Treadmills Suck: The Effects of Cardio on Muscle Growth

Lets face it, no one really means it when they say they love doing cardio. I get asked questions regarding the effects of cardio on muscle growth; is it good or bad to do cardio when you are trying to build muscle? Lets go over some of the effects cardio has on your muscle growth.

Cardio – Aerobic exercise that extends over a period of time.

Energy Expenditure

When you do any type of cardiovascular training, your body burns energy in this order:

  1. Carbohydrates – Stored as body fat and directly from foods
  2. Fats – Stored as body fat and from foods
  3. Proteins – Muscle burned for the amino acids

To preserve muscle, you want to burn only carbohydrates and fats; when your body starts burning protein for energy, you start burning muscle for energy.

Benefits of Cardio for Muscle Building

  • Improves metabolism
  • Helps burn fat
  • Improves appetite
  • Maintains or improves conditioning
  • Improves recovery from strength training

Improves Metabolism

Your metabolism is very important when it comes to building muscle and losing fat. Cardio stokes your metabolism and will burn fat and build lean muscle. Metabolism is the most important thing to control on a drug-free athlete because it determines how well you lean up and how much muscle mass you can build and maintain on a cut.

Helps Burn Fat

You must keep your heart rate elevated to burn body fat while doing cardio. There are many magical formulas saying 50-80% of your maximum heart rate is best, or you only do cardio in a fasted state. All of this is valid and studies support these claims, but for all intents and purposes, if you are breathing heavy but able to carry on a conversation still, you are working hard enough.

Improves Appetite

Studies show and personal experience shows that cardio will improve your appetite. There are also studies showing cardio will reduce appetite, but I have not run across anyone who claims that. Improving appetite is important because the more healthy foods you eat, the more muscle mass you will gain. I always suggest to people who have problems eating enough to gain weight to do a couple 20 minute cardio sessions a week. This has proven to increase appetite and helped people eat more calorie dense foods to gain weight.

Maintains or Improves Conditioning

Conditioning is something everyone should work on. If you have to go up a flight of stairs and it takes 5 minutes to catch your breath, your conditioning is not very great. Cardio increases cardiac output and efficiency. Your heart is a muscle and working a muscle makes it stronger.

It is harder to preserve muscle and improve conditioning at the same time because high intensity rowing, running, or sprinting are all exercises that burn a lot of energy.

Improves Recovery from Strength Training

Cardio raises your heart rate and this pumps more blood and nutrients through your body. Low to moderate cardio is a great form of active recovery. When you use cardio for active recovery, you give your muscles more nutrients by increasing blood flow and this increased blood flow also helps flush lactic acid.

Drawbacks of Cardio for Muscle Building

  • Burns calories that could be used for muscle growth
  • “Over-training”

Burns Calories That Could be Used for Muscle Growth

Cardio burns calories and if you do not eat enough to cover the calories burned during cardio, you have a chance of stalling your muscle growth. If your sole purpose is to build muscle and you are not eating enough food to stay in a calorie surplus, you will not build any muscle.


Over-training is a topic I try to stay away from, but there is a correlation between muscle building versus the amount of stress you have. Stressors include anxiety, depression, or physical exercise, so when you push your body to the max doing strength training, add in long sessions of cardio, and aren’t getting enough food and sleep, your body won’t be able to recover fully.

This sort of over-training stalls your progress and could lead to getting sick. Unless you are able to add more rest, food, and relaxation to your life, there is a possibility of over-training if you add in too much cardio.

If I want to only build muscle, why would I want to do cardio?

There are many benefits to adding cardio to your routine regardless of your goals. Light to moderate cardio for 20 minutes post work out 2 days a week will improve your recovery, help your metabolism, and increase your appetite. This means that not only will you burn bodyfat but you will build muscle and improve your conditioning at the same time. If you do not want to lose any more body fat, eat extra calories to make up for the extra calories burned.

If you are looking to burn body fat and limit muscle loss, ensure your diet is up to par and you are eating plenty of protein. Light to moderate cardio for 20-30 minutes post work out or 3-4 times per week is going to be a great start. You will burn a moderate amount of calories and keep from using your muscles for energy. Cardio combined with a great workout routine and eating plenty of healthy foods will give you great results..

If you are very overweight and your main objective is to lose as much body fat at a safe rate willing to sacrifice some muscle mass, I invite you to try to walk for an hour every morning in a fasted state. Studies suggest that doing cardio in the morning in a fasted (nothing eaten since dinner) state, your glycogen and carbohydrates are lower so your body will burn body fat for energy. Walk at a moderate pace that you can keep for an hour; going ‘all out’ and pushing yourself too hard will result in sub-optimal fat burning.

