TRAINING INSIGHTS
Cardio, Strength, and Muscle Growth
Cardio Pt.2
By Shane O'Dwyer
The previous article made the case for regularly performing cardio for general health. Even so, plenty of dedicated lifters would argue that, for all its benefits, cardio kills gains. Sure, cardio might lower your risk of dying from cancer and add a few high-quality years to your life — but if that comes at the cost of lean tissue, or worse, your bench press, the trade isn’t worth it. But what if cardio actually led to better gains in the long run?
To be fair to the skeptics, the interference effect is a real, documented phenomenon in which endurance training can blunt strength and muscle-growth adaptations. Strength and aerobic training each activate distinct cell-signaling pathways that drive different adaptations. A full breakdown of these pathways is beyond the scope of this article, but a review by Vainshtein and Sandri (2020) describes how these pathways govern the building and breakdown of muscle protein, the metabolic regulation of muscle mass, and the way mechanical tension from training alters cell signaling — both immediately and over time.
One of the best-described of these is the mTORC1 pathway, triggered by mechanical tension and loading (Schiaffino et al., 2021). Activating it kicks off an energy-intensive process of building new proteins and remodeling muscle tissue to prepare it for future strain. The AMPK pathway, triggered by aerobic training, works in the opposite direction: it conserves energy by favoring fat and carbohydrate metabolism over protein synthesis. On paper, these two pathways can look like they’re pulling in opposite directions — but in practice, they interact in far more nuanced ways, and oversimplifying that interaction leads to some faulty conclusions. Rather than getting lost in the mechanisms, it’s more useful to focus on the outcomes that actually matter.
The Real Trade-Off: Training Time, Not Muscle Loss
In the real world, maximizing aerobic capacity and performance can genuinely impact muscle growth and strength, and vice versa. But very few people who claim cardio should be avoided to protect muscle are actually maximizing anything. Rather than fixating on how AMPK signaling might interfere with protein synthesis, it’s more useful to think about the trade-offs of how you spend your training time. As we’ve noted, maximizing any single training outcome takes a serious time investment — you simply have to put in the work. Most people with demanding careers aren’t going to have the hours in the day for both high-volume resistance training and serious distance running.
Fueling adds another layer of difficulty, since eating enough to support both muscle growth and high-volume aerobic work takes real time and appetite. A protein-rich, calorie-sufficient, carb-forward diet sounds simple in theory, but it can be genuinely difficult to execute when you’re also trying to fit in grueling training sessions.
As we’ve been emphasizing, though, improving cardiorespiratory fitness while building strength and muscle is nowhere near the same thing as maximizing both at once. High volumes of aerobic training, especially performed directly before lifting, can plausibly interfere with resistance training — but there’s also evidence that improved aerobic fitness can enhance muscle growth over the long run.
How Cardio Can Actually Support Muscle Growth
One adaptation to endurance training is greater capillary density — essentially, a denser network of small blood vessels delivering oxygenated blood and fuel to working muscle, and clearing metabolic waste more efficiently. This improved capillary network may actually enhance resistance-training adaptations over time (Tan et al., 2023; Thomas et al., 2022). A study by Snijders and colleagues (2017) didn’t test endurance training directly, but instead compared participants based on how well-capillarized their type II muscle fibers already were. Type II fibers are the “fast-twitch” fibers that respond most dramatically to resistance training with strength and size.
Type I fibers, by contrast, are more fatigue-resistant and better suited to long-duration endurance work, but they respond less to growth-focused training. In the Snijders (2017) study, healthy older men with higher baseline capillarization in their type II fibers saw significantly greater muscle fiber growth after 24 weeks of progressive resistance training than those with lower baseline capillarization. Notably, the resistance-training program itself didn’t change capillarization — whether through prior aerobic conditioning or simple genetic luck, the more capillarized participants were already primed to grow.
That study is an intriguing proof of concept, but a paper by Thomas and colleagues (2022) offers more direct evidence that aerobic adaptations can contribute to strength and size gains. The researchers used a clever within-subject design: each participant had one leg randomly assigned to one of two conditions.
One leg did 10 weeks of resistance training only. The other spent 6 weeks doing single-leg cycling — 45 minutes, three times a week — before starting the same resistance program. Unconventional as it sounds, this design strengthens the study considerably, since each participant serves as their own control.
As in the Snijders (2017) study, greater baseline capillary density was again linked to greater increases in muscle size (Thomas et al., 2022). These are compelling findings, but the more practical question is whether performing cardio and resistance training together, in the same ongoing program, actually holds back results — since most of us aren’t going to take six weeks off from the squat rack purely to set up future gains. Fortunately, a comprehensive meta-analysis by Petré and colleagues (2021) reviewed the available concurrent-training literature to answer exactly that. The good news: for untrained and intermediate lifters, there was no measurable interference effect on strength when cardio was performed in the same session as resistance training.
The slightly less good news: more highly trained individuals did show a noticeable drop in strength adaptations when they performed demanding cardio in the same workout as resistance training. But here’s the silver lining — that effect disappeared once cardio and resistance training were separated by about six hours.
How to Combine Cardio and Strength Without Sacrificing Gains
You might be thinking, “that’s not realistic — who has time for two training sessions in a day?” Fair point. If that describes you, it’s worth remembering that you’re also unlikely to be trying to maximize any single fitness quality to begin with — and that’s completely fine. There are trade-offs to maximizing performance in any area of life, and while there are ways to train more efficiently, becoming truly elite at anything demands serious time and effort. Going from average to genuinely good, on the other hand, takes far less.
Simply doing cardio on a separate day from resistance training is a practical, low-friction solution — especially if you’re already training three to four days a week. Intensity and volume matter too. A leisurely walk after a lower-body day is unlikely to hurt your gains in any meaningful way. Nor should a moderate 20-minute stationary bike session after an upper-body day. But immediately following a heavy squat session with a 15-mile run is a different story — at minimum, expect a sluggish run and significant soreness the next day.
Building a cardiovascular base can also improve recovery, both within and between resistance training sessions. Poor aerobic capacity can become a limiting factor in how much high-effort training volume you’re able to complete in a single workout. Likewise, better delivery of oxygen and fuel to tissue tends to speed up recovery between sessions. None of this requires elite-level conditioning — a modest aerobic base is enough to see these benefits.
Since training volume is a key driver of muscle growth over time, being fit enough to recover well — both between sets and between sessions — can plausibly lead to better long-term results. Someone with poor cardiorespiratory fitness will likely struggle to complete meaningful resistance training volume if they’re gasping for air after a set of bicep curls. These are small investments that compound. Regular cardio may or may not directly drive more muscle growth, but it will absolutely make you a healthier person — improving recovery, body composition, and your ability to keep doing the things you enjoy for longer. The claim that cardio kills gains, without any further nuance, doesn’t hold up.
References
Pelland, J. C., Remmert, J. F., Robinson, Z. P., Hinson, S. R., & Zourdos, M. C. (2026). The resistance training dose response: Meta-regressions exploring the effects of weekly volume and frequency on muscle hypertrophy and strength gains. Sports Medicine, 56(2), 481–505. https://doi.org/10.1007/s40279-025-02344-w
Petré, H., Hemmingsson, E., Rosdahl, H., & Psilander, N. (2021). Development of maximal dynamic strength during concurrent resistance and endurance training in untrained, moderately trained, and trained individuals: A systematic review and meta-analysis. Sports Medicine, 51(5), 991–1010. https://doi.org/10.1007/s40279-021-01426-9
Schiaffino, S., Reggiani, C., Akimoto, T., & Blaauw, B. (2021). Molecular mechanisms of skeletal muscle hypertrophy. Journal of Neuromuscular Diseases, 8(2), 169–183. https://doi.org/10.3233/JND-200568
Snijders, T., Nederveen, J. P., Joanisse, S., Leenders, M., Verdijk, L. B., van Loon, L. J., & Parise, G. (2017). Muscle fibre capillarization is a critical factor in muscle fibre hypertrophy during resistance exercise training in older men. Journal of Cachexia, Sarcopenia and Muscle, 8(2), 267–276. https://doi.org/10.1002/jcsm.12137
Tan, J., Krasilshchikov, O., Kuan, G., Hashim, H. A., Aldhahi, M. I., Al-Mhanna, S. B., & Badicu, G. (2023). The effects of combining aerobic and heavy resistance training on body composition, muscle hypertrophy, and exercise satisfaction in physically active adults. Healthcare, 11(17), 2443. https://doi.org/10.3390/healthcare11172443
Thomas, A. C. Q., Brown, A., Hatt, A. A., Manta, K., Costa-Parke, A., Kamal, M., Joanisse, S., McGlory, C., Phillips, S. M., Kumbhare, D., & Parise, G. (2022). Short-term aerobic conditioning prior to resistance training augments muscle hypertrophy and satellite cell content in healthy young men and women. FASEB Journal, 36(9), e22500. https://doi.org/10.1096/fj.202200398RR
Vainshtein, A., & Sandri, M. (2020). Signaling pathways that control muscle mass. International Journal of Molecular Sciences, 21(13), 4759. https://doi.org/10.3390/ijms21134759