of external distractions, less sense of achievement) are often cited as one of the key reasons that it feels subjectively harder to produce power inside vs out. There’s some decent scientific evidence to support this theory…
For example, Irvine et al. (2022) found that indoor riding results in a higher perceived mental workload (and thus potentially more mental fatigue), and Slapsinskaite et al. (2016) showed that outdoor riding resulted in more external thoughts and fewer task-related thoughts, which shows outdoor riding is more ‘distracting’, meaning you focus less on the pain or discomfort.
However, it’s clear that the power mis-match between indoor and outdoor FTP isn’t purely mental. It’s not something you can just grin and bear your way though, or distract yourself from with some musings or a movie. Bertucci et al., (2012) showed that cycling economy was 11% lower when athletes rode their own bikes indoors on a turbo trainer, vs riding outdoors. In other words, for power output was 11% lower indoors vs out for the same oxygen consumption. So there’s clearly something going on physiologically too.
There seems to be two major factors at play in causing this drop in economy…
2. Biomechanical
Firstly there are a lot of biomechanical differences when riding indoors vs out. This is true, even if you’re riding exactly the same bike both indoors and out, or have meticulously replicated your bike fit to the millimetre.
Firstly, and perhaps most obviously, when riding indoors, you’re generally unable to move the bike beneath you and use your upper body to help generate power the same extent. Instead, you’re held in a fixed position, using smaller sample of muscles, which don’t get as much of a rest.
Secondly, depending on your set-up, your trainer will probably hold you in either a horizontal or a slightly nose-down position, as if you were descending). This is a harder position to generate power in for most, compared with riding in an ascending position. Since most people will test outdoor FTP or perform hard efforts on a climb, this can give rise to a notable difference in power output.
Indeed, Valenzuela et al. (2022) showed that in professional cyclists, power output can differ by between 0.4-3.6% when comparing flat riding and climbing. If you’re in a slightly nose-down position on your trainer, this discrepancy is probably exacerbated further.
Another difference comes down to the way resistance is applied throughout the pedal stroke. Older trainers (particularly wheel-on trainers) will apply 360 degree resistance. In other words, you’re pushing against something throughout the entire pedal revolution.
In contrast, when you’re riding outside, there are resistance ‘dead-spots’, where your muscles are able to take a very short break from having to turn the cranks. Bertucci et al., (2012) showed that this difference in resistance (also known as ‘inertia’), resulted in different peddling dynamics, which helped explain some of the drop in economy indoors.
More modern trainers will typically use a flywheel (or alternative electronic technology) to create pedalling inertia and produce a closer approximation of outdoor riding - i.e. to provide a better ‘road feel’. However, there will still be differences between indoors and out, in terms of how the resistance is applied, and how muscles are activated at different points of the pedal stroke.
Linked with these differences in resistance and inertia, there’s also the fact that power output and muscle activity seem to be more variable when riding outdoors, which appears to be associated with less fatigue (Blake & Wakeling, 2012; Jeffries et al., 2019).
In other words, the fact that power output is more variable outside seems to be a good thing in reducing fatigue, perhaps because it allows for brief ‘micro-recoveries’.
Finally on the subject of biomechanics, indoor cycling is also linked with higher cycling cadence, which is well known to affect the efficiency with which power can be generated on the bike (Lipski et al. 2022; Jeffries et al., 2019).
3. Overheating and dehydration
Another reason cycling economy can be impaired when riding indoors is that there’s an increased risk of overheating and dehydrating. When you’re riding outside, there is considerable air flow past the skin, even on a still day. This allows the body to cool effectively, both through evaporation of sweat from the skin, and ‘convection’ (i.e. transfer of the body’s heat directly to the air).
Even when a fan (or several fans!) is used inside, the air flow past the skin is much less than experienced outside, which limit’s your ability to stay cool. Couple this with the fact that the environmental temperature is often higher when riding inside (particularly in the winter), you have a recipe for overheating!
When body temperature rises, the body (or more specifically the central nervous system) begins to initiate a safety mechanism, increasing perceived effort level, and forcing you to reduce power output in order not to raise body temperature to dangerous levels (Nybo, 2010).
To further compound the issue, as there is less air flow indoors, the body relies more on sweating in order to keep the body cool which exacerbates the risk of dehydration. Dehydration reduces the heart’s stroke volume, leading to elevated heart rate, and a reduction in VO2max, which also contributes to impaired performance. Indeed, just a 2% body weight loss due to dehydration has been shown to substantially impair performance (Nybo, 2010).
