The muscles themselves had grown just as much; the brain simply couldn't access that growth
For generations, gym culture has enforced a quiet dogma: cardio and muscle-building are rivals, not allies. Researchers at the University of São Paulo have now tested that belief against evidence, tracking sedentary young men through sixteen weeks of combined and isolated training. What they found is less a revolution than a clarification — the body can build muscle and endurance at once, though the nervous system, not the muscle itself, bears the cost of doing both.
- A deeply entrenched fitness belief — that aerobic exercise sabotages muscle growth — has been directly challenged by a controlled Brazilian study published in a leading physiology journal.
- The 'interference effect' theory, which warned that cardio would divert the body's protein resources away from muscle hypertrophy, failed to hold up when put to a 16-week experimental test.
- Muscle mass grew equally in both the weightlifting-only and the combined training groups, forcing a rethink of how the fitness industry has long advised people to structure their routines.
- A real but narrower cost was identified: combined training did produce slightly lower strength gains, traced not to muscle tissue but to neuromuscular fatigue — the brain's reduced ability to recruit muscle fibers after aerobic exertion.
- The findings suggest that people chasing both cardiovascular health and muscle growth no longer face a forced choice, though they should calibrate expectations around peak strength performance.
For years, a quiet rule governed gym culture: if muscle is the goal, avoid cardio. The logic seemed biological — aerobic exercise would compete with strength training for the body's resources, forcing a trade-off between endurance and size. A study from the University of São Paulo has now put that assumption to the test.
Researchers followed 19 sedentary men in their late twenties across four months. One group lifted weights twice a week — leg press and extensions, two to three sets each. The other group did the same strength work but added four weekly sessions of high-intensity interval running. When the results were published in the Journal of Applied Physiology, both groups had gained the same amount of muscle mass.
The old theory, known as the interference effect, held that aerobic exercise would trigger mitochondrial protein production, crowding out the synthesis needed for muscle growth. Study co-author Carlos Ugrinowitsch explained that the body was thought to prioritize aerobic adaptation as a survival mechanism. The data simply didn't support it.
There was, however, a subtler finding. The combined-training group showed slightly lower strength gains — and Ugrinowitsch believes this is precisely why the myth has endured. Observers noticing weaker performance likely assumed muscle growth was also suffering. But the real culprit was neuromuscular fatigue: after aerobic sessions, the nervous system struggled to recruit muscle fibers at maximum effort. The muscles had grown; the brain just couldn't fully access them under peak load.
The distinction reframes what concurrent training actually costs. Cardio still delivers what weights cannot — cardiovascular conditioning, caloric burn, improved oxygen capacity. The study, funded by the São Paulo Research Foundation, suggests that building muscle and building endurance are not mutually exclusive goals, only that chasing both may slightly temper the upper ceiling of raw strength.
For years, fitness enthusiasts have operated under a simple rule: if you want to build muscle, don't do cardio. The reasoning seemed sound—aerobic exercise would steal resources from muscle growth, forcing your body to choose between building endurance and building size. A study from the University of São Paulo has now dismantled that assumption, at least partially.
Researchers at USP tracked 19 sedentary men, averaging 28 years old, over a four-month period. Half of them performed weightlifting twice weekly, focusing on leg press and leg extension exercises—two to three sets of twelve repetitions each. The other half did the same strength work but added four weekly sessions of high-intensity interval training on treadmills, alternating between intense sprints and longer recovery periods. The results, published in the Journal of Applied Physiology, showed that muscle mass increased equally in both groups. The long-held belief that combining strength and aerobic training would compromise muscle growth turned out to be wrong.
Carlos Ugrinowitsch, a professor at USP's School of Physical Education and Sport and one of the study's authors, explained the old theory. Researchers had believed that aerobic exercise triggered the production of mitochondrial proteins—the cellular machinery for generating energy—which would compete with the protein synthesis needed for muscle growth. This "interference effect" assumed the body would prioritize aerobic adaptation because it was essential for survival. In practice, this meant combining cardio and strength training in the same routine should produce less muscle growth than doing each separately. The data suggested otherwise.
But the study revealed a more nuanced picture. While muscle gain was identical between groups, the combined-training group did experience slightly lower strength gains. This finding likely explains why the myth persists. Ugrinowitsch suggested that people observing smaller strength improvements in combined training may have mistakenly assumed muscle growth was also compromised. The actual mechanism, however, had nothing to do with protein synthesis. Instead, it involved neuromuscular fatigue—a breakdown in communication between the brain and muscles. When performing maximum-effort lifts, the nervous system couldn't recruit muscle fibers as effectively after aerobic training sessions. The muscles themselves had grown just as much; the brain simply couldn't access that growth as efficiently during peak exertion.
The distinction matters because it reframes what concurrent training actually does. Ugrinowitsch emphasized that combining aerobic and strength work remains fundamental to overall health. Cardio increases heart rate and blood oxygen, burns calories, and strengthens the cardiovascular system—benefits that isolated weightlifting cannot provide. The study, supported by funding from the São Paulo Research Foundation, suggests that people pursuing both muscle growth and cardiovascular fitness don't need to choose. They can pursue both simultaneously, though they should expect their maximum strength gains to be slightly more modest than if they trained with weights alone.
Notable Quotes
The study showed that muscle increase was equivalent in both groups, contradicting the belief that combining strength and aerobic exercise would compromise muscle growth.— Carlos Ugrinowitsch, USP professor and study coauthor
The combination of aerobic and strength training is the foundation for good health.— Carlos Ugrinowitsch
The Hearth Conversation Another angle on the story
So the old rule about cardio killing muscle gains—that's just wrong?
Not entirely. Muscle growth itself isn't compromised. But strength gains are slightly lower when you combine the two. That's the real finding.
Why would strength go down if the muscles are growing the same amount?
It's a nervous system issue, not a muscle issue. The aerobic training creates fatigue in how your brain communicates with your muscles. When you try to lift heavy, your nervous system can't recruit the fibers as well.
So the muscles are there, but you can't use them as effectively?
Exactly. It's like having a bigger engine but a weaker ignition. The muscle tissue develops normally, but the neural drive to activate it is diminished.
Does that mean people should still avoid cardio if they only care about strength?
If pure strength is the only goal, yes, isolation training would be better. But most people care about health too—heart health, endurance, calorie burn. The study suggests you don't have to sacrifice muscle growth to get those benefits.
What made researchers want to test this in the first place?
The interference effect was treated as gospel in fitness circles, but it was based on theory, not solid evidence. They wanted to see if the theory held up in real people over real time.