[chrisseaton]

You are born onto the earth, so your reference point starts as the earth.

In reality the earth is moving through the solar system, so your starting reference point is in motion already.

If you get into a vehicle and start moving at speed relative to the earth, then that vehicle becomes your new reference point when the speed becomes stable.

When you step onto a treadmill your speed relative to the earth, your previous reference point, doesn't change, so the reference point doesn't change. If someone were to drag the treadmill behind a vehicle with you surfing on it, then it would become your reference point.

It's confusing to talk about your inner ear being in your feet because they're in a running motion, so they're being shaken up all the time. They'll experience constant acceleration and deceleration relative to the earth as your foot goes back and forward in a running motion. If you had little dice on strings in your feet you wouldn't be able to tell what was going on with them as the dice would be flapping all over the place.