"My toenails turning black happened before," said Jessica Horne, 26, an amateur athlete who will be running for the second time in the ING New York City Marathon this Sunday.
"But it wasn't until this intense, long-distance marathon running that I discovered everything else," Horne said.
Horne is just one of thousands of non-elite athletes joining the marathon boom in the United States. And as more amateur athletes take up the sport, more people like Horne are discovering the "everything else" side effects that come from pushing their bodies literally to the limits -- black toenails, missing toenails, stomach problems and bloody appendages.
In 1970, the New York Marathon only had 127 runners. By 1976 it was 2,090. Last year, 39,265 people started and 38,607 finished.
"This sport is exponentially increasing," said Dr. Lewis Maharam, medical director for the ING New York City Marathon and a columnist at runnersworld.com.
As an athlete during high school, Horne had already experienced the infamous "black toe" before she started training for a marathon.
Black toe, technically called subungual hematoma, is just a simple bruise from repeated impact that turns into a pool of blood under the nail.
Bruce Williams, president of the American Academy of Podiatric Sports Medicine, ran in six marathons and treated hundreds of marathoners in his career.
"We've seen lots of things from lots of blisters, black toenails, in-grown toenails, stress fractures," said Williams. "Just about everybody is going to get a blister."
Unlike many other sports where athletes tear ACLs or break bones, the majority of injuries in marathons tend to be overuse injuries, said Riann Palmieri-Smith, a certified athletic trainer at the Bone and Joint Injury Prevention and Rehabilitation Center at the University of Michigan in Ann Arbor.
"One of the biggest things that happen when people injure themselves is they're really running too far at a low intensity, or they are rapidly increasing their intensity," said Palmieri-Smith.
All that rubbing and pounding can cause tendons to rub over joints (called ITB or illiotibital band syndrome), inflamed muscles in the foot, foot stress fractures, skin chaffing on skin and nipples rubbed so raw from moving shirts that they bleed.
"You want to have as little on the body as possible," said Horne.
To help with chaffing, she uses a product called 'glide' on her skin. Luckily, Horne said, she doesn't have a problem with bleeding nipples.
"That makes me just shiver," said Horne. "It mainly happens to men because women have sports bras. A lot of the guys that I train with on my team wear Band-Aids to help."
Horne runs with the Team for Kids, which has raised millions of dollars to improve the lives of children in the United States, as well as in the Cape Town region of South Africa.
As for her black toe, Horne said she never lets it slow her down.
"It never really bothers me; it hurts in the beginning when my toenails turn black and blue," said Horne. "Another toenail grows underneath and the black and blue one falls off."
Black toe can also happen to mountaineers, hikers, and other athletes.
A great documentation of a black toe experience can be found on the Starling Fitness Blog.