Well, you can sit on a treadmill but you can't run on a chair (at least very well).
Depends i'd say, whether it is an active treadmill.
Walking or running and getting nowhere. I have a feeling that treadmills have some symbolic value for hopelessness or being stuck in a rut.
That's probably why it is a reocurring theme in nightmares.
@The Stranger or being like a hamster
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