Men, here's another reason to work up a sweat: It boosts your sperm count.

According to new research, couch potatoes who watch lots of TV have fewer sperm than men who exercise moderately or vigorously each week.

Sperm count is a measure of semen quality, which has mysteriously declined in U.S. men in recent decades. Low sperm count is linked to infertility as well as testicular cancer, prostate cancer, and cardiovascular problems later in life.

That's why scientists have been searching for ways that men can improve their sperm—including changing daily behaviors.

"We know little about how lifestyle may impact semen quality," said study leader Audrey Jane Gaskins, a doctoral student in nutrition at the Harvard School of Public Health.

Her discovery that two changeable behaviors—exercise and TV watching—"could have a big impact on sperm count is pretty exciting," she said.

"Impressive" Sperm Finds

For the study, Gaskins and colleagues asked 189 young men, mostly college students from the University of Rochester in New York, to fill out questionnaires on their physical activity, diet, stress, and other lifestyle factors.

Each man then provided a semen sample in a medical clinic. (See "Sperm Recognize 'Brothers,' Team Up for Speed.")

The results showed that the men who reported exercising more than 15 hours a week had 73 percent higher sperm counts than men who exercised fewer than 5 hours a week. And men who watched more than 20 hours of TV had 44 percent lower sperm counts than men who watched little to no TV.

These are "pretty impressive differences," said Gaskins, whose study appeared February 4 in the British Journal of Sports Medicine.

The team also ruled out smoking and being overweight—two clinical causes of low sperm counts—as contributing factors among participants in its study.

Why couch potatoes produce less sperm is unknown, although there are two theories, Gaskins said. One is that exercise produces more antioxidant enzymes that can prevent a natural process called oxidative stress from damaging cell membranes in the body. This damage can disrupt the creation of new sperm cells. (Find out how a man produces 1,500 sperm a second.)

The other reason is somewhat controversial: That when men watch TV, their scrotums get squished against their bodies, making that region hotter and possibly preventing new sperm from being made.

Some research has shown that sperm production slows if the scrotum temperature rises 1.8 to 3.6ºF (1 to 2ºC) above normal body temperature, Gaskins said. But other studies have also shown that warmer scrotums have no bearing on sperm creation.

Get Moving, Men

But the study also raises some more questions about sperm count, experts noted.

Oddly, the sedentary subjects' sperm didn't show any changes in their physical structures or in how well they swam—two soft indicators of sperm health, noted Phillip Mucksavage, a urologist at Pennsylvania Hospital in Philadelphia who was not involved in the study. (See "Sperm Tracked in 3-D-A First.")

Though there are fewer of them, "the sperm that are there still look good."

Mucksavage added that the study's results would have been stronger if the scientists had found other sedentary activities—such as sitting at a computer or reading a book—had the same affect on sperm count as did watching TV.

What's more, said T. Mike Hsieh, a urologist at the University of California, San Diego, the study doesn't have any implications for fertility, one of the main reasons men are concerned about sperm count.

That's because one semen sample is not enough to determine fertility. That requires a more thorough analysis, including multiple semen samples and blood work, said Hsieh, who wasn't involved in the study.

It's not like if you "stop watching TV you'd go from infertile to fertile," he said.

But all the experts agreed on one thing: you should get active if you aren't already.