Cardio
Cardio is a tool that can be implemented at any time. And it’s not just meant to lose weight body fat. Cardio also can be used to improve your cardiovascular conditioning which in turn will allow for you to breathe easier in general and during your training. And the more oxygen we can deliver to the muscles when training the more we can withstand and push. There are two types of cardio, steady state and hiit. Steady state simply means your doing a consistent low/moderate pace at which your body will use fat as its primary fuel source. Hiit or high intensity interval training will overall burn more calories but some of those calories will be glycogen. Now depending on your glycogen storage you might be able to supply this demand, but if you don’t have enough glycogen on hand this is where your body has the potential to break down protein and or muscle tissue and convert it to glycogen in order to fuel your energy requirements. The time when hiit is more appropriate is if you're severely overweight and the goal is too just overall lower total body weight as fast as possible to improve health conditions. Other than that steady state should be your go to cardio when it comes to dropping body fat and retaining muscle tissue. Now when varies from person to person. Now when it comes to fasted cardio you can never go wrong, As long as you don’t eat or drink anything(other than water, lemon water, acv) your body is going to favor fat highly throughout your cardio session. Now post workout can be argued that it has the same effect because of the fact you have the potential to burn through your carb stores while training. The problem is a lot of people don’t train hard enough to drain those stores, also some people take carb powders intra workout. This is feeding your body carbs while your training to increase performance. A lot of people will argue that those carbs go to training but alot of the times the amount of carbs taken in during training is more than needed so when going to do post workout cardio you might be burning some fat but not anywhere near as much as if you weren't loading with carbs while training. This doesn’t go to say drinking intra carbs is bad because it does have an effect on performance because it’s immediate energy but say you finish your carbs right at the end of a session then you hop on a piece of cardio equipment, chances are you still have alot of carbs shuttling around your body. So if your training hard and not consuming carbs during training, post workout cardio might not be a issue for you but to eliminate any issues with what is being used a primary fuel source i'm a fan of fasted cardio. The amounts vary from person to person. Some people need more and some need lees depending their goals and where they currently stand. But everyone can do some type of cardio a few times a week if not daily and see not negative effects to muscle tissue. 30m of steady state stationary cardio, riding your bike or even taking your dog for a walk will have some type of positive benefit to your cardiovascular system and physique.