Just turn one upside down and turn the peddle and you will see. The rotation of the peddles transferrs rotation to the wheels by a chain. The sproket onthe wheel is smaller so your feet don't have to move too fast.
It takes about 1/10 pf a horsepower to run a bike 15 MPH. How much power can you make ?
Time yoursefl running up the stairs and use this formula...
HP = number of steps x height of one step x your weight / time / 12 / 33000