BERLIN (Reuters) - After 15 days sitting on top of a two-and-a-half-meter (eight-foot) high pole, a momentary shift of his buttocks shatters Benjamin Buettner's dreams of glory in the 2003 World Pole-Sitting Championships.

The contestants must stay on the pole day and night, squatting on a 40 by 60 cm (15 by 23 inch) board -- about the size of a broadsheet's front page. They may come down for only 10 minutes every two hours to perform their ablutions.

"If you get back up there a second late, you're out," said organizer Klaus Mueller from the Heide Park Soltau amusement park in western Germany where the event is staged.

Ladders allow competitors to go up and down and also supply them with food and drink or whatever distractions they need to while away the hours on their bird-like perches.

To make sure no one cheats when the rest of the world has gone to bed, electronic sensors attached to the board monitor the pressure of backsides. Video cameras run constantly, as 22-year-old ambulanceman Buettner found out.

"He tried to dupe us by pressing on the board with his hands and stealing a few seconds of respite on the ladder, but the camera caught him and the judges threw him out," said Mueller.

This year's field of iron-bottomed contestants includes the reigning world pole-sitting champion, a 27-year-old mechanic who happens to be a Pole. Egypt, Hungary and Germany were also represented, but for some the stakes were too high.

With the contest entering its 17th day, only six of the 10 starters remain in the fray. They are bracing for a long wait.

"The current world record stands at 196 days. This year they want to crack the 200-day mark," Mueller said.

That would mean sitting on the pole until mid-November.

The unusual sport dates back to 1952, when villages in the Dutch province of Holland were flooded and inhabitants sat on top of poles until rescuers arrived.

The Dutch are the sport's purists.