(Adds dropped word "of" in fourth paragraph)
By Dorene Internicola
NEW YORK, July 29 (Reuters) - Triathlons, the three-prong running, biking and swimming races, are more popular than ever in the United States and fitness experts say anyone who trains properly can complete one.
Runners hail the triathlon, which offers a whole body, cross training workout, as the new marathon.
"Triathlon is any multi-sport competition that has swimming, biking and running at different distances," said Emily Furia, who has been competing in the triathlons for more 10 years.
Participation in triathlons in the United States is at an all-time high, according to USA Triathlon, the sport's governing body in the United States. The group's membership has swelled from around 100,000 in 1998 to 550,446 last year. Anyone wanting to participate in any of its 4,300 races a year must be a member.
The Sports & Fitness Industry Association, a trade organization, estimates the total number of triathlon participants rose 59 percent from 2008 (1,251,000) to 2011 (1,992,000), according to the SFIA.
Furia, a senior editor at Bicycling magazine, said the phenomenon is a natural outgrowth of the rise of other endurance sports, such as marathons and long-distance cycling.
"I personally don't do the Ironman," Furia said of the challenging, long-distance triathlon, which includes a full marathon run. "But competition isn't why a lot of people get involved."
There are many types of triathlons, ranging from the easiest, known as the sprint, to the most challenging - the Ironman.
Furia said many people can accomplish the most popular, the sprint triathlon, which includes an approximately 1640-foot (500-metre) swim, 12-mile (19 km) bike ride and 3-mile (5 km) run, "with a huge sense of accomplishment."
More challenging is the Olympic distance, typically a 0.9-mile (1.5km) swim, 25-mile (40 km) bike ride and 6.2-mile (10 km)run, which became an Olympic sport in 2000.
The Ironman race, with a (2.4 mile (3.4 km) swim, 112 mile (180 km) bike ride and 26.2 mile (42 km) run), is the most challenging.
"We're trying to attract individuals who thought they couldn't do it," said USA Triathlon spokesperson Chuck Menke, whose organization also sanctions events where the focus is fun rather than competition.
"We believe you should prepare and train but we also believe anyone can do it," he said.
Dr. Mark Kelly, an exercise physiologist with the American Council on Exercise, said the multi-sport training can reduce overuse injuries by not stressing one body part too much.
"Swimming gets upper body and some core, cycling is leg strengthening, and running, the most common, has some pounding on the joints," he said. "Cross-training has interesting benefits and it's fun because of the variety alone."
Kelly emphasized that preparation and pacing are the keys to success.
"You should really know those distances and have as few surprises as possible," he said. "And you really need to practice that bike-to-run transition, especially with the legs, it can be a rude awakening."
Chicago-based trainer Kai Karlstrom helps clients at Equinox fitness centers gear up for their first triathlon.
The transition, he agrees, is an art in itself. But the open water swim is where the novice is most likely to fail.
"People practice in a pool with lane lines," he said, a necessity when training for a summer sport in a Chicago winter. "In open water there's no lane line. If they don't practice open water sighting, they'll zigzag and swim almost twice as much they need to."
People are motivated to train for a triathlon, he added, because at the end of it they can show off what they've learned.
"It's as simple as finishing it, beating your time, sharing the camaraderie," he said.
Furia advises newcomers to put extra training time into their weakest sport and on race day to push hardest with the best.
"In other words," she said. "Train your weakness, race your strength." (Editing by Patricia Reaney)