The answer to this question lies in what your specific goals are. If maximizing muscle gain is the goal then you only want as much cardio as you need to help your stamina inside the gym. If your goal is to complete a marathon, well then cardio is going to be your primary focus.
It’s a Matter of Overkill
The issue isn’t a matter of being a good form of exercise or not. Cardio is great. It increases your endurance, produces more mitochondria, helps with your VO2 max, and increases your overall aerobic capacity. Cardio is very good for the heart.
The issue I see too often these days is that cardio is no longer seen in that same way. Let’s put it this way. Lifting is seen as a form of exercise to get you bigger and stronger. Yoga is seen as a form of exercise to get you more mobile and flexible. Cardio SHOULD be seen as a form of exercise to strengthen your heart and build your aerobic capacity. For the general population it isn’t.
It’s instead seen as this one and only tool to burn the most amount of calories in the shortest amount of time so you can (hopefully) lose weight. This completely abuses the idea of cardio. Not only that, people will do hours upon hours of cardio weekly thinking more is better. What they are really doing is driving themselves into the ground.
Cardio Burns Less Calories
In the beginning, yes. Cardio burns more calories than say weight lifting. Over time, your body gets more and more efficient at burning LESS and LESS calories over time. It wouldn’t make sense for your body to not learn to get more efficient at burning calories from a survival standpoint. The end result of this is, people will stop seeing the scale move, and have to add even MORE cardio just to elicit the same rate of fat loss. You are working against yourself, and it only gets harder the more you do it.
Resistance Training Burns MORE Calories
Lifting weights on the other hand does the opposite. It may burn less calories in a given hour (but let’s also keep in mind that is because you spend most of your session resting vs lifting weights), but over a longer timeline it will burn more calories. Muscle is calorically expensive to have. The more of it you have, the more calories you burn at rest! Doesn’t that sound so much better?
Check out my article Resistance Training is the Key to Long Term Fitness Success as a Woman.
Cardio Burns Through Muscle
Not only does resistance training increase your metabolism, but it preserves or continues to gain muscle. Cardio does the opposite. Running is not a strong enough muscle building stimulus to grow or even maintain muscle. Add on top of that being in a deficit of calories like on a diet, and you are just asking your body to burn through muscle and hold onto fat. Remember, your body is built for survival. It would prefer to hold onto fat and burn through calorically expensive muscle if you aren’t using it.
If you are focused on building muscle, limit the amount of cardio you do to 2-3 days a week. If you do too much, you are now sending conflicting signals to your body as to what to prioritize. Your overall goal should be “how can I get my body to burn more calories on its OWN?”
How Cardio Should Be Used
Here would be the ideal setup. Your main focus regardless of goals should be to add 2-3 days of resistance training for overall strength and wellbeing. Whether you want to get big or not, we need our strength especially as we get older and become more prone to loss of bone density and muscle mass. In fact, it’s the leading cause of injury. So our focus needs to be keeping the body healthy and strong, while also building that metabolism back up so you can eat more calories.
Once you’ve incorporated that, unless running is your main focus, incorporate a movement goal vs a cardio goal. Figure out how many steps you currently average out and add 3,000 to that. Make it your goal every day to hit that amount of steps. So if my watch said I was only moving 7,000 steps a day, now my goal is 10,000 steps. Go for 10 minute walks after each meal, or park a little farther at the mall. What this does is it changes the way we see cardio. Instead, it becomes a healthy lifestyle habit, where you are finding reasons throughout your day to get up and move instead of being sedentary. This is huge!
Once you’ve hit that goal, THEN, and only then would I say it's even necessary to add cardio in the form of additional running outside or on the treadmill. We need to change our relationship with running, and turn it into a more positive habit you can sustain 6 months from now. Any additional cardio will be because you couldn’t hit your goal, or because you simply enjoy doing it.