On top of all this, dehydration also further limits the body’s ability to stay cool, by reducing sweating and blood flow to the skin (Nybo, 2010). So, it’s easy to get caught in a vicious cycle of heating and dehydrating unless you’re careful about your indoor set-up and hydration strategy.
Adjusting FTP for indoor riding
Sadly, there’s no one-size-fits-all rule to determine the difference between your indoor and outdoor FTP (Lipski et al., 2022). So it’s not possible to simply reduce your indoor FTP by ‘X’ Watts for example.
Does this mean you need to test FTP both indoors and out?
If you’re really keen to test both indoors and out, then you absolutely can. This will give you some hard data on how big your power gap is. However, we think it’s not essential to test FTP in both settings…
If you train fairly regularly indoors (e.g. a few times per week), our suggestion would be to perform all your testing on the trainer. This allows for better power control and a higher-quality test. Then when riding outside, we’d suggest using your indoor FTP as a basis, but iterating your power output based on your perceived effort level (RPE), breathing rate and heart rate.
For example, if you’re performing a set of intervals outdoors, perhaps aim for 5-10W higher than you’d usually target indoors, and see how this feels. If all is good, try increasing another 5W and so on, until you know what power you can hit outside for a particular session.
Athletes in our experience tend to find that the training zones that manifest from their indoor FTP work fine when riding indoors, since most zones allow for quite a wide range in power. You can simply aim for the upper end of your power zones outside, and the lower end indoors, for example.
Training zones always need iterating based on an individual’s strengths and limiters, and day-to-day fatigue, so these zones are only a rough approximation anyway, and you should always pay attention to how a particular session feels subjectively anyway.
Taking this approach to testing and FTP-setting has the benefit that you’ll always be using the lower of your two FTP values, which helps prevent overtraining and can buffer against the tendency for common testing protocols to overestimate values in amateur athletes.
If you’re only riding indoors occasionally, then we recommend testing FTP outside, and then using perhaps 85% of this power as your indoor FTP, and seeing how this feels. Again, it’s always better to stay conservative, then iterate your power based on how you’re feeling.
How to improve your indoor power
Below, we’ve set out some tips for indoor training that should help you narrow the gap between your indoor and outdoor power.
Some of you will know that we work closely with Wahoo Le Col, the world’s leading e-racing team, and so the following advice combines both theoretical and practical advice gleaned from training for and competing within elite indoor competitions…
Try not to use ERG mode all the time. While this is a great option if you just want to catch up on the latest box set without having to keep an eye on power, or you need some extra accountability to push through some tough intervals, in our experience, most people find riding in ERG mode is harder than riding without. Firstly, it can drive your riding cadence down below your natural cadence, which can make things feel tougher. Secondly, the micro-variation in your power will be very minimal, which we know from the scientific literature seems to be linked with greater fatigue. And finally, it can make a session more mindless. Having to concentrate on keeping your power in range tends to make a session go faster, because you have something you need to focus on.
Related to the point above, you can set yourself mini challenges to make time pass faster. So for example, see how close you can get your normalised power to a particular target. This works well on a ride that has several segments (for example see how close you can keep your normalised power on each of the sectors on Alpe du Zwift). Or try doing something similar with cadence.
As obvious as it might sound, playing music has been shown to increase motivation and performance during aerobic exercise (Jia et al., 2016; Brooks & Brooks 2010), so this is certainly a credible (albeit often-used) strategy.
Having something entertaining to look at is also often helpful. That might be watching a TV show or film, or perhaps watching competitive cycling (many people find this particularly motivating). Zwift is also a good tool, which combines having something to watch, with a social aspect too!
Obviously, it’s important to keep your training area as cool as possible, use good qualify fans, and make sure you’re staying hydrated with plenty of cold liquids to avoid overheating and dehydration.
Make sure your indoor bike set-up is as close as possible to your outdoor set-up to minimise biomechanical differences. Ideally you should use a trainer with a good ‘road feel’ (e.g. heavy flywheel or more modern virtual ‘flywheel').
If you’re using different power meters indoors and out, then make sure they are both well calibrated.
Make sure your training is purposeful, and has clear goals. Check out our training plans if you’re looking for an evidence-based plan to follow.
