High Intensity/Cardio Workouts - For People who have knee issues and/or live in an apartment

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I know it's possible, and I have been modifying workouts to suit the fact I live in an apartment and also have been advised to not do any high impact exercise because of my knee....but I wondered if it would be an interesting challenge to create a workout which is low impact but still high intensity and/or a good cardio workout....I love getting a sweat up, so find the level 3 difficulty a little pedestrian in some cases......just a thought...love your work BTW - Thank you :)