when does the body start to break down muscle for energy?

when does the body start to break down muscle for energy? is it after all your glycogen stores have been used? i know also it depends on what type of activity one is performing that dictates the order in which muscle protein is used for energy, but is muscle the last used energy source when the body is at resting? is that why ketosis works? the body will try to use carbohydrates, but because the body only has fat and protein to use, it uses fat, thus preserving muscle? i hope i don't sound ignorant, im just trying to learn some physiology.

your body burns fat at rest, glucose and glycogen during intial strees(depends on intensity of how long until depleted) then your body moves to fat. Thats why you hit a wall during cardio. you reach a point where the glucose is gone, you start to become weak for a little bit because there is no glucose and the process to turn fat into glucose is a longer process. but once the fat gets converted to glucose you get a second wind. after awhile your body can't turn the fat into glucose and starts breaking down muslce tissue through catabolism and uses the glucose from that for energy.

muscle is not used for energy until all the other possible glucose and gylcogen sources are depleted.

When you enter the "death zone" climbing mount Everest.