TLDR;
On Tuesday we broke down the physiology of hybrid training and why running doesn’t melt all your muscles away. Today I’m explaining where this belief actually came from, why it spread so easily, and how context changes everything:
The gym-bro hangover: For years, having huge muscles was treated as the epitome of fitness online. Cardio was framed as the enemy of that look, and running has only really exploded online in recent years.
Most people are exercise scientists: Most people get their fitness "knowledge" from short clips. When enough influencers repeat the same phrase, it becomes accepted as fact simply because "everyone says so."
Context is everything: If your primary goal is stepping on a bodybuilding stage, high running mileage might be a hindrance. If your goal is overall fitness, athleticism, and health, running and lifting belong together.
Big muscles got mistaken for fitness
On Tuesday, we looked at the actual science behind concurrent training: Robert Hickson’s 1980 experiment, what the interference effect actually is, and why local fatigue (not molecular sabotage) is the real bottleneck.
That’s what the lab data tells us.
So why is the phrase "running kills your gains" still repeated like absolute gospel every time someone laces up a pair of running shoes?
Because it didn’t start in a physiology lab. It came out of early internet gym culture.
For a very long time online, "fitness" meant exactly one thing: bodybuilding.
Getting huge, standing under harsh gym lighting, and carrying as much mass as humanly possible was the unquestioned pinnacle of being in shape. In that world, anything that burned calories without adding bulk to your biceps was seen as a threat.
Cardio and long-distance running was treated as the ultimate sin. The joke was always that jogging for 20 minutes would somehow strip off all your hard-earned muscle overnight.
It’s only really in the last few years that running, trail running, and endurance sports have started becoming genuinely popular on social media. But for a decade before that, the gym-bro narrative had complete control of the fitness feed.
And once a belief like that settles into the culture, it’s remarkably hard to shake.
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Repeat it enough and it becomes truth
Let’s be honest: most people are not educated in exercise science or human physiology. And there’s nothing wrong with that; most people have jobs, families, and lives, and they just want to stay in decent shape.
So where does the average person get their fitness knowledge?
Social media. (Though whether what circulates on TikTok and Instagram counts as actual "knowledge" is definitely up for debate.)
Social media platforms are built to reward certainty, simplicity, and catchy hooks. A thirty-second clip saying "STOP RUNNING, IT'S EATING YOUR MUSCLES" performs a hundred times better than an unsexy video explaining glycogen depletion and systemic nervous system fatigue.
When someone opens their phone and sees ten different fitness influencers all repeating the exact same talking point, the human brain takes a shortcut:
Well, everyone else is saying it, so it must be true.
That’s how a total myth gets accepted as an unquestioned rule of thumb.
At its core, the whole "running kills gains" panic is simply a misunderstanding of fundamental bodily functions, wrapped in a catchy social media hook.
Your body is not a delicate machine that panics the second your heart rate stays elevated for an hour. It adapts precisely to the demands you consistently place on it.
It completely depends on what you're training for
Like almost everything else in health and fitness, whether running interferes with lifting comes down to one simple question:
What are you actually training for?
If your number one, non-negotiable priority is maximizing muscle hypertrophy (muscle growth) if your goal is to step onto a bodybuilding stage or carry maximum size, then yes, running 50 or 60 kilometers a week is probably going to be a hindrance.
Not because it kills gains, but because it builds a different type of muscle.
Doing a hard tempo run right before heavy squats will leave your legs fatigued, reduce the mechanical tension you can generate, and compete for recovery resources. In that specific context, running isn't helping your goal.
But if your goal is overall fitness, longevity, cardiovascular health, and being a capable human being?
Then running doesn't kill your progress at all. In fact, combining the two builds a level of work capacity, joint resilience, and stamina that you simply cannot get from lifting weights alone.
The problem isn't the physiology. The problem is people applying the hyper-specialized rules of competitive bodybuilding to normal people who just want to be strong, fit, and healthy.
Don't let an algorithm dictate how you move your body. Be clear about your own goals, give each session the recovery it deserves, and don't mistake social media consensus for scientific fact.
If you're trying to figure out how to balance strength work with high running mileage without burning out, you're always welcome to book a 1:1 call with me. It’s completely free and no strings attached.