Have you ever noticed that, after stopping using a treadmill and dismounting, the floor feels like it is still moving underneath your feet? It almost feels like you are skating.
What exactly is this effect, and why is it peculiar to treadmills? Why don’t you feel the same effect after running a short distance around a track?
I can’t give you a firm answer, but it might have to do with the conflict between senses. Your legs tell you you’re running, but your eyes say you’re not moving. Your brain adjusts to that paradox, and then has to re-adjust when you step off the machine.
I have felt similar reactions, myself. Sometimes, when getting off a sailboat onto terra firma, it feels like the ground is moving under my feet. Also, when I run on a treadmill, I have trouble keeping my balance.
I don’t think it’s only treadmills. I fall down if I use one of those gazelle thingies and then step off. It completely disorients me. I’ve never used a stairstepper, but I would expect the same problem.