Based on the level of NEAT -how much cardiovascular exercise needed will vary with each person. NEAT is Non-Exercise Activity Thermogenesis. NEAT is the energy expended for everything we do that is not sleeping, eating or sports-like exercise. It ranges from the energy expended walking to work, typing, performing yard work, undertaking agricultural tasks and fidgeting. Even trivial physical activities increase metabolic rate substantially and it is the cumulative impact of a multitude of exothermic actions that culminate in an individual’s daily NEAT. So it is not surprising that it varies person-to-person.
If the objective is to build muscle, steady state cardio is not necessary because the plan would not be to burn calories but instead build lean muscle mass. Building lean muscle mass requires that you are in a caloric surplus. However, HIIT cardio could be beneficial when the objective is to build muscle. This is because the level of intensity will be greater at a shorter period of time. Cardio done at a higher intensity for a shorter period of time will not only help you maintain your muscle, but can actually help you build muscle mass. When you train at a slow and steady pace for a longer period of time, you are training your muscle fibers to be more aerobic and have greater endurance- you’re stimulating SLOW twitch muscle fibers.
Think about it from a common sense perspective. The statement that slow and steady cardio for longer periods of time is best for maintaining muscle mass is similar to saying that curling 5-pound dumbbells for 30 minutes straight will build more muscle than curling 40 pound dumbbells for sets of 10 reps with rest between sets