Why Cardio and Weight Lifting Both Help Maximize Muscle


The fitness world has always been split down the middle between those who love to lift and those who love doing cardio. Most people who seek to get in shape choose to either build muscle by lifting weights and training with short, intense workouts, or burn fat by hitting the treadmill and exercising at a moderate intensity for longer workouts.

Obviously, for well-rounded fitness, you need a blend of both approaches; but past research has shown that aerobic exercise can lessen muscle and strength gains—a heartbreaking compromise for lifters, and fuel for grizzled gym veterans who like to warn skinny newbies that doing any cardio will keep them small forever. But, good news: Recent studies indicate that the so-called “interference effect” that aerobics has on muscle isn’t what we thought. And that means a lifter can have his weights and his treadmill, too. Here’s how to do it.

Why cardio got a bad rep

When you do steady-state cardio (e.g., aerobic training, including long runs, cycling, etc.), you run down your body’s energy reserves. That causes the activation of a compound called AMPK. In vitro and rodent studies over the past 20 years have shown that, to protect your body by preserving its energy stores, AMPK inhibits a protein called mTOR, which is the body’s main driver of muscle growth.

Add to that the findings of a study that showed legs-press strength was reduced more in a group that lifted and did aerobic training than in guys who only lifted, and you can see how the anti-aerobics movement gained steam.

In other words, it would seem that at least slow cardio (as opposed to high-intensity interval training, or HIIT, which is cardio but anaerobic in nature) would sabotage your ability to make strength gains. Right?

What the new science says

Wrong—at least according to long-term human studies. A recent review in Sports Medicine found evidence to suggest that, while the interference effect does damage muscle signaling within the body, there’s practically no proof it harms actual muscle gains over time. It’s the difference between smelling smoke and seeing fire.

Another study in PLOS ONE this year pitted a resistance training-only group against subjects who lifted and did steady-state cycling for 30 to 60 minutes. Not only did both groups improve their max legs press by about the same amount, but the lifting-plus-cardio guys actually gained much more muscle than the lifting-only guys—and of course that’s in addition to improved aerobic capacity. Why they gained more muscle isn’t clear, but it may be because aerobic training activates type I endurance muscle fibers, while heavy lifting works mainly type II fibers.

And how does HIIT affect lifters? The Journal of Strength and Conditioning Research found not only that it didn’t hinder gains in strength or muscle mass, but that it also actually led to improved body composition.

How you can strike a balance

“The idea that 30 minutes of cardio is going to eat up muscle makes me laugh,” says Don Saladino (driveclubs.com), a trainer to celebrities and athletes in New York City. “The body is more resilient than we think.” He does, however, caution that over-training is a possibility with any exercise, and that if you mix cardio and lifting, you should consider how much you can recover from.

Ask yourself how active you are out of the gym—e.g., do you work at a desk, or do manual labor?—and how much your diet and sleep are on-point. But 1-2 HIIT sessions and two or more low-intensity aerobic workouts on top of three to four lifting days per week should work well for most people.

Skeptical? Consider this: Any given week, Saladino uses various aerobic and anaerobic cardio methods ranging from walking to battling ropes—and he looks like a Roman gladiator.

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