High Intensity Interval Training (HIIT) is rocket-fuel to your muscles which is why the Rocket Fuel workout is here to help you reach new performance levels. Because you're always chasing your own performance horizon there is never an easy setting on this, though the difficulty level is subjective. Do it in a half-hearted way and you instantly reduce the benefits it will give you. Go full-out in each 20-second burst and you'll soon feel the burn and the benefits.