For 51 weekends a year, Stage Fort Park in Gloucester, Mass., is a picturesque New England playground. Overlooking the Atlantic Ocean on the edge of this gritty, working-class seaport, the park is a harborside haven, with picnic benches, sandy beaches, a baseball field, volleyball nets and spectacular views.

However, during one weekend in early fall, hundreds of Spandex-clad, adrenaline-fueled cyclists descend on Stage Fort for a two-day ritual of maniacal — and some would argue masochistic — bike racing, and the park's bucolic setting becomes a blur. That, in short, is the essence of cyclocross, better known to practitioners as simply 'cross.

"There's almost no pacing in cyclocross; it's just full-speed from start to finish," said Pete Webber, a former professional mountain bike racer from Maine (and a 2011 inductee of the Mountain Bike Hall of Fame) now living in Colorado. "To sustain the effort, you have to block out the pain, dig deep inside and find a way to keep hammering. The best riders are the ones who can find the concentration and courage to keep going even when they're dying inside."

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The Gran Prix of Gloucester races have been held on achingly beautiful clear days, in torrential rains and even the occasional snow squall. That variety is one of the sport's sublime draws.

"I raced 'cross when it was 80 degrees and when it was eight degrees on a sheet of ice," said Jen Dial Santoro, a two-time New England Cyclocross Series champion now living in Utah, and the first American woman to notch a top-five finish in a World Cup (fourth; Nommay, France in 2000). "Like the Alaskans are said to have multiple words for snow, I'm sure most 'cross racers have multiple words for mud, because it's all different. Some riders do better in thick, heavy mud, while others do better on dry, hard and fast courses. Still others are masters of ice.

"Because you can have anything at a 'cross race, it makes the outcome very exciting," she added. "Any rider could have the stars align and have a great day in conditions that are good for them."

The stars are definitely aligning for cyclocross, the sport's fastest-growing discipline in the United States, according to data compiled by USA Cycling. Paul Boudreau, race director for the Gran Prix of Gloucester, said his event mirrors that growth, beginning as a one-day event with 150 racers in 1999. Last year, the Gran Prix drew 1,000 racers on both Saturday and Sunday.

"'Cross is still the punk rock of cycling, a little edgy and alternative," said Boudreau. "The people who are attracted to 'cross tend to really be obsessed about it."

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Over a five-month season, from Labor Day to the World Championships (in the Czech Republic this year) in late January, cyclocross showcases cycling's most race-specific discipline, often in the worst conditions.

"Since the cyclocross season coincides with the unpredictable weather of fall and winter, bad weather is expected and embraced," said Webber. "Racers strive to be tougher than Mother Nature, and the events go on regardless of rain, ice or snow. Remember, harder is better in 'cross."

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The hard-core racing that defines cyclocross can now be found throughout North America, expanding from traditional hot spots in New England, the Northwest and Boulder, Colo., to include the entire Eastern Seaboard, the West Coast and even the Great Plains of the Midwest. It culminated with the first-ever World Championships held in the United States, in Louisville in the winter of 2013.

"For Louisville to happen was a dream come true for lots of us who have worked on this dream for a long, long time," said veteran 'cross racer and race promoter Adam Myerson of Massachusetts. "Europe will take United States' cyclocross seriously now."

Cyclocross has European roots, where it blossomed in the early 1900s as an off-season training regimen for road racers. It is commonly referred to as "steeplechase with bikes," though Myerson likens it to an off-road rally on two wheels. Webber described it as "motocross without the motors."

While road bikes and mountain bikes can be used for both competitive and casual pursuits, cyclocross is all racing, all the time. The bikes, which look like road bikes on steroids, are utilitarian and the ride of choice for many commuters. But the discipline itself is all about the starting gun and the finish line.

"If you're going to do it and do it well, you will suffer because you are going full-throttle, all-out effort for a full 45 minutes," said Selene Yeager, Bicycling magazine's "Fit Chick" columnist. "The pro men go for an hour — that's real suffering. Being in the red for that long is an exercise in existential suffering. You have to transcend it and push through it and not let it consume you."

Most 'cross races are held on tight, twisty courses that challenge the bike-handling skills and tactics of the racers, and feature natural and man-made obstacles employed to produce the sports' signature maneuver: a rapid dismount, a short sprint to jump over barriers or run up an incline, followed by an equally quick remount.

"Because 'cross is hard in a ridiculous way, it becomes fun," said Myerson. "It almost has this performance art absurdity to it. Try to explain to people that you're running with a bicycle. It's just strange to watch someone running through a muddy field with a bike on their shoulder. That suffering, combined with the oddity of it, is part of what makes it so interesting."

Another attraction is that every 'cross racer competes on the same course, so first-timers and novices can test themselves over the exact same terrain as regional and national champs. The looped, cloverleaf-style layouts also make the races much more accessible for spectators. And since they can be squeezed into urban areas, more fans tend to join in.

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"A great cyclocross race is made for crowds," said promoter Richard Fries. "It's like a racecourse laid out in a supermarket."

Races are categorized according to age and ability levels, but all are grueling, with competitors pedaling (and running) to their thresholds. Still, practitioners say it's an exquisite pain, where the glow of the subsequent endorphin rush far outlasts the quivering thighs and throbbing temples. Races are relatively brief, allowing combatants to recover quickly and compete on consecutive days.

"Cyclocross is the crack of sports," said 54-year-old Eiric Marro, a longtime racer from New Hampshire. "You do it once, and you can never turn back. For a person with a competitive nature and a need to push themselves to their physical limits, cyclocross is highly addictive."

Of course, misery loves company, and cyclocross is defined by a tight-knit community that is sometimes lost in other cycling disciplines. Foul weather makes for fast friends, and competitors often huddle together in cars, campers or under makeshift tents and lean-tos.

"'Cross also has a welcoming atmosphere which stems from all the abject suffering," said Marro. "If you're the race winner or barely finishing on the lead lap, you all suffer the same. Nobody sits in or drafts. As soon as you stop pedaling, you stop. The discipline of 'cross is pure. You get out of it whatever you put in. No frills or shortcuts."

At its heart, 'cross tests that all-too-human fight-or-flight response. The sport is so difficult that most would quit, if not for the other competitors and the gallery that typically lines the course, brandishing cowbells, exhorting the racers.

"There's no question that camaraderie and community are integral to cross," said Webber. "The venues are compact, there are races for every ability and the whole affair is a mixing pot of bikes, families, friends and mud. The mutual encouragement, the elbow-to-elbow racing and the shared pain are why racers get addicted to the sport."