Movie Name: Naan Ee (I am the Fly). Originally made in south Indian languages of Tamil 
and Telugu and later dubbed into other languages. The movie was a huge success.

The story goes like this. 
Young hero was in love with heroine. A rich tycoon gets attracted to her and he became jealous on the young man and kills him to win her. The young man takes rebirth as a house-fly and starts chasing the villain to take revenge.

Subsequently, the fly manages to convince the girl that he is her former lover and that the tycoon killed him. The girl, who is a micro artist (who can make a sculpture out of a rice grain and the like), teams up with the fly and make plans to kill the tycoon. As the first step, she starts to build up fly's muscle strength, and the cassette treadmill is one of the exercise machines she made for the fly.

Video as available at the time of posting (watch at 01:30 and 02:14):