If you use power for running or cycling, one common issue that can come up is differences in your data when comparing indoor training to outdoor training. While this post will focus on bike power, read below to learn a few of the reasons why this issue occurs and some ways to help minimize the impact on your training.
What are the impacts, and why is this important?
Let’s say you do an FTP test inside via your smart trainer and determine a value of 300 (watts) from that data. You then use that figure to calculate your zones into an app like Training Peaks, which incidentally only supports one FTP value for a road bike and has no separate settings for indoors. Using that information, you ride inside all winter, and then in the spring, you start riding outside consistently using your bike power meter, which (unbeknownst to you) measures your power values lower (or possibly higher) compared to your smart trainer. You now do a structured workout on the road and either can’t hit the targets because they are too hard or find that they are way too easy. In both cases (low or high), you are now working a different training zone than the workout called for and not getting the proper stimulus. So how do you fix this? The simplest way is to do another FTP test outside with the bike power meter and get an updated threshold power value. You can then use this information to reset your zones from that data, but it still leaves you with the issue of managing two different FTP values, plus potential inaccuracies in calculating fitness metrics such as TSS, CTL, etc… To figure out your best approach to dealing with this issue, let’s first examine why the values may differ.
The primary reason this occurs is obvious, two different power meters are being used, which is usually the most significant reason you will have a power mismatch. Typically in these scenarios, a rider uses a smart trainer indoors and a bike power meter outdoors (i.e., pedal, crank, crankarm). While the style and brand of the power meter will impact the accuracy, most newer quality power meters usually measure within 1-2% accuracy, plus you also need to factor in where they register the power from on the bike. However, in all cases, to attain the most accurate reading and minimize device differences means you are also running the proper procedure to zero out, zero offset, or “calibrate” your meters. This action sets a value related to the state of no load being applied on the meter (remember, bike power meters are strain gauges that measure force/torque). To perform this step, follow the manufacturer’s recommendation and apply the setting through your bike computer or a mobile app. For more on zero offset and calibration, check out the article by Cyclist Hub, where the differences are summed up nicely. This step should be done periodically, regardless of method or device, especially when taking your bike off and on a trainer.
How can you check if your power meters are reading differently? If it is not easily discernible through everyday riding, I recommend you dual record a workout and compare the two files. You can do this by recording one meter on your bike computer and the other via Zwift or another 3rd party app. This method will give you the best indication of how well the two meters track. An excellent tool for this is the DCR Analyzer – here is an article on how to use it.
Screenshot comparing a client’s Garmin Vector 3 pedals to a Wahoo Snap trainer. In this example, meters aligned well at high intensity but not at lower intensities.
Remember that even highly accurate meters, which may be from different manufacturers, and may additionally be measuring power from different areas on the bike, will not align perfectly. Nevertheless, if you remember to do the zero offset process often, they should be reasonably close. If they are not, I usually recommend to my athletes that if they are using a smart trainer and an app like Zwift, to use their bike power meter as the power source and the trainer as the controllable source. This method allows you to have all your power data recorded from the same meter, and while it will still have some differences compared to outside, it may be the best balance in minimizing the issue.
Screenshot of Zwift setup using a pedal-based meter and a smart trainer.
Is it ok to have two different FTP values?
You may have various reasons to maintain two values, including a dedicated indoor bike or a standalone smart bike (Stages, Wahoo, Atom). Keep in mind, though, that no matter what the FTP value is, it loses the “functional” impact when it is not established correctly. While you can maintain two values across bikes and indoor vs. out, the method for establishing those values on each meter must be sound. Therefore improvements and progression (done through proper training) are likely accurate regardless of having to maintain the two values. Just remember, their can be a downside to regularly switching back and forth between power meters and using two different FTPs. Metrics like TSS (Total Stress Score) may report either higher or lower than prescribed since most training platforms only support creating power zones based on a single sport or bike. In this case, you are still working the correct zones (again, as long as the FTP values are set correctly for each device) but having to deal with the extra headache of accurately maintaining your post-ride data to match prescribed values.
Not sure how to test FTP – I can help!
Why does the difference exist?
As stated prior, I do recommend, whenever possible, using the same meter inside and out to reduce the variability between devices; however, this may still not reduce all the differences in power. Many articles have indicated that this difference between the power you can generate indoors vs. outdoors occurs due to environmental conditions such as heat and humidity. However, a recent research article has thrown a wrench into that theory. According to the authors, Lipski et al., while there were statistically significant differences (specifically lower power indoor vs. out) in an elite cycling population who performed maximal tests ranging from 60 secs up to 14 mins, it was not due to sweat rate or air temperature differences. This result demonstrates that temperature may not be the reason behind the inability to generate equal power indoors. Further, while the authors conclude that indoor and outdoor values (notably CP and W’) can not be used interchangeably, there appears to be no clear conclusion as to why the differences in power occur. Unfortunately, my request for the full research article has gone unanswered so far, so I can not say if their methods had any limitations or how it can be applied to minimize the differences. However, a good deal of research still points to core temperature negatively impacting prolonged performance and power output. Considering my own anecdotal experience and subjective feedback from dozens and dozens of clients, all of whom report universally better performance when keeping their indoor training space cooler. So it may be that short-duration, single-interval power is not impacted, but prolonged and/or repeatable power is. Therefore, keeping yourself cooler indoors may indeed allow for better-sustained power output, in turn, better resembling what you can generate outside. Certainly, this is an area that requires more research in the future.
Besides core temperature, there may be other issues that impact the values, again, regardless of the power meter. These variables include the slight biomechanical differences between being locked into a stationary trainer vs. the body’s more natural movement outside and possibly even mental aspects such as motivation, perception, or neural drive.
Hopefully, this post gives you some insights into why there are differences and ways you can mitigate the impact when training both inside and outdoors. In the end, if you are unsure that your values align, remember the saying, “When in doubt, test it out!” That way, you can ensure you are going off the most valid numbers for your situation and tracking improvements. In the meantime, stay cool! 🤣