Your body has an amazing way of adjusting out on a run, and for men that includes what’s going on below the belt.

“Your body has ways of adapting to prevent bad things from happening to your penis and testicles,” says Jeffrey Thill, M.D., an Orlando-based urologist who has completed five world marathon majors.

Even if your running habit puts your man parts in some uncomfortable situations—like a long run on a sweltering afternoon without adequate support—you don’t have much to worry about, adds Tobias Köhler, M.D., a urologist and chief of male infertility at Southern Illinois University.

“Your testicles’ ability to draw up toward your body is called the cremaseric reflex,” Köhler says. “And that’s just one of the ways your penis and testicles adapt to prevent injury.”

But there are still a few concerns regular runners need to be aware of, Köhler and Thill say. There are also a few perks that should encourage men who don’t run to dust off their running shoes.

Running Could Cause Testicle Pain

Roughly one out of every seven men experiences some testicle pain caused by a varicocele—a kind of enlarged, malfunctioning vein inside your scrotum, Köhler says.

Varicoceles are genetic, and if you have one, the muscle clenching involved in running can cause blood flow to back up and enlarge the varicocele, which leads to extra pain.

Some guys also experience pain that defies explanation. “I have men come in worried about cancer, but a lot of the time their pain just doesn’t have an identifiable cause,” Köhler says.

But even in those unexplained cases, running can heighten your agony, he says. Wearing compression shorts or regular running shorts with a liner that keep your “boys” secure from jostling can help.

Too Much Compression Can Be Problematic

While some support can make your run more comfortable, sporting too-tight compression shorts or leggings can restrict blood flow to your groin, which could lead to testicle pain, Thill says.

“If you feel pain or numbness, your body’s letting you know that’s too much,” he explains.

The good news: There’s really no concern when it comes to compression and fertility issues, he adds.

Running Can Supercharge Your Erection

Anyone who runs regularly—a couple days a week or more—will improve their blood flow and heart health, Köhler says.

“And since we all know your heart’s main purpose is to pump blood to your penis,” he says, jokingly, “strengthening your heart will also strengthen your erection.”

If you’re currently a couch potato, he says taking up running will lead to improved erections within a few months. A recent study from Duke University School of Medicine found that men who reported 18 MET hours/week of exercise—similar to two hours of hard running—will benefit your erection.

Running Can Also Strengthen Your Sperm

“We have lots of evidence to suggest physical activity—and running in particular—may improve a man’s fertility,” Thill says.

One recent study appearing in the British Journal of Sports Medicine found men who run or perform other moderate-to-vigorous exercise 15 hours or more a week have sperm concentrations up to 73 percent higher than sedentary men. The active dudes also have higher sperm counts.

Both Thill and the study authors say regular exercise increases your body’s levels of testosterone, which could explain the sperm-health benefits.

But Too Much Running Can Harm Your Sperm

While running strengthens your swimmers, there’s some evidence men who run a ton—roughly 67 miles or more every week—may suffer a 28 percent drop in circulating testosterone, as well as lower semen quality.

That’s according to an older study from the University of Connecticut. Some newer research has raised similar concerns.

The evidence is far from conclusive. But “it seems you can get to a point where if you run too many miles, it kind of creates an inflammatory response throughout your body that’s not helpful for sperm health,” Thill says.

Köhler agrees, and says exercise puts stress on your body.

“That stress is helpful even in regular doses,” he says. “But for guys who are always marathon training and really running a long way every day, there may be some negative effects to that physical stress, including lower testosterone and impaired sexual function or fertility.”

The same Connecticut study found testosterone and sperm quality peaked among runners who logged about 35 miles a week—though that was a rough average.