Can cardio (eventually) make you bigger?

A recent study found that, under the right circumstances, aerobic training might actually increase muscle growth.
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When we discuss aerobic training on Stronger By Science and in MASS Research Review, we’re usually discussing how to best mitigate the interference effect (the finding that a combination of resistance training and aerobic training may lead to less muscle growth and smaller strength gains than resistance training alone). However, a recent study found that, under the right circumstances, aerobic training might actually increase muscle growth. So, let’s dig into the study, and examine whether cardio may actually be a useful tool to improve hypertrophy results in the long run.

The present study by Thomas and colleagues compared the effects of aerobic training and resistance training versus resistance training alone (1), much like a typical concurrent training study, but it didn’t involve a concurrent training intervention. Rather, one condition involved six weeks of aerobic training followed by 10 weeks of resistance training, and one condition only involved the 10-week resistance training intervention. In a typical concurrent training study, the resistance and aerobic training would be performed at the same time.

Fourteen young, recreationally active subjects completed the present study. For each subject, one leg was randomized into the aerobic + resistance training intervention, with the other leg completing just the resistance training portion of the intervention. Thus, each subject could serve as their own control. For the first six weeks of the study, subjects performed moderate-intensity single-legged cycling three times per week, for 45 minutes per session, with their leg that had been randomized into the intervention with an aerobic training component. The resistance training-only legs did not undergo any structured exercise intervention during these first six weeks. Then, for the next 10 weeks, subjects completed a standard bilateral resistance training intervention, consisting of squats, leg press, knee extensions, hamstring curls, and calf raises for sets of 10-12 reps, with the last set taken to failure. This isn’t explicitly spelled out in the study, but training loads were presumably increased when the subjects completed more than 12 reps on their last set, in order to keep the subjects within the desired rep range. Muscle biopsies to assess fiber size, capillary density, satellite cell content, and myonuclei content were performed at the start of the study, following the aerobic training portion of the intervention, and following the resistance training portion of the intervention. Furthermore, following both the aerobic training and resistance training portions of the intervention, squat and leg press 1RM were assessed, and leg fat-free mass was assessed via DEXA. The design of the study is depicted in Figure 1.

Graphic by Kat Whitfield

Overall, capillary density increased to a greater extent in the legs performing six weeks of aerobic training before the resistance training intervention. Capillary density was assessed two ways: capillaries per fiber (C/Fi), and capillaries per unit of fiber perimeter (CFPE). Of these two measures, CFPE is generally the more informative measure, since it accounts for not just the number of capillaries, but also the size of the muscle fibers. The benefits of aerobic training were more apparent for measures of CFPE than C/Fi (Figure 2).

Graphic by Kat Whitfield

Increases in type I fiber cross-sectional area, type II fiber cross-sectional area, and mean fiber area were generally larger in the legs performing aerobic training prior to the resistance training intervention (0.05 < p < 0.10 for all comparisons; Figure 3). The legs performing aerobic training before the resistance training intervention tended to experience larger increases in satellite cell and myonuclear content as well.

Graphic by Kat Whitfield

Overall, increases in leg fat-free mass didn’t differ between conditions, and subjects experienced large increases in squat and leg press 1RM strength (Figure 4).

Graphic by Kat Whitfield

Finally, the researchers separated out “high responders” and “low responders” (the 10 legs experiencing the largest change in particular outcomes, and the 10 legs experiencing the smallest change in particular outcomes) for a series of exploratory analyses. They found that a) the legs experiencing the largest changes in satellite cell content generally experienced more hypertrophy than the legs experiencing the smallest changes in satellite cell content, b) the legs experiencing the most hypertrophy had greater capillary density than the legs experiencing the least hypertrophy, c) the legs with the greatest capillary density prior to resistance training experienced more hypertrophy than the legs with the lowest capillary density prior to resistance training, and d) the legs experiencing the most hypertrophy generally experienced larger increases in satellite cell content than the legs experiencing the least hypertrophy (Figure 5).

Graphic by Kat Whitfield

Overall, this study suggests that a higher degree of local aerobic fitness pre-training may augment skeletal muscle hypertrophy following a resistance training intervention. The authors specifically suggest that increases in capillary density are particularly important, which comports with prior research. A 24-week study by Snijders and colleagues found that baseline levels of capillary density were predictive of training responses in older men (2). The men with greater capillary density at baseline experienced larger increases in satellite cell content and fiber hypertrophy than the men with lower capillary density. The present study found that these results generalize to younger subjects, and the present study also suggests that increases in capillary density due to prior aerobic training can also augment hypertrophy and satellite cell responses (i.e., it’s not simply a matter of people with innately higher levels of capillary density experiencing more hypertrophy).

For the past five years, the Snijders study has been in the back of my mind. We focus a lot on factors that can increase rates of muscle growth, but I don’t think we adequately consider the factors that limit muscle growth. Sooner or later, we all reach the point where rates of muscle growth slow way, way down. Why does this happen?

