Have you ever struggled to achieve the same indoor and outdoor power outputs?
If so, you would not be alone.
This begs the question if indoor and outdoor power output does indeed differ. And if so, how to we work our way around this discrepancy in controlling training intensity?
A study by Lipski and colleagues in International Journal of Sports Physiology and Performance took an interest in this question (1).
Is indoor and outdoor power output the same?
Lipski and colleagues wanted to see if indoor and outdoor power output differed across a range of efforts from 1 to 14 minutes.
To investigate the problem they put 14 male cyclists from UCI World Tour and UCI Continental level through a series of maximal effort tests. Test durations were 1, 3, 5 and 14 minutes and were completed indoor and outdoor over four separate sessions.
The outside tests were performed at a slight uphill with 2-3 percent incline.
What did they find?
There was a significant difference in power output between indoor and outdoor tests.
In summary, mean maximal power (MMP) during tests were 4.2-8.8% lower in the indoor setting compared to outdoor. These differences were statistically significant (meaning they likely represent a true difference, as opposed to being caused by chance).
For a more detailed illustration of individual rider responses, see the below graph off of the original paper (embedded from Dajo Sander’s twitter).
As the illustration demonstrates, there were considerable individual differences in how much power differed in the two test conditions.
One subject had a difference between indoor and outdoor tests of 152, 48, 37 and 36W for MMP time durations of 60s, 180s, 300s and 840s, respectively…”Lipski et al, IJSSP 2022
How does this affect training intensity?
To illustrate how the above power changes affect exercise intensity estimates, the authors calculated separate critical power (CP) estimates from indoor and outdoor results, respectively.
Critical Power is often described as the highest intensity without a progressive loss of homeostasis (2). For the sake of argument, think of CP as a value close to your functional threshold power (FTP). The only difference being that CP more accurately correlates with the physiological turn points between different energy systems.
The indoor trials resulted in a critical power of 330W. Whereas the critical power estimate from the outdoor trials were 348W. In other words, an 18W and 5.5% difference. Again, the difference was statistically significant.
This finding is relevant with regards to your exercise intensity.
Imagine doing a long indoor threshold interval session with 5×10 minutes at close to 100% of critical power (or FTP), based off of a power estimate that is 18W too high (off an outdoor test).
Anyone who has ever performed a threshold interval session will testify that such a difference in power output will make a big difference to the perceived effort of the workout. It is the kind of difference that may well push an intended moderate intensity workout up into the high intensity realm.
The same can be true in the opposite direction, leading to underestimating your intended exercise intensity.
Why does indoor and outdoor power differ?
The short answer is, we do not know.
However, the authors do engage in discussing potential reasons.
Firstly, they found no difference in air temperature and sweat rate between indoor and outdoor testing. As such, temperature did not seem to affect the results in this study.
Secondly, they did find a difference in self-selected cadence between the two test scenarios. Specifically, cadence was 5-6 RPM higher indoors compared to outdoors. This could indeed be a contributing factor, as studies have shown that elite riders perform better at their most efficient cadence.
Beyond this, it is probably too soon to suggest with certainty why indoor and outdoor power differs so considerably.
The main take-away from this study is that indoor and outdoor power is not interchangeable.
As such, it is probably wise to consider doing new power tests whenever changing from indoor to outdoor training. And vice versa.
- Lipski ES et al. Differences in performance assessments conducted indoors and outdoors in professional cyclists. International Journal of Sports Physiology and Performance, 2022;17(7):1054-1060
- Jamnick NA et al. An examination and critique of current methods to determine exercise intensity. Sports Medicine, 2020;50(10):1729-1756