According to a study out of Ohio State University-
Two groups of patients with broken arms were researched and analyzed--one group was told to imagine themselves exercising and the other group was told nothing. After the casts were removed, those patients who were asked to imagine themselves exercising had twice as much strength in the now healed arm than those who did not imagine exercising at all.
"The imagination of motion, which occurs in the same part of the brain where the impulse to move originates, does have physical benefits."
Of course, this is not a substitute for actual exercise, but imaginary exercise has proven to show improvement in performance of those who have highly physical jobs such as athletes, musicians, surgeons, etc.
So go home, sit on your couch, and imagine yourself curling those 45 pound dumbbells. (Then get up and get moving for real!)