> Your legs are moving but your head / vestibular system isn’t experiencing acceleration that it expects with those leg movements
That's also true on a treadmill. I think the bigger problem would be that the world appears to be moving (unlike a treadmill) and you don't feel any acceleration.
You don't get motion sick on a treadmill because you're not wearing a VR headset. There's no forward motion of your head captured with your eyes to mismatch what your vestibular system is feeling.
To add to your point, there is a somewhat unpleasant sensation once you get off the treadmill, i.e., even if you are standing in place, it feels like you are moving forward.
This is incorrect. You don’t get motion sick on a treadmill because the small accelerations and decelerations match what your eyes are seeing. There is motion.
If you actually use a treadmill, or watch motion capture, you’ll see that this isn’t the case. It’s a lot of slight accelerations and decelerations. The variation change from person to person, and more experienced runners are able to minimize them, but they’re still present.
> It’s a lot of slight accelerations and decelerations
Isn't that always true? If I pay attention to it, I notice a lot of tiny movements of my head, as I just sit here typing.
"You don't have acceleration" is just talking about the big accelerations from 0 mph to roughly 3 mph to 0 mph, not implying that you're completely motionless as if frozen in ice.
Acceleration is change in velocity and change in time, either can effect the apparent amount of acceleration. The velocity you accelerate to or from is just an implementation detail.
That's also true on a treadmill. I think the bigger problem would be that the world appears to be moving (unlike a treadmill) and you don't feel any acceleration.