How do you become the #1 fitness trainer on YouTube? Mix equal parts sex appeal, production values and brutal at-home workouts - and you get married couple Zuzana and Frederick Light.
Their 12-minute routines regularly rack up millions of views. Their top workout has reached almost 9 million plays.
We asked Zuzana and Frederick about the rigors of staying in killer shape and managing a monster YouTube fan base.
Credit: Bodyrock.tv