I was just thinking the other day “why does an exercise machine need a motor”? Shouldn’t YOU be doing all the work? Wouldn’t it be more of a workout if there were no motor assist? - Jinx 
You see? I knew it all the time! 
The one at our fire station has two motors-one is the belt pace motor, and the other in the incline degree motor.
WAG: the motorized ones will keep you walking/running at a steady pace.