Stuart Bradford

At 245 pounds and 6 feet 3 inches, Jeffrey West knows he looks more like a football player than a runner. But that hasn’t stopped him from competing in nine marathons in the past three years.

“When I go by, people say, ‘Go, big fella, you can do it!’ ” Mr. West, 45, of Carol Stream, Ill., said with a laugh. He plans to be one of 45,000 runners in this weekend’s Bank of America Chicago Marathon. “But I know at the end of the race most people don’t want to get beat by the big guy who doesn’t look like he can run very fast.”

To runners, Mr. West is a Clydesdale, a name (referring to the large, powerful draft horses) that is typically used to describe race participants who weigh 200 pounds or more.

While most running events allow runners to compete in age divisions, a number of road races and triathlons also offer participants the chance to compete in weight divisions, which allows athletes to compare their performances against athletes with a similar build. Clydesdale runners argue that a number of sports, like boxing and wrestling, focus on weight-based competition, and it also makes sense in the sport of running, where slighter-framed runners have a clear advantage over those carrying an extra 50 pounds or more.

“As a Clydesdale, you know you’re never going to win a race,” says Mr. West, who has run nine marathons and whose best time is 4 hours 51 minutes. “But can I compare myself to another 45-year-old runner who weighs 145 pounds? Not really.”

Clydesdale running dates to the late 1980s when a Baltimore-area accountant analyzed 20,000 runners in 10-kilometer races and marathons. The analysis showed that once men reach about 170 pounds, their performance declines relative to athletes of about the same age with a slighter build. The findings suggested that, compared with his peers, a 210-pound man who runs a 10-kilometer race in 51 minutes (about an eight-minute-per-mile pace) is performing as well as a 150-pound man who runs the distance in a speedy 38 minutes (about a six-minute-per-mile pace).

The research persuaded a few small races to create awards based on weight divisions, and eventually larger races like the Marine Corps Marathon and the Portland Marathon began allowing runners to gauge their performance by weight category.

The USA Clydesdale and Filly Racing Federation (www.clydesdale.org) lists races on its Web site that offer weight divisions. Paul Collyer, the group’s founder and a race promoter from Somerville, Mass., says there’s no official tally of Clydesdale participation, but the category is becoming more popular. “Now a lot of big fit guys and big fit women are taking part,” says Mr. Collyer, who is also a Clydesdale runner.

And research shows that people who are overweight or even obese still can be fit. Studies at the Cooper Institute in Dallas have found that it’s better to be fat and fit than thin and sedentary. The studies showed that the death rate for men and women who are thin and don’t exercise is at least twice as high as that for obese people who stay active.

Devon Pearson, 28, a lieutenant in the Naval Reserve in West Chester, Pa., will run in the Clydesdale division of the Marine Corps Marathon on Oct. 31, a race he hopes to finish in about four hours.

“I’m on the slow end if you compare me to a 28-year-old runner who is 145 or 155 pounds,” he says. “But I compete in my division for sure.”

The Marine Corps Marathon offers three Clydesdale weight divisions for men starting at 185 pounds, 200 pounds and 224 pounds. For women, two weight categories start at 155 pounds and 170 pounds. (Like many other races, the marathon calls its female weight category Athena, for the Greek goddess.)

Although some races require weigh-ins, the Marine Corps race director, Rick Nealis, said he stopped the practice after he watched several runners step on a scale near the registration desk. Once he recalled a runner who was a pound shy of his weight group.

“I heard someone make the comment, ‘Go eat pasta and come back and weigh in,’ ” Mr. Nealis said. “I was thinking, this has got to be wrong. We shouldn’t be doing this. Now we don’t weigh in for the Clydesdales. I go with the integrity of the runner.”

Some running clubs hold their own weight-based competitions. The Chicago Area Runners Association offers its members a Clydesdale and Athena circuit, which scores runners based on their performance in 16 local races, including the Chicago Marathon.

Not every runner embraces the weight group. Katy Moeller, 41, of Boise, Idaho, ran the New York City Marathon in 1999 weighing 220 pