If you’re a bodybuilder or even just a casual gym-goer trying to pack on muscle, chances are you focus almost exclusively on resistance training (and correctly so). Nevertheless, cardiovascular training shouldn’t be overlooked even when muscle building is your primary goal, and there’s quite a bit of science that actually shows cardio can increase muscle growth when done in conjunction with resistance training.

 Keep reading to learn the scientific reasons why you should be doing cardio year-round for better muscle growth.

Physiology of Different Types of Exercise

Exercise is generally classified in two different forms: Endurance training (cardiovascular exercise) and resistance training. Cardio and endurance training generally rely on aerobic (oxygen-dependent) metabolism since they involve lower intensity carried out for an extended duration, whereas resistance training (especially weight lifting for strength and muscle building) requires anaerobic (oxygen-less) metabolism and high intensity/powerful movements carried out for a short duration.

 Research indubitably demonstrates that incorporating both cardiovascular and resistance training is ideal for health and longevity (amongst the general population). If you’re reading this I realize you might be a little more concerned with building muscle and staying lean than the average person, but that doesn’t mean you should only focus on one form of exercise (like many bodybuilders do, at least for a large part of the year).

 The molecular mechanisms that underlie different forms of exercise remain rather misunderstood. However, it is clear that the myostatin and IGF-1 systems are quite important for building and maintaining muscle mass and power, along with various androgens and even estrogen.

 Note: The following gets a little “sciencey”; if you get lost don’t worry, there are simple takeaways provided at the end of each section.

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Molecular Regulators of Mitochondrial Biogenesis: A Game-Changing Discovery?

Recent research shows that a family of transcriptional coactivators - known as the peroxisome proliferator-activated receptor gamma (PPARγ) coactivator (PGC-1) family - are the “master regulators” of mitochondrial biogenesis. If you’ve read up on the benefits of high-intensity interval training (HIIT), you may be familiar with mitochondrial biogenesis - the process by which your cellular mitochondria increase in number.

 Mitochondria are essentially like the “power station” of cells, being responsible for energy production and substrate metabolism. In general, the more mitochondria you have, the higher your metabolic rate.

 The PGC-1 family of proteins are indiscriminate coactivators, meaning they interact directly with specific steroid hormone receptors and promote transcription - the first step of gene expression.

 There are at least 15 known coactivators in the human body, but the PGC-1 family appears to be especially crucial for regulating our response to exercise (especially endurance exercise/cardio). New findings suggest that one of the PGC-1 proteins -  PGC-1α4 - actually stimulates skeletal muscle hypertrophy (we will touch more on this below).

 Section Summary in Layman’s Terms: Mitochondria are key components of your cells for elevating metabolic rate, and one way to increase them is by performing HIIT. A special group of proteins in your body known as PGC-1 proteins regulate how your body responds to exercise (by increasing mitochondria, muscle hypertrophy, fat loss, etc.)

PGC-1 Proteins: How They Work and Why You Should Care

The PGC-1 family of proteins don’t bind directly to DNA; rather, they recruit a number of cofactors to transcription complexes. The PGC-1α gene was the first identified member of this family and is now known to code for multiple proteins in response to exercise.

 A specific protein of the PGC-1 family, known as PGC-1α1, is stimulated primarily by endurance exercise and regulates many physiological adaptations, such as mitochondrial biogenesis, fiber-type switching, and protection against muscle catabolism. However, in gain-of-function research, PGC-1α1 does not increase anabolism in skeletal muscle mass (nor does it increase strength).

 PGC-1α1 is also induced by certain environmental stimuli, especially cold exposure and fasting.

While the role of PGC-1α1 in regulating mitochondrial biogenesis is well-established, recent research has identified a different protein coded by the same gene (PGC-1α) that regulates skeletal muscle hypertrophy - PGC-1α4

 Section Summary in Layman’s Terms: One of the PGC-1 group of proteins - known as PGC-1α1 - mainly controls how your body responds to endurance exercise (cardio), but not resistance training. However, recent findings show that the same gene that codes for PGC-1α1 also does so for a protein - known as PGC-1α4 - that controls how your body responds to resistance training. (You’ll learn why this is useful knowledge in the next section.)

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PGC-1α4: An “Endurance” Protein that Helps Build Muscle?

PGC-1α4 is a novel protein that does not regulate the same set of genes stimulated by PGC-1α1, but rather acts on the myostatin and IGF-1 pathways, both of which are well-known regulators of skeletal muscle hypertrophy and strength.

 Human research shows that PGC-1α4 increases by about 150% after resistance training. However, both PGC-1α4 and PGC-1α1 increased even more in skeletal muscle tissue after a combined exercise routine where subjects performed both weightlifting and cycling.

 This suggests that the most effective exercise regimen for maximizing skeletal muscle remodeling is a combination of resistance training and cardio. Cardio induces the proximal promoter of PGC-1α, while resistance training induces the alternative promoter.

 Key Article Takeaway: Your body increases production of synergistic proteins that control both muscle hypertrophy and mitochondrial biogenesis (and many other physiological adaptations) to a greater degree when you perform cardio and weight lifting together (as opposed to one or the other only).

 Basically, it behooves you to do cardio year-round in conjunction with weight training, even if you’re in the “offseason” as a bodybuilder and trying to pack on mass. Don’t be lazy! Even 20-30 minutes of moderate-intensity cardio 3-4 times a week will only help you build size (plus it will keep you leaner than if you do no cardio at all).