CHAPEL HILL -- The Cash sisters can skip rope 170 times in a minute, wearing their arms down to jelly, slapping welts on their legs.

It's an offbeat hobby that started when they were schoolgirls, then grew into a mania.

Flips. Leapfrogs. Dives to the floor. Makenna and Suzanne Cash jump rope like linebackers hit tackling dummies.

People sometimes chuckle when they mention their competitive jump-rope team, Skipsations, which practices three times a week. But in July, the Chapel Hill team flies to South Africa for a shot at the world title -- facing off against the most intense, no-prisoners jump-rope teams in the world.

"My team is pretty chill," said Suzanne, a 19-year-old N.C. State University student, "but some teams are like a cult. Jump rope takes over their lives. It's kind of insane."

The International Rope Skipping Federation boasts more than 10,000 members worldwide, and it's growing. The Cash sisters' mother jumps, too.

In the world of offbeat sports, jump-roping is hard to match for obsessiveness. How many people sweat over what kind of jump rope to use: plastic, beaded or wire?

The skipping-rope federation dates the pastime to the ancient Egyptians and Chinese, who had to hop over strands of hemp while they wove them into rope.

Most people think of Sylvester Stallone in the "Rocky" movies, jumping rope between chasing chickens and punching sides of beef to train for the ring, but the point of big-time skipping is to dream up tricks and routines.

On a whim Sunday, the Cash sisters did pushups over a twirling rope while a teammate hopped over them.

They use wire ropes for the speed jumping, which sometimes smack against legs and leave red marks. It ain't hopscotch.

"People laugh at first," said Makenna, 22, who starts at N.C. State's College of Veterinary Medicine in August. "But then I tell them I've been to Belgium and Australia for competitions and they're like, 'Whoa.' "

Last year, team members got a free trip to Turkey, where they performed along with a contortionist act. The team has about 30 members, not all of whom compete internationally.

Win or lose, when they finish in July, they will have jumped rope competitively on four continents -- stepping in perfect time, linked by a fast-flying tether.