WILL CARDIO BURN MUSCLE?? To put it simply... NO!
High intensity cardio will not burn muscle for 99% of the population! In fact, programs incorporating high intensity cardio work incredibly well with weights (aka strength or resistance) training.
There are 4 states we want the body to achieve: fit, strong, flexible and relaxed. Many regular gym users will focus on just the one: strong. Ignoring just one state will cause issues for your body. But there is a common misconception about cardio that bodybuilders have held for years.
If you are a professional bodybuilder with less than 6% body fat, AND you are not eating carbs, your body may convert protein from the food you eat into energy for fuel, rather than repair and maintain muscle tissue. But if you don’t meet both these conditions, you have nothing to worry about!
The fear is often based around the fact that cardio will help to remove fat from the body. Fat is 3x the size of muscle, so as the fat disappears, the body will visibly shrink. Many bodybuilders have worked hard for months or years to 'bulk up' and the thought of shrinking is horrible!
If your goal is to get as big as possible, which will include fat as well as muscle, then by all means stay away from cardio. If you'd rather just look 'buff', get a 6-pack, and have all those lean hard muscles on show, then will cardio burn muscle and waste all your previous gains? No, cardio is a recommended addition to your exercise regime. And cardio will provide the 'fit' state for your body as well... so you just need to sort out the other 3 🙂