Local aerobic fitness and capillary density have always seemed like logical answers to me. At the most basic level, the size of organisms and individual cellular structures within organisms are strongly associated with energy requirements and rates of energy production (3). As muscle fibers grow, all of the “stuff” (contractile proteins, organelles, metabolic machinery, etc.) inside of the muscle fibers gets further away (on average) from the capillaries providing oxygen, energetic substrates, and signaling molecules to the muscle fibers, and clearing waste products from the muscle fibers. So, to avoid an intracellular energy crisis, it makes sense that fibers either need to a) stop growing, or b) experience an increase in capillary density to allow for further growth.

Of course, both the Snijders study (2) and the present study by Thomas and colleagues (1) don’t fully validate my little pet theory. They both examined rates of muscle growth in individuals who weren’t already resistance-trained, after all; they weren’t focused on increasing the hypertrophic potential for lifters who’d already hit a wall. However, I do think they suggest that capillary density and local aerobic fitness are more important for muscle growth than (I suspect) a lot of people realize.

Furthermore, it’s worth acknowledging that the results of the present study probably aren’t fully attributable to the increases in capillary density observed during the aerobic training intervention. For example, a prior study by Kazior and colleagues employed a standard concurrent training intervention, and the concurrent training group experienced considerably more hypertrophy than the resistance training-only group (4). However, the changes in capillary density observed in that study were pretty underwhelming, suggesting that some other aspect of aerobic fitness (or the training intervention itself) contributed to the findings. In any biological system, a particular observed outcome (in this case, muscle hypertrophy) is the result of dozens of inputs and competing cellular signaling pathways. So, even if increases in capillary density are a factor that could augment hypertrophy results, that certainly doesn’t imply that there aren’t other adaptations to aerobic training that can also improve hypertrophy outcomes.

The obvious question is, “what can we actually do with these findings?” We’re in an awkward situation where the results of aerobic training seem to have a positive impact on hypertrophy, but aerobic training itself generally has a neutral-to-negative impact on hypertrophy in resistance-trained individuals. As I see it, there are two potential options:

  1. You could try alternating periods of concurrent training and periods where you only perform resistance training. During the periods of concurrent training, it’s alright if your rate of muscle growth slows down a bit, because the goal is to make your next training period consisting solely of resistance training more effective. Ideally, you should include aerobic training modalities that train both the lower body (cycling, jogging, etc.) and the upper body (arm cycling, swimming, etc.) so that most major muscle groups receive a local aerobic training stimulus.
  2. Work in more high-rep training for accessory exercises. Since high-rep training leads to disproportionate increases in relative strength endurance (as I discuss in one of my other briefs this month), it presumably causes some of the same local aerobic adaptations as more traditional forms of cardio. So, if you’re training for hypertrophy, instead of doing sets of ~6-12 reps for most exercises, you could stick to sets of ~6-12 reps for the primary compound exercises you use to gauge progress, but opt for sets of ~25-30 reps for your accessory exercises. Set-for-set, this approach should be similarly effective for hypertrophy in the short term (on average), and it may set the stage for greater hypertrophy in the long run.

To be clear, I’m not positive that either of these approaches to training will definitely lead to greater muscle growth in the long run, but they’re my best guesses for how trained lifters might apply the findings of the present study. More importantly, I’m really excited about this line of research moving forward. I think it has the potential to not only tell us how to make training more effective in the short term, but also to help us learn more about why muscles eventually stop growing in the long term.

Note: This article was published in partnership with MASS Research Review. Full versions of Research Spotlight breakdowns are originally published in MASS Research Review. Subscribe to MASS to get a monthly publication with breakdowns of recent exercise and nutrition studies.

References

  1. Thomas ACQ, Brown A, Hatt AA, Manta K, Costa-Parke A, Kamal M, Joanisse S, McGlory C, Phillips SM, Kumbhare D, Parise G. Short-term aerobic conditioning prior to resistance training augments muscle hypertrophy and satellite cell content in healthy young men and women. FASEB J. 2022 Sep;36(9):e22500. doi: 10.1096/fj.202200398RR. PMID: 35971745.
  2. Snijders T, Nederveen JP, Joanisse S, Leenders M, Verdijk LB, van Loon LJ, Parise G. Muscle fibre capillarization is a critical factor in muscle fibre hypertrophy during resistance exercise training in older men. J Cachexia Sarcopenia Muscle. 2017 Apr;8(2):267-276. doi: 10.1002/jcsm.12137. Epub 2016 Aug 4. PMID: 27897408; PMCID: PMC5377411.
  3. Gillooly JF, Brown JH, West GB, Savage VM, Charnov EL. Effects of size and temperature on metabolic rate. Science. 2001 Sep 21;293(5538):2248-51. doi: 10.1126/science.1061967. Erratum in: Science 2001 Nov 16;294(5546):1463. PMID: 11567137.
  4. Kazior Z, Willis SJ, Moberg M, Apró W, Calbet JA, Holmberg HC, Blomstrand E. Endurance Exercise Enhances the Effect of Strength Training on Muscle Fiber Size and Protein Expression of Akt and mTOR. PLoS One. 2016 Feb 17;11(2):e0149082. doi: 10.1371/journal.pone.0149082. PMID: 26885978; PMCID: PMC4757413.
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