CARDIO AND STRENGTH TRAINING workouts are typically thought to be useful for distinct purposes: Running is for moving your body, burning calories, and training your heart, and hitting the iron is for building strength and muscle. That’s the supposed order of the world. That said, you might see a jacked guy on the track or trail and wonder about his routine. Can you build muscle by simply running?

The short answer is no—at least not in the same way that strength training can. You shouldn’t expect that you can experience the same level of muscle growth by going on runs that you would by lifting weights. But there are some circumstances where you might see some muscle growth from running.

“If you’re untrained and you do some running, you will see additional lean body mass,” says Mike Nelson, Ph.D., C.S.C.S., an associate professor at the Carrick Institute. “But if you are more trained, running probably isn’t going to be a big enough stimulus to directly increase lean body mass.”

Even if you’re already trained, certain types of running can help increase the size of your quads, and sub in for some hard-to-find gym equipment. And pounding the pavement may also improve the results you see from your lifts in the gym. Nelson and Percell Dugger, a Nike running coach and founder of Fit for Us, explain how.

How Running Does (and Doesn’t) Build Muscle

There aren’t a ton of studies on the topic of running and muscular hypertrophy, but the few that exist back Nelson up: In a small 2017 study of untrained 19- and 20-year olds, running high intensity intervals three days per week for 10 weeks increased the size of the subjects’ quadriceps by 10 percent. A much older study that tested older, untrained men saw guys in their 60s and 70s increase their quad size by nine percent after six months of steady state running.

The reason these types of muscle gains don’t continue, Nelson says, is related to the amount of time your muscles are under tension. When you’re strength training, the muscles you train are under stress for the entirety of a repetition.

“Running creates more of an impulse load or impact load, which is going to be really, really brief,” Nelson says. Even though the impact can be huge—four to six times your body weight—the impacts don’t add up to as much time under tension as you’d get from a few sets of resistance training exercises, he says.

Just because the brief impacts of running aren’t building significant muscle size in other parts of the body doesn’t mean they aren’t making the legs stronger, according to Dugger.

“To think that [world champion marathoner] Eliud Kipchoge isn’t strong, you’d have to have a very limited understanding of what strength is and what it means,” he says. A belief that running will make you less strong, he says, holds many men back from doing cardio training that could benefit their health and well-being—and their strength training.

Just because running doesn’t make you bigger, Dugger says, doesn’t mean it will automatically make you smaller. In fact, it could help your resistance training work better.

How Running Can Improve Muscle Building (and Reveal Your Muscle)

While running doesn’t build a ton of muscle mass on its own, it may help with muscle growth in the long term, Nelson says—so long as you’re pairing it with resistance training.

There are two reasons this may be the case: First, running increases your aerobic capacity, or your ability to recover quickly from a bout of work. “If you look at someone who has a horrible VO2 Max or aerobic capacity, they’re probably going to be limited by just how much volume they can do in the gym because they can’t recover,” he says. Running builds this capacity, so you’ll have more gas in the tank to do more sets, more reps, and more volume when you’re lifting. And all that adds up to more muscle.

The second reason is also related to endurance. When you run, “you may have more local capillarization,” Nelson says. When you do aerobic training, your body makes more capillaries in your muscles. This increase in small blood vessels increases the blood flow to your muscles.

Why that matters: “Let’s say you’re doing a set of leg extensions, for example. At some point, you’re not getting enough blood flow there, so you’re not getting enough oxygen to the muscle. Therefore, it becomes painful and you have to stop,” he says. “In theory, if you can get more blood flow to that muscle, you can do more work and build more muscle.”

Don’t be so concerned that supplementing your strength training with running will cut into your potential for gains, as is typically thought to be the case. A 2021 systematic review of 43 studies published in Sports Medicine concluded that “concurrent aerobic and strength training does not interfere with the development of maximal strength and muscle hypertrophy compared with strength training alone.”

Basically, this means that you can still make strength and muscle gains while you train for endurance, too. Just remember, though, that your runs will not be the direct cause of the muscle growth.

How to Use Hills to Build Muscle

Running can not just enhance your muscle building, but when done in certain ways, can even aid parts of your strength training routine, Dugger says. One of those ways? On a hill.

“For someone who has been doing sled pushes, go find a steep hill, and run up it as fast as you can,” he says. “I promise you: Your heart and your mind will feel like you’re carrying a 500-pound sled or wearing a 50-pound vest.”

For guys who don’t have access to that equipment, or who want to get outside, this kind of hill work can offer the same high-intensity, muscle-building finishers as expensive sleds, he says. Nelson agrees. Compared to sprinting on flat land, which can lead guys to suffer injuries when they’re not used to super speeds, hill work provides similar, intense work while making trainees go slower, he says.

One caveat, he says: Running downhill is a completely different beast. This type of work is focused on different muscles than when you ascend. Your quads are working on the way up, while your lower legs (think calf and shin muscles) will bear more of the burden on the way down.

So if you want to swap in hill sprints, choose a steep hill to run up a few times each week. But be smart: Walk back down the hill before sprinting again.