A lot of people hate on cardio, cuz it will kill yer gainz, brah.

Or something.

Question: How big do you need to be?

Life is a series of compromises. You can’t be an elite marathoner and be huge. Likewise, you can’t be a competitive bodybuilder and be a fast in the marathon. But you can be muscular and be a pretty fast marathoner. It’s all a continuum of deciding where to place your focus. Also, genetics.

Yes, lots of aerobic exercise is going to inhibit muscular growth. Inhibit, not stop. If your goal is to get as massively muscular as humanly possible, marathons are not for you. Any aerobic exercise should be limited. Conversely, if you want to run marathons really fast, extra muscle mass will weigh you down and not help you fly along the 26.2 mile course.

But there can be a happy medium, if you so desire.

Doing something like qualifying for the Boston Marathon is no easy task. Most people don’t run. Most runners don’t run marathons. Most marathoners don’t qualify for Boston. It’s a small percentage of a small percentage of a small percentage that can say they have qualified to run Boston.

That’s kind of, sort of, a little bit elite. Not competitive. Were not talking anywhere near earning prize money here, but it’s a damn tough test of your endurance to make the qualification time, and you don’t need to be a waif to pull it off. Yes, the Schwarzenegger-sized would be hard pressed to run a Boston qualifying time, but again, how big do you need to be?

When I was 45 and training hard for my qualification marathon—the one that would allow me to run Boston—I was easing off on the weightlifting a lot just due to being too damn tired. I’d been a dedicated runner for 10 years averaging at least 30 miles a week, and I spend many hours a week on a road bike as well. During that qualification training, at most, 25% of my exercise time was dedicated to lifting.

So, at age 45, lifting weights only about two hours a week, during peak marathon training, I took this biceps selfie:

Definitely not huge, but far from scrawny. And no, I hadn’t pumped it up. I hadn’t lifted in two days when that shot was taken.

Of course, that photo is just an anecdotal example. It’s N = 1. My genetics are such that I am not very good at everything, and through hard work can be mediocre at everything. I’ll never be massive, or have Herculean strength, or be an elite endurance athlete. I’m genetically destined to be middling.

But just how anathema is aerobic exercise to muscular gain?

Steps Forward and Backward
If you desire larger muscles, it’s a “Well, duh” that resistance training is required. But if you also do aerobic training, does this undo your efforts? Answer: Somewhat. If you have a challenging lifting program that promotes increases in strength and size, then this will beinhibitedby large amounts of aerobic activity. The more aerobic activity (lots of running), the greater the inhibition.

But inhibition doesn’t mean it completely “killz gainz.” It you’re starting out with both resistance and aerobic training, “inhibition” means you’ll gain less muscle than if you did no or little aerobic training.

But to get the full story, I needed to talk to the man. Brad Schoenfeld, PhD, is the man when it comes to the science of building muscle.

So, what about these steps forward and back?

“It comes down to specifics of the protocol, as well as genetics and lifestyle,” Brad said. “Building muscle and endurance training are at opposite sides of the continuum. To build muscle you have pathways that promote anabolism, and the aerobic pathways are generally catabolic, and they interfere with each other. When you jack up aerobic training to a certain degree it can interfere with muscle building pathways.”

“At a basic level they’re not good for one another,” he said. However, high-intensity interval training does have certain anaerobic aspects that are more friendly to building muscle.

Brad explained that it comes down to how much aerobic activity you’re doing. He doesn’t recommend that bodybuilders run marathons, that’s for sure. But there is more to it than volume of aerobic training: “Genetics, nutrition, sleep and stress can all affect muscle growth. You can’t give a cookie cutter because it will vary from person to person.”

But this doesn’t change the fact that controlled studies repeatedly show that lots of aerobic exercise will interfere with the ability to build muscle. “It’s going to blunt anabolic potential,” Brad told me.

What’s interesting is that running does seem to have a greater negative impact on muscle building than cycling does. That being written, Brad said that running three days a week for about 40 minutes each time probably won’t inhibit muscle growth that much. So moderate running is okay. Training to qualify for Boston, however, is not a great idea if you’re looking to pose on stage in a Speedo.

I have no such bodybuilding aspirations. I like being muscular, and I can run a fast marathon and still be a good size. I never had the desire to be huge. Also, I likerunning long distances.

But Brad explained it’s important to understand that the body adapts to what it is given. I’m kind of muscular and kind of good at marathons. If I eschewed one for the other I could get better at the one I focused on. If I wanted to be bigger I could quit all aerobic training, focus on lifting and maybe start taking creatine. If I wanted to run my fastest marathon ever I could cease all lifting and drop the “useless” extra biceps, pecs and deltoids weight and get a little faster. It’s all about finding compromises I’m happy with. I have reached a happy medium.

These are my genetics. Yours are different. Brad advises making adjustments to your training based on your own desires and your own results. But you can lift and do marathons, and while you won’t be huge, you might still qualify as “big.”

Just one last thing you should know is that …

Power takes a Pounding
Here’s a quote from Essentials of Strength Training and Condition: “Chronic high-volume running creates a catabolic response that can lead to muscle degradation and reduction in power. It is not uncommon for elite aerobic endurance runners who are training for high mileage to be able to jump vertically only several inches.”

Agreed. Don’t ask me to play basketball with you. I can’t jump worth shit.

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James S. Fell, MBA, writes for the Los Angeles Times, Chicago Tribune, Women’s Health, Men’s Health, AskMen, the Guardian, TIME Magazine and many other fine publications. His first bookwas published by Random House Canada in 2014. He is currently working on his next book, which is about life-changing moments.