Penis fractures are about as fun as they sound (and apparently, that sound is often a “crack” followed by a blood-curdling scream).

If you want to avoid a case of the “boner snaps,” it’s probably best to stay extra cautious while enjoying the “cowgirl position.” Brazilian researchers have conducted a study, published in Advances in Urology, which found that the “women on top” position during heterosexual sex caused the highest amount of penis fractures among a sample group.

Of course, the human penis doesn't actually have a bone. So what exactly is going on when a penis fractures? When aroused, blood flows to the erectile tissue and causes it to balloon up. This increases pressure and causes the penis to become rigid and hard. Suddenly forcing the penis to bend when it's in this state (such as hitting the perineum during sex) can rip the surrounding tissue that’s holding the pressure in the tissue. The rip causes an unpleasant “crack” sound. The result is pain, more pain and pretty severe bruising. Occassionally, it can also cause damage to your urethra, so if you are unfortunate enough to suffer a penis fracture, doctors always recommend you take a trip to the hospital.

The study looked at all of the penis fractures between January 2000 and March 2012 that were reported to the three main emergency hospitals in the city of Campinas, Brazil. The population of this area is over 3 million people, so you’ll be relieved to hear they only received 44 patients over the study period.

The majority of penis fractures were found to be sustained during heterosexual sex, accounting for 66.7 percent of patients that the hospital saw. A further 14.3 percent were caused by “penile manipulation” and 9.5 percent were caused by homosexual intercourse. The cause of the remaining 9.5 percent were classified as “unclear.” Perhaps it’s best to leave it that way.

In hetrosexual intercourse, the cowgirl position was responsible for half of penis fractures, 28.6 percent were experienced during “doggy style” (that was the researchers' official medical terminology) while 21.4 percent occurred with the man on top. 

In homosexual intercourse, both "main positions" appeared to be very similar: Half of the patients reported being on top and the other half were in the “doggy style” position.

The research also found that mean age of sufferers was 34.5 years.

As cringingly horrible as a penis fracture sounds, you’ll be pleased to know that the long term effects of a penis fracture were relatively mild. Just two patients experienced long-term erectile dysfunction and four reported minimal penile curvature, which is totally harmless in the majority of cases.

Stay tuned to IFLScience for all the latest penis news.