Several studies now show that dogs can be powerful motivators to get people moving. ( I write this as my 2 year old lab paces back and forth, what we fondly call her "sharking", as we wait for the pavement to cool for her evening walk).
Check out this great article from the NY times: http://well.blogs.nytimes.com/2011/03/14/forget-the-treadmill-get-a-dog/
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