What about high intensity interval training?

High intensity interval training or HIIT for short, is a great way to add cardio into your routine and not spend 30 minutes to an hour doing boring cardio. The main goal of HIIT is to give you the benefits of traditional cardio without sacrificing muscle mass. Studies have shown an increase in anaerobic activity using HIIT. HIIT improves aerobic capacity and will burn more calories than traditional cardio.

High intensity cardio has been shown to support heart and circulatory health. A stronger heart and better circulation is a great combination to build huge muscles. HIIT is great because it forces the body into an anaerobic state and also stimulating fast-twitch muscle fibers. Since burning protein takes oxygen and anaerobic means “without oxygen,” the body cannot burn protein for an alternative energy source.

Slow-Twitch Muscle Fibers

Slow-twitch muscle fibers are more efficient at using oxygen to generate fuel for extended muscle contractions over a long time. They fire slower than fast-twitch fibers and they can go a long time before they fatigue. Marathon runners, distance bikers, and endurance athletes have an abundance of slow-twitch muscle fibers which allow them to do their sports for hours.

Low intensity cardio such as walking or jogging promotes the growth of slow-twitch fibers.

Fast-Twitch Muscle Fibers

Fast-twitch muscle fibers use anaerobic metabolism to create fuel and are much better at generating bursts of strength or speed for a short amount of time than slow-twitch fibers are. Fast-twitch muscle fibers are essential to being a good power athlete such as a powerlifter or a sprinter. These muscles generally produce the same amount of force per contraction as slow-twitch fibers but they are able to fire more rapidly and fatigue much quicker.

To maximize muscle growth, exercise must stimulate fast-twitch fibers. The traditional means for accomplishing this is using strength training; in order for fast-twitch fibers to grow, they need to be activated.

Why do I care about the types of fiber types?

If you are looking to simply lose bodyfat or start doing endurance sports, traditional low to moderate intensity cardio will promote slow-twitch fiber growth which is not optimal for muscle size and power.

So if you are a powerlifter, sprinter, or a power athlete , traditional cardio will de-train your strength training and will effect your performance. In order to compliment strength training, HIIT cardio such as sprints or jumping rope will be the most beneficial.


I suggest adding cardio to your routine regardless of your goals simply because there are benefits for every goal, weight loss, muscle building, and endurance; just know your goals and learn how to achieve them.


If you’ve found this article helpful, leave a comment below along with answers to these questions:

  1. How often do you do cardio per week?
  2. What is your favorite activity to do for cardio?

19 thoughts on “Treadmills Suck: The Effects of Cardio on Muscle Growth”

  1. Great article, in reference to cardio in the morning you state “going ‘all out’ and pushing yourself too hard will result in sub-optimal fat burning.” Would you avoid HIIT training or high intensity cardio in the morning in a fasted state?

    1. HIIT is great, I’ve found that LISS works best for me and other clients.

      I would invite you to try out a month of each and see which you’ve gotten better results with.

      If you do, I would love to hear your results.


  2. Hi i am doing your 5 day dumbbell only workout and all is going good. I came upon this article and was wondering if it should be done right after the weight lifting sessions or should I wait a bit and say down a protein shake and then do the cardio. Thanks.

    1. Cyrus,

      That’s more of a personal preference than anything. Try both ways and see what you like best. I sometimes get an upset stomach if I down a shake and then do cardio so it’s pretty well up to you. The “timing” that a lot of people talk about has some merit but for 99% of the people at least that come to this site don’t need to worry about it.

      Try out doing it both ways and let me know which way you prefer.


  3. Daniel Junior - USA

    I Do it, before starting with my exercise in order to warm up my body… 10-20 Min max to activate my system.
    It works perfectly for me… highly recommended.
    Have a great day!

    1. That’s great. I had some ankle issues that keeps me from freely walking all of the time for now and I have been using more of an active warmup more or less some aerobics instead and it works great for myself as well.

  4. Well I have to tell you. I lost a lot of weight doing cardo. But I also lost a lot of muscles. My man boobs were worse abs side of my body lose skin not to saggy. I took protein. I also talked to trainers who conformed that cardio will take muscles away and also drain you! Told me to do it only once a week, or for me right now. Not at all. To fine another exercise that uses weights it doesn’t have to be heavy and do it on a cardio day that should do the trick. So I will try that and let you all know 🙂

    1. Thank you for sharing your experience! I like to tell people to use cardio as a “workout for your heart” instead of a calorie burner. If you couple cardio with lifting and eating plenty of nutritious foods, you won’t have an issue. Keep your ticker healthy, but don’t lose your muscles doing it :).

      Good luck!

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