Why is Riding on a Trainer Harder?
The main reasons for this are as follows:
1. Rolling resistance is different
The flywheel will decelerate your bike a lot more quickly than the road. This requires constant pedaling, which requires constant pedaling, which is an increased level of muscle activation.
2. Reduced rate of convective cooling
On a trainer, the usual source of convective cooling is provided by a fan. When riding outside, the athlete is generating "wind" that is equal to their speed moving forward. This overall reduction in your ability to shed body heat (reduce core temperature) will increase the RPE of any session.
3. Lack of "moving" or "shifting" around on the bike
While riding outside, we are constantly shifting in position for various reason (comfort, change of intensity, terrain, traffic, etc...). When we are on the trainer, those small and often subtle shifts are not required, leaving "comfort" as the main reason to change body position. As discomfort in the seat area increases, so does RPE.
4. Lack of "moving" or "rocking" the bike
When we are outside, we move the bike underneath us. Not only in a forward fashion by pedaling, but from side-to-side. When we do this outside, it changes the recruitment pattern of the muscles being used, to include activating additional muscles that are not used while "sitting" on the trainer.