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First, let’s take a peek at how sperm is analyzed. Most studies look into at least these three things when judging your seed:

1. Sperm count, or how many millions of sperm there are in a milliliter of ejaculate. According to the , more than 15 million per milliliter is normal.

2. Motility, or how well those suckers swim. At least 32 percent of them should have good forward motion.

3. Morphology, or how many do not have defects, like double tails, double heads, large, or small heads.  At least four percent of your sperm should be normal looking.

How these parameters will affect your fertility, however, is not entirely straightforward. A high score on sperm count, for example, may make up for low motility because the overall number of good swimmers in your ejaculate may equal that of someone with average sperm count and motility.

And now to answer your question:

“It’s becoming more clear that one of the only activities that seems to be detrimental is cycling,” says Harvard fertility researcher . Several studies have linked time in the saddle to —the more time you ride, the more demented your swimmers get. that men who had less than four percent normal-looking sperm were consistently riding more than 186 miles per week. It’s still not entirely clear whether it’s the tight shorts, friction, or position on the bike that leads to bad looking sperm. “It’s probably due to the mechanics of it,” Gaskins says.

Additionally, she says, poor sperm quality is often observed not only in avid cyclists, but also marathon runners because of the “severe energy imbalance” from which both types of athletes often suffer. published in 2004 found men whose body mass index was less than 20 had a 36.4 percent reduction in total sperm count (the number of sperm in the entire ejaculate), and a slight reduction in the number of normal sperm. .)

“One thing about semen quality that’s nice is it’s a three-month exposure,” Gaskins says. “So you’ll likely be able to change [any problems] if you change your behavior. You’re not necessarily hurting yourself in the long term, we don’t think as of right now.” 

THE BOTTOM LINE:  If your BMI is less than 20, or you cycle more than 186 miles per week, you may up your risk of having poor quality sperm. But if you cut back the mileage or increase your BMI for at least three months, any damage you’ve done, researchers believe, can be reversed. There’s not much evidence that any sport besides cycling will hurt your sperm quality.