How Does Cardio Affect Muscle Building?


When it comes to cardio and muscle building, people often choose one over the other. People who want to gain muscle tend to stick with resistance and weight training and those who don’t want to gain muscle go for the cardio.


Sometimes the lines can become a little blurred between the two and misconceptions can lead to people not achieving their goals and being misinformed.


So what’s the score with cardio and muscle building?

Does cardio even build muscle?

When a muscle is torn it has to repair itself. Any form of cardio that challenges the muscles from swimming to boxing will lead to the growth of necessary muscle as a natural means of adaptation.

It wont lead to excess muscle growth like targeted resistance training will but your body will have to adapt to develop at least some surplus muscle to ensure it can meet the continuous endurance demands of the activity performed.

You also need rest and recovery time between workouts to allow your body to rebuild and increase your muscle mass.

Also, ensure you are consuming enough protein to enable your muscles to repair, without ample protein it wont matter if your training consists of resistance or cardio you wont develop further muscle size.

Bodybuilding and cardio

Bodybuilding and cardio is probably where a lot of the confusion comes from. When bodybuilders need to cut down for a competition, or photo-shoot they incorporate more and more cardio into the mix.

Bodybuilders will weight train to build and maintain the muscle they have and perform cardio to strip down the body fat. Yes, it’s an awful lot of effort and bodybuilders mainly use cardio for fat burning and cardiovascular fitness, not to build muscle.

On ya bike…

Ever noticed the muscular thighs on a cyclist? It’s time you got off the standard treadmill and onto a bike instead!

In a study, which compared cycling to running, it was discovered that steady state running caused far greater declines in muscle growth than cycling [1].

This echoed a 2009 study in another lab setting, which found that uphill walking produced greater decrements in strength than cycling [2].

Sprint training and muscle building

It’s important to look at one of the most effective forms of cardio for building muscle – sprint training.

Have you ever noticed when watching sprinting on TV or in the Olympics that the runners have huge, carved up quads and calves?

These guys and gals wont be hammering their legs in the gym with heavy weight training every week. Instead they are sticking with their cardiovascular exercise but are constantly training the fast-twitch muscle fibers, which as a result, provides optimum growth of skeletal muscle tissue.

What are fast-twitch muscle fibres? Often known as type II fibres these fibres fatigue faster than type 1 and are primarily used in powerful bursts of movements like sprinting, whether it’s on a bike or on the track.

In a recent study testing students who sprint trained for a 6-week period it was found that type II fibres increased from 32 to 38%.

This significant increase indicates that it is possible to achieve a fibre type (muscle building) transformation with high-intensity cardiovascular training. [3]

Sprinting is a form of anaerobic cardio and technically so is resistance training because it is performed without oxygen. Anaerobic glycolysis is still the major source of power for both, although aerobic metabolism makes a very small contribution.

When cardio becomes bad for muscle building…

It’s a topic that definitely needs covering. Performing cardio activity too frequently and too intensely, or for too long can certainly prevent you from gaining muscle and even start to break it down.

This is where the myth comes from that cardio doesn’t build muscle it diminishes it, because people overtrain.

Have you ever noticed the difference between marathons runner’s physiques compared to sprinters physiques?

Metabolic conditioning and specific cardio methods such as sprinting, rowing and cycling can also be one hell of a way to pack on some quality muscle on your lower body.

Exercises classes, which are also classed as cardio, can be excellent to develop muscle tone too.

Remember if you are training it, you are breaking it, so you need to repair it to grow with ample rest and protein in between sessions.

How long does cardio need to be to have an affect?

Plodding along on the treadmill for an hour isn’t as affective as you probably thought.

A recent study conducted by researchers at The University of Western Ontario demonstrated that doing just 4 – 6 30-second sprints burns more fat over time than 60 minutes of incline treadmill walkingYes, less is actually more, it just depends upon what type of cardio you choose to do, and intensity is the key to effectiveness [4].

At around 50% of your max heart rate, your body burns a ratio of 60% fat to 40% glycogen. At 75% of your max heart rate, the ratio is 35% to 65%, and at even higher intensities, the ratio is even lower.

HIIT cardio is also particularly good for getting rid of stubborn abdominal fat, including the dangerous accumulations of visceral fat.

It is a fact that the body uses fat as the primary source of fuel during lower intensity exercise. About 60%of the calories burned come from fat.

If you are burning more calories than what you are consuming your body will eventually tap into your protein and muscle stores and this will cause you to actually lose muscle instead of tone or build it.

Cardio and hormones: what happens in your body when you do cardio?

Actually a lot of things go on in your body when you perform cardiovascular activity.


Cardio is also excellent for releasing the feel good chemicals known as endorphins. Endorphins interact with the receptors in your brain that to reduce your perception of pain and make you feel great. Cardio is therefore definitely a mood enhancer.

Growth factors

Growth factors (hormone-like compounds within your body) such as hepatocyte, fibroblast, and insulin deliver signals to the satellite cells to move to the damaged muscle area, fix the damage following exercise, and regulate muscle mass growth, respectively.


Known as the happy hormone is released more when you work out regularly. As well as elevating your mood it also allows you to enjoy better concentration, more energy and a better nights sleep.

Be smart with your cardio

If you want to burn fat then opt for short bursts and interval training as this has been proven to be more affective than prolonged steady state cardio. If you are looking to use it as a muscle building method then ensure you crank up the intensity and you are consuming enough protein to repair the damage.

Hopefully you feel more clued up about cardio and it’s effects on your body and muscle building and you are ready to start winning and SMASH all of your fitness goals.



[1] Wilson, J.M., et al., Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises. Journal of strength and conditioning research / National Strength & Conditioning Association, 2012. 26(8): p. 2293-307.

[2] Gergley, J.C., Comparison of two lower-body modes of endurance training on lower-body strength development while concurrently training. J Strength Cond Res, 2009. 23(3): p. 979-87.

[3] Jansson, E., et al. “Increase in the proportion of fast‐twitch muscle fibres by sprint training in   males.” Acta Physiologica 140.3 (1990): 359-363.

[4] Macpherson, R. E., et al. “Run sprint interval training improves aerobic performance but not maximal cardiac output.” Med Sci Sports Exerc, 43.1 (2011): 115-22.


Personal trainer and professional fitness writer, Betsy has dedicated her life to fitness. Having a career in competitive swimming for ten years she then found her love of weight training and bodybuilding in the gym environments and helps people transform their bodies day by day.
I have to edit them again cos I only have the old version of the bios so bear with me.

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