Myth: Cardio Reduces Muscle Tissue
So many people have inherently believed that cardio will cause muscle loss. AGAIN, I’m here to clear up the myths and misinterpretations. There are many facets to this discussion, and i’m going to highlight a few. For starters, ‘cardio’ (we will call it) does not result in skeletal muscle atrophy. Period. There is no biological reason as to why, but there are some indirect reasons why it may cause muscle loss that I will allude to after I clear up why it directly will not cause us to lose any hard earn muscle.
What is Cardio:
Cardio is commonly referred to as running, cycling, swimming or any other from of physical activity that elevates our heart rate and normally performed under endurance like conditions. Cardiovascular training has endless benefits, but for the purpose of this article, i’m going to stay along the tracks of cardio induced skeletal muscle atrophy. The biomechanics of cardio training are not too dissimilar to that of resistance training (in fat cardio has been shown to be a form of resistance training). We still have t produce muscular contraction and we still have to control forces. Do you see where I am going with this. How can the same mechanism drive SMH and also atrophy? It cannot.
Muscular contractions whether they are as a result of the bicep producing force move an external load, or whether is the quadricep group producing force to negate ground reaction forces during running, or to move the external load (pedal) during cycling, will have zero difference in how that muscle contracts. Overload the quads enough during cycling under the right conditions, and you will respond with SMH. Or, overload the hamstring/glutes enough during some uphill running under the right conditions, you will respond with SMH, just like you would overloading the hamstrings on a hamstring curl in the gym. You will probably be starting to realise how cardio and resistance training are not so different after all.
There are certain conditions necessary for muscle protein synthesis (MPS) to occur, an adaptive response to exercise. We muscle expose tissue to a demand which causes muscle protein degradation (MPD). The stimulus must be a certain intensity to elicit the amount of damage required, too consequently enable MPS as a result. There are dietary factors also involved that must be accounted for to elicit MPS. Although MPS does not, I repeat, does not solely produce hypertrophy, MPS is a necessary stepping stone to produce the desired SMH. Essentially, what I am trying to say, is if we are exposed to cardio (controlling for required intensity), and we undergo the required conditions for stimulating MPS, while also implementing certain other conditions for SMH such as caloric surplus and stimulation of anabolic pathways, cardio could be a tool for producing muscle. This pretty much covers the fact that cardio does not directly inhibit muscle growth, and also does not create skeletal muscle atrophic conditions. One study, over 20 years ago by Schwarz et al. established that 6 months of running/walking showed a 9% increase in quadricep cross sectional area. 
There are different training conditions we might want to emphasise to elicit certain muscle fibre type development, such as higher intensities ‘normally’ associated with type 2b fibre growth and low-loads inducing more slow-twitch muscle fibre growth  (i say normally as this is a very deep topic that has a few considerations when we train under different conditions i.e fatigue/failure and the consequent use of multiple fibre types.) There will also be many different benefits to low load, cardio based resistance training that may indirectly promote SMH in future. One of these is that low-load endurance type training will stimulate our PGC-1a gene that induces mitochondrial biogenesis. If the function of our mitochondria is improved will will see an all round increase in levels of performance, energy metabolism and endless other benefits.
A few other point to note regarding the benefits of low-load cardio bases training:
a bout of aerobic exercise restores the anabolic response of muscle proteins to insulin by improving endothelial function and Akt/mTOR signaling 
aerobic exercise improved leg blood flow and amino acid delivery to skeletal muscle 
Why might cardio elicit a ‘loss of gains’?
So we have established that cardio DOES not 1) inhibit SMH, and 2) does not cause direct loss skeletal muscle. However, there are going to be situations when cardio may have a detrimental effect to muscle gain. I am going to highlight two potential reasons.
1. My first reason is that it may push us into a caloric deficit. If other variables are not tracked and controlled for, implementing cardio as a form of exercise may induce a caloric deficit. This will have potential negative effects if the goal is SMH. However, if the cardio is also supplemented with added caloric intake, we will potentially see greater improvements in our pursuit if skeletal muscle hypertrophy in comparison to an exercise regime without cardio. This point all boils down to energy balance. Without the activation of anabolic pathways in response to a caloric surplus, we will not see SMH. The cardio itself will not be the sole reason why SMH is inhibited but the overall impact it has on our daily energy intake/output.
2. Substitution of weight training for cardio will be my second point. If cardio is added into an exercise regime while not replacing our normal weight training session, and controlling for added energy expenditure, we will see numerous benefits. If cardio sessions replace weight training sessions, then we will have our second reason how cardio may reduce optimal hypertrophic response. Remember, cardio will not cause muscle mass reduction, but training with weights, specific machines and equipment suited to supplying the best tools for hypertrophy biased training will obviously be our go to protocol for inducing maximal SMH. If these sessions are replaced with cardio, you will not lose muscle, but you will definitely not be maximising your efficiency to put on as much tissue as possible.
In conclusion, we have established that cardio does not kill our gains, but under certain conditions, the introduction of cardio may prove to be sub-optimal if we are looking for maximal time-utilisation to produce an optimal hypertrophic response. We have uncovered that adding cardio to your training will undoubtedly improve overall performance and also be a huge contributor to maximising your returns in all attempts to build as much muscle as possible.
Luke French | Health & Physique Coach
Published by Luke French, 4th July 2020
 Schwartz RS, Shuman WP, Larson V, Cain KC, Fellingham GW, Beard JC, Kahn SE, Stratton JR, Cerqueira MD, Abrass IB. The effect of intensive endurance exercise training on body fat distribution in young and older men. Metabolism. 1991;40(5):545–51.
 Grgic J, Homolak J, Mikulic P, Botella J, Schoenfeld BJ. Inducing hypertrophic effects of type I skeletal muscle fibers: A hypothetical role of time under load in resistance training aimed at muscular hypertrophy. Med Hypotheses. 2018;112:40-42. doi:10.1016/j.mehy.2018.01.012
 Geng T, Li P, Okutsu M, et al. PGC-1alpha plays a functional role in exercise-induced mitochondrial biogenesis and angiogenesis but not fiber-type transformation in mouse skeletal muscle. Am J Physiol Cell Physiol. 2010;298(3):C572-C579. doi:10.1152/ajpcell.00481.2009
 Fujita S, Rasmussen BB, Cadenas JG, et al. Aerobic exercise overcomes the age-related insulin resistance of muscle protein metabolism by improving endothelial function and Akt/mammalian target of rapamycin signaling. Diabetes. 2007;56(6):1615-1622. doi:10.2337/db06-1566
 Konopka AR, Harber MP. Skeletal muscle hypertrophy after aerobic exercise training. Exerc Sport Sci Rev. 2014;42(2):53-61. doi:10.1249/JES.0000000000000007
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