Finally, stick with it. The gap between your indoor and outdoor power will start to narrow as you get used to riding inside, and you begin to acclimate to a warmer riding environment, and your muscles adapt to the change in biomechanics.
Is training indoors bad for outdoor performance?
A final concern you might have is whether training predominantly indoors might hamper your outdoor performance.
The answer to this is ‘yes and no’.
While you’ll be able to build fitness indoors that will translate to outdoor riding, the ‘best practice’ would be to make sure you spend increasing amounts of time riding outside as you get closer to your target events or competitions.
This will allow chance to train the muscles and ligaments that are worked less when riding inside (key ones being your upper-body), and for you to generally get used to the different sensation of riding outdoors and work on your bike-handling skills.
A note on other performance measures…
A final word… Throughout this article we’ve focussed predominantly on FTP, since it’s the most commonly tested performance measure, and most cyclists use FTP to derive training intensities and zones.
However the principles discussed above apply equally to other measures of performance, such as critical power, maximal efforts of shorter and longer duration (e.g. 5-mins and 1-min).
There may also be an indoor/outdoor difference in physiological measures such as VO2max, or maximal lactate steady state power due to lower efficiency and potential dehydration, but these differences are likely smaller, since mental factors don’t play into these measures.
References
Inglis, E. C., Iannetta, D., Passfield, L., & Murias, J. M. (2019). Maximal lactate steady state versus the 20-minute functional threshold power test in well-trained individuals:“Watts” the big deal?. International journal of sports physiology and performance, 15(4), 541-547.
Sullivan, S. A. (2019). The Effect of Bicycle Crank Length on Maximal Oxygen Uptake and Ventilatory Threshold in Trained Cyclists (Doctoral dissertation, The University of North Carolina at Chapel Hill).
Pinna, M., Roberto, S., Milia, R., Marongiu, E., Olla, S., Loi, A., ... & Crisafulli, A. (2014). Effect of beetroot juice supplementation on aerobic response during swimming. Nutrients, 6(2), 605-615.
Lipski, E. S., Spindler, D. J., Hesselink, M. K. C., Myers, T. D., & Sanders, D. (2022). Differences in Performance Assessments Conducted Indoors and Outdoors in Professional Cyclists. International Journal of Sports Physiology and Performance, 17(7), 1054-1060. https://doi.org/10.1123/ijspp.2021-0341
Valenzuela, P. L., Mateo-March, M., Muriel, X., Zabala, M., Lucia, A., Pallares, J. G., & Barranco-Gil, D. (2022). Road gradient and cycling power: An observational study in male professional cyclists. Journal of Science and Medicine in Sport, 25(12), 1017-1022.
Jeffries, O., Waldron, M., Patterson, S. D., & Galna, B. (2019). An analysis of variability in power output during indoor and outdoor cycling time trials. International Journal of Sports Physiology and Performance, 14(9), 1273-1279.
Bertucci, W. M., Betik, A. C., Duc, S., & Grappe, F. (2012). Gross efficiency and cycling economy are higher in the field as compared with on an Axiom stationary ergometer. Journal of applied Biomechanics, 28(6), 636-644.
Jia, T., Ogawa, Y., Miura, M., Ito, O., & Kohzuki, M. (2016). Music attenuated a decrease in parasympathetic nervous system activity after exercise. PloS one, 11(2), e0148648.
Irvine, D., Jobson, S. A., & Wilson, J. P. (2022). Evaluating Changes in Mental Workload in Indoor and Outdoor Ultra-Distance Cycling. Sports, 10(5), 67.
Blake, O. M., & Wakeling, J. M. (2012). Muscle coordination during an outdoor cycling time trial. Medicine and science in sports and exercise, 44(5), 939-948.
Nybo, L. (2010). Cycling in the heat: performance perspectives and cerebral challenges. Scandinavian journal of medicine & science in sports, 20, 71-79.
Slapsinskaite, A., Garcia, S., Razon, S., Balague, N., Hristovski, R., & Tenenbaum, G. (2016). Cycling outdoors facilitates external thoughts and endurance. Psychology of Sport and Exercise, 27, 78-84.
Mieras, M. E., Heesch, M. W., & Slivka, D. R. (2014). Physiological and psychological responses to outdoor vs. laboratory cycling. The Journal of Strength & Conditioning Research, 28(8), 2324-2329.
Brooks, K., & Brooks, K. (2010). Enhancing Sports Performance Through The Use Of Music. Journal of exercise physiology online, 13(2).
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