LONDON — The picture of the Olympics so far, an image that swept across the Internet and introduced the sports world to the term “quad-off,” might be one featuring two cyclists in their underwear with shorts around their ankles, their thunderous thighs prominently displayed.


The size of those thighs invited comparisons to large, cylindrical objects like barrels, telephone poles and ham hocks. Each thigh, on its own, seemed bigger than a female Olympic gymnast.


The image, a try-this-on-for-thighs comparison between one German cyclist nicknamed Gorilla (Andre Greipel) and another nicknamed Mr. Thigh (Robert Förstemann), underscored a more serious notion. Namely that cyclists, particular track sprinters, rely on quadriceps, in all their massive, veined glory, to power them to success. Förstemann’s thighs, each comparable to a watermelon, measured 34 inches — wider than his waist.


“The picture is definitely real,” said Benjamin Sharp, the high-performance endurance director for USA Cycling. “Cyclists have strange shapes: big quads, small waists and big butts. It’s hard to find pants.”


He paused, then added, “It’s funny we’re talking about this.”


Olympic track cycling started Thursday, inside the velodrome, where seemingly everything, including the design of the arena, the track inside and the athletes’ helmets, emphasized the sleek and the slim. Everything, that is, except the thick, bulky and somewhat frightening quadriceps of the competitors.


To cycling teams and their support staffs, this is less an Olympic oddity and more a necessity specific to their sport. The British track cyclist Chris Hoy, who collected his fifth career gold medal in the men’s team sprint Thursday, noted recently that his thighs measured 27 inches, or size 8 for a woman’s waist. Of course, Hoy can also cover a kilometer on his bicycle in under a minute.


Hoy’s former teammate Jamie Staff could fit his wife’s skirt snug around one thigh when he won a gold medal at the Beijing Games.


“Insane,” said Staff, now a USA Cycling coach. “Basically, going quick on a bike as a sprinter is about leg speed and leg strength. Naturally, quads help.”


Beth Newell, a United States national champion from California, is considered something of an international quad expert in cycling circles. She first measured the muscle when a skinny friend suggested that Newell’s quads were bigger than the friend’s head.


In a 2007 blog post Newell outlined the steps for proper quadriceps measurement. One: wrap string around the thickest part of the leg. Two: align string with tape measure. Three: pull string taut. From then on, Newell became, in her own words, “a crusader to glorify the big quad,” updating her Web site with her latest measurements and those of her competitors.


Improper measurement drove Newell bonkers. Unlike their track counterparts, road cyclists, as endurance riders, often balk at the reality of their thigh size, she said, when they should celebrate it instead. Her advice: Go for girth.


“It isn’t like measuring waistlines for skinny minnies,” Newell wrote in an e-mail. “This is about bragging about massive (sometimes) muscular quads! The people making those comments obviously didn’t get the point. Go big or go home!”


Athletes tabbed the baseline measurement for an acceptable sprint cyclist’s thigh at 60 centimeters, or 23.6 inches. Newell cited the American cyclist Jennie Reed as someone she envied in that regard.


Hoy noted in recent interviews that some of his competitors’ thighs made his look skinny. Perhaps there was a bit of thigh envy involved. Regardless, no one disputes that the biggest thighs in cycling belong to Förstemann, whom Newell referred to as Quadzilla.


“Those German track sprinters are pretty much legendary,” she wrote. “I don’t think any of them have names, even. They just get referred to by their quad size.


“Herr Achtzig to the line,” Newell wrote, using the German for Mr. Eighty.