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 Forstemann), 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. Forstemann’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 U.S. 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 website 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 email. “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 Forstemann, 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.

To be fair, participants did note that quad size does not necessarily correlate to speed. Imagine a sumo wrestler, for instance, on a race bike. Victoria Pendleton of Britain, winner of nine world titles and of a gold medal on Thursday, is but one example of the regular-size-thigh cycling set.

Lack of quad size, though, can doom even the most promising young riders, the equivalent of a weight lifter with small biceps. Big thighs, the participants said, serve as an intimidation factor before races, a way to send a message to competitors without speaking, with no more than a quick flex.

Newell said some riders rubbed warming oil on their quadriceps before races, because the shine accentuates the size of the muscles, giving them that bodybuilder cut. She argued against team uniforms that included thinning stripes, a “serious no-no” in the quad game.

“I mean, we pedal hours and hours every week — it’s a full-time job — so often, our bodies are seen as a measurement of our ability,” she wrote.

Much of a cyclist’s thigh growth, Sharp said, is natural, the byproduct even of particular positions on the bike. Sprinters who sit hunched over, more compact, tend to use their quadriceps more, and because they spend thousands of hours in the same position, the body conforms accordingly.

Staff said much of his thigh expansion came from workouts. He loved that part of cycling, in particular the squats. His exploits in that lift still live in videos on YouTube. At his peak, Staff said, he could squat about 529 pounds.

Even now, when he is retired and coaching, his thighs compare favorably to those of the athletes on his team. He could still win a quad-off, which is cycling’s version of the male model walk-off in the movie “Zoolander.”

Food helps, too: massive quantities of protein, enough calories to feed a large village. Newell recommended living with noncyclists, so as to finish what they do not eat. She also advocated drinking microbrews and having dessert with breakfast. She said an Australian coach gave his sprinters chocolate cake between intervals to enhance quadriceps growth.

This often leads, of course, to what cyclists call the pants problem. For years, Newell pestered one company executive with her campaign for “keirin cut” jeans, roomier in the quad, less so in the waist. Keirin is a type of cycling race.

Hoy told the British newspaper The Guardian that women often approached him and said they did not want to cycle because their thighs would look like his. Those thighs, he tells them, give him the power to accelerate.

But Hoy must also buy pants two sizes too big to accommodate them. Sharp said a Brazilian jeans store in Philadelphia, now defunct, used to cater to cyclists, who would descend on it from all over the world.

Newell insisted that if all else fails her brethren embrace quad size, in the one place where the term “thunder thighs” is actually a compliment, a measure of speed and pride.

“Your friends love to hear about your muscles,” she wrote. “Pull down your pants to show them your strong quads and muscle definition. Make them grab your legs in public.

“We can all be winners here.”

© 2012 The New York Times Company