This article was previously published in January of 2016. With the recent uptick in indoor training, it was worth updating.
There is a commonly reported complaint about indoor wind trainers; power output is lower than the road. While Zwift is an awesome indoor training platform, the aforementioned comment has been showing up a lot in my feed, along with suggestions on why power is different indoors. Zwift had previously posted a video by coach Hunter Allen, who provided his own explanation for this discrepancy. For the sake of brevity and transparency, I recommend watching the video yourself. While some of his insights were correct, I found his claim that power is 20-30 W lower indoors dubious at best; that’s about 10% for most of us. That’s massive, and highly circumspect. I was also left wondering if he, perhaps, meant to say higher, not lower, which would be more plausible. Here’s my breakdown and counter-points to his explanation: 1) There’s no evidence that power is consistently 20-30 watts (10%) lower. That’s a huge impact in one direction, and one I’ve never seen; 5-10 W is more plausible. Moreover, I’ve got enough data to show that power could be higher, lower, or actually not much at a given effort.
2) Power is a work rate over the whole pedal stroke. In simplest terms, if it takes me 1 sec to complete a revolution and I generate 20 Nm in one direction or 5 Nm in 4 directions, my rate of work is the same.
3) The “feel’s different” mechanism is over-rated, and cannot account for a difference of 20-30 watts. If that were the case, we would see a feels different response more often, including most notably, in the lab on big ergometers. We do not. Again, it’s more likely the different feel is an effect of the response, not the cause.
4) Work output should be largely independent of psychological factors. I could imagine difference of 1-2% for a placebo/nocebo effect, but not 10%. If your head is losing you 10% you’ve got some serious issues!
5) Related to 1, power could be higher or lower due to a greater resistance which causes you to increase force (torque) at a given cadence (e.g., lower tire pressure, more pressure on tire-drum contact). In other words, you try to pedal at a common cadence but work a lot harder, and hence fatigue. The opposite could also be true.
6) If you train at a power 30 W lower, then you are training at a lower load, period! This is a problem with training in super hot weather or altitude. If you train at altitude all the time you actually detrain.
7) You can and should train with and without a fan. The added heat load has beneficial adaptations.
Issac Newton may have the answer!
In summary, while I think Hunter hits many of the right points, his explanations fall short. Moreover, riding indoors is more akin to riding up a long climb, which as most attest, yields a higher average power output. And that is what I believe we’re ultimately talking about. On the road, even a flat road, Newton’s 1st and 2nd Laws help us understand the major factors involved here, namely inertia and momentum. On a trainer, or riding uphill, both are more stable, thus we need to input a constant effort to maintain our speed. In contrast, on a flat or rolling road, this is changing constantly, resulting in an ebb and flow of resistance and thus power. Need more evidence? Consider normalized power (NP), which accounts for the highly variable power readings during road riding. Ever notice that NP is higher than average power on most road rides, but the same or even lower on a long climb or riding indoors?
The reality is that power indoors is likely higher due to the constant resistance and lacking coasting (i.e., no zero’s), but should not be in the 10% range. However, I will agree with Hunter that we cannot assume that indoor and outdoor training prescriptions are equivalent. However, by actively rating your exertion, you can make those adjustments on the fly, and then look back at your NP to see what the difference was!
What are your thoughts? Drop me a line or comment!