The Importance of Cardio ….. for Muscle Growth

While it should be common knowledge that performing cardiovascular activity is a proven and effective tool for burning fat, it is not so well known that it can also support the muscle building process, if applied appropriately. Currently, there appears to be an abundance of bro-science circulating regarding the topic of cardiovascular training.  Some would advocate avoiding cardio all together when attempting to add muscle mass.  As doing cardio does not directly stimulate an increase in muscle size, it may not be immediately clear how it can play a role in building bigger muscles.  The following article is intended to discuss the application of a well-designed cardio program to support further muscular growth and development, and yes, even in your off-season.

The Lesser Known Benefits of Cardio Training

In response to an aerobic exercise program, muscle cells actually undergo a cellular re-modeling process in order to become more efficient at providing energy.   Some key adaptations which occur at the cellular level in response to aerobic exercise are an increase in the muscle capillary density, increased myoglobin content, increased number and size of mitochondria, increased functional vasodilation[1], increased stroke volume, increased muscle glycogen stores, increased muscle fat stores, and decreased blood lactate levels at the same relative intensity[2],[3i].  Many of these cellular adaptations to aerobic exercise can also be a benefit to a resistance training program as well.

Improved Insulin Sensitivity with HIIT cardio

High intensity interval training, as little as 3 x 20 minutes per week has been shown to improve insulin sensitivity in studies. This is key for both athletes on a contest prep or off-season clients looking to make lean gains and bring up body parts. When you consider that your off-season should be focused on trying to consume more carbohydrates, continuing with your HIIT cardio after your show is over just makes a ton of sense because you’ll reap more benefit from the carbs and let’s be honest – who doesn’t want to enjoy a higher carb intake during their off-season!?  I don’t mean continuing with prep-levels of cardio year round, but I do recommend all clients keep up with a minimum of 2-3 cardio sessions all year

Stimulate the Appetite

One limiting factor for many athletes during an off-season is the struggle to consume a larger amount of food and calories. By continuing with your HIIT cardio during your off-season phase, you will keep your metabolism charged and appetite primed for a slightly higher caloric intake to support lean gains. Note that this effect is typically seen over the 24 hours after a HIIT session and not immediately after a workout. So if you feel nauseous after a cardio session don’t think you did it wrong.

Improved Recovery between Sets and Workouts

The amount of time it takes a trained muscle to recover from a workout and regenerate itself in between sets is highly dependent on the level vascular innervation to each muscle.  Through the adaptation of an increased oxygen and blood delivery system to the muscles with improved cardiovascular conditioning, recovery time between workouts and between sets can be decreased. In other words, the more tension you can apply to your muscles in that 60-minute workout, the more stimulus you have for growth to occur.

Improved Nutrient Delivery to Muscles

A highly developed vascular system serves as the supply route to the muscles, carrying nutrient-rich blood and oxygen to the damaged tissues, as well as carrying waste products away from the muscle.   This well-developed vascular system serves as the means to ensure a nutrient-rich anabolic environment, one which is conductive to muscle growth.

Improved Lactate Threshold

One of the factors which forces trainers to stop a heavy set is the accumulation of lactic acid in the muscle.  The aerobic adaptation of  decreased levels of blood lactate at the same level of intensity allow muscles to perform at high intensities without succumbing to the fatigue causing effects of lactic acid.  This is likely due to an increase in size and number of mitochondria, where the rapidly accumulating lactic acid is collected and recycled into useable energy for the cell.   As result, sets can be pushed to new limits with an improved ability to manage elevated lactic acid levels.

Improved Pumps

The goal of any great weight training workout is to lift heavy weights while maintaining strict technique and achieve an intense  pump.  The increased vasodilatation ability and increased stroke volume adaptations obtained from aerobic exercise effectively combine to improve the body’s ability to achieve a muscle stimulating pump. In addition to these adaptations,  cardio training can also help to regulate your blood pressure to a normal range, which is also a key factor for blood flow and muscle pumps.

Improved Muscle Cell Storage Content

The size of a muscle is dependant on the size (hypertrophy) and number (hyperplasia) of muscle fibers located within.  While it is difficult to cause an increase in number of muscle fibers beyond childhood, the primary means to increase the size of an adult muscle is to cause cellular hypertrophy of muscle fibers.  An aerobic training adaptation which could contribute to the increased size (hypertrophy) of muscle fibers is the increased ability to store more glycogen and fatty acids within muscle tissue.  This training effect is designed to allow for more readily available fuel during intense training.  A secondary benefit is that an increased amount of stored fuel in the muscle may contribute to an increased size of the muscle.

Don’t be Scared of a Little Cardio Bros

As demonstrated, avoiding aerobic activity due to the fear of muscle loss is a huge mistake.  Realistically, it would take an excessive amount of cardio done at a high intensity to result in any significant amount of muscle loss.  The loss of muscle has more to due with an inadequate diet and supplementation regimen to support the demands of a high intensity training program, rather than a few cardio sessions each week. The benefits of adding 3-4, 20 minute sessions of H.I.I.T. cardio to your current training program are too impressive to be ignored.  For those paranoid about losing even an ounce of hard-earned muscle, go with 30 minutes of moderate intensity to preserve most of your muscle glycogen and still get some of the above mentioned benefits to support you muscle building efforts.



1 Lash, J.M. and Bohlen, H.G. (2005). J Appl. Physiol 72 (6) 2052

2 Holloszy, J.O., et al. (1984). J Appl.Physiol 56(4): 831-838

3 Hickson, R.C., et al. (1981) Med. Sci. sports Exerc. 13(1): 17-20

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