I apologize for the cryptic title. There is a purpose to it. Treadmills are clearly a vehicle for exercise. The term is also used in another context–the hedonic (happiness) treadmill.
Let me address that more obtuse reference first. Also known as hedonic adaptation, the hedonic treadmill refers to the human tendency to return to baseline levels of happiness in the face of major positive or negative events in their lives.
The implication is that those who seek happiness predominantly through the acquisition of material possessions may find a temporary jolt of enjoyment but will ultimately return to their baseline level of contentment in life.
The treadmill analogy implies they are running in place. They are not "getting" more happiness.
This is a concept heavily endorsed by the Positive Psychology movement. Which also notes that our personal levels of happiness seem to be genetically determined (50%), determined by outside circumstances (10%) and by our personal choices (40%).
There is much more to explore regarding this subject but I will address the more conventional treadmill. A recent NYTimes ( July, 6, 20011 ) by Gretchen Reynolds addresses this issue. She refers to both human and animal researchers who have demonstrated that regular exercise reduces anxiety and the tendency for depression.
The rat studies were particuarly interesting in that rats which were pre-treated with regular (not excessive for them) exercise were more "stress resistant" when exposed to bullying rats. They exhibited far fewer neurotic anxiety type responses.
Ah so what can we take away from the two treadmill analogy?
For certain, regular exercise changes our brain's reaction to stress.
Secondly, the hedonic treadmill concept may reveal to us the futility of seeking happiness through purely material possessions.
Now no one argues that poverty is a desirable state of being. But the excessive fixation on the world of "things" does not bring the contentment we seek.
So be quick to jump on the first treadmill, beware of the other.