Heart Rate and Keto

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Hey :) So I just have a question and I hoped someone could answer.
This is for people doing the Keto Diet (Note: not Atkins just a general Keto meal plan) who eat very few carbs and so don't have a lot of stored carbs.
When doing my Zumba classes I tend to reach and stay at a heart rate high for several minutes that is well past the "fat-burning zone" so in a person with a balanced diet this would cause the body to burn stored carbs instead of stored fat. I'm just curious being as people doing Keto don't have a lot of stored carbs, does the body continue to burn fat or will it burn muscle?
I just don't want to make progress then watch my new muscles burn away.