There are a few things that change when you ride indoors on a trainer as compared with on the road. A common one is heat. Since you’re not moving forward when on a trainer body heat tends to accumulate. Your core temperature may rise causing you to sweat more. At the same time, blood is shunted to the skin to seek some cooling. This dual demand for your blood—providing energy for the muscles while also cooling—causes you to work harder thus wasting energy. This problem is easily solved—if you have enough fans.
And there are other energy wasters when riding a trainer such as the bike not swaying side to side as you pedal. On the road it does, albeit only slightly when seated, but a great deal more when standing on a climb or sprinting. But what happens indoors is your bike remains stationary while your body moves slightly side to side. That also wastes energy.
Of all the indoor energy wasters the most uneconomical has to do with pedaling skill. This changes a lot from what it is like on the road and can result in a significant decrease in power output. Your functional threshold power (FTP) may be decreased by 10% or more.
Pedaling skill has a lot to do with the “dead spot” in your pedal stroke. This occurs when the pedals are 12 and 6 o’clock in the rotation. When pedaling you come to this position twice in every pedal stroke or about 10,000 times per hour. During this brief but frequent interlude while riding outdoors on flat terrain the rear wheel continues to rotate because of momentum. But when on some trainers—especially friction, fluid, and magnetic trainers—the rear wheel immediately slows down when the pedals are in the dead spot since there is no tension on the chain. It’s not a gigantic slowdown but it’s enough to waste energy because you have to apply a great deal more force on every stroke to keep the pedals turning to maintain your power and the speed of the rear wheel. And that’s happening about 10,000 times per hour. That makes for a lot of wasted energy. But if your indoor training machine has a flywheel then you’re much less likely to see any change in economy when indoors versus outdoors. The weight of the flywheel boosts the momentum of the rear wheel so that it behaves more like it does when on the road.
If you don’t have a flywheel, over time your pedaling technique will adapt and slightly “correct” how you pedal. You’ll learn to engage the pedal earlier in the downstroke. That could take a few weeks. In the meantime your training is less effective. But it’s possible to speed up the adaptation by consciously adjusting how you pedal when indoors. The good news is that this new adapted pedaling technique will also improve your road-riding energy economy.
In Part 2 I’ll get into the “how-to” of improving your pedaling skills for both indoor and outdoor riding.