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. Recently Zwift 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, his explanation overall just didn’t make sense. Most notably, Allen’s claim that power is 20-30 W lower indoors; that’s about 10% for most of us. That’s massive, and highly circumspect. 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 “feels different” mechanism is meaningless, 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.
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.
What are your thoughts? Drop me a line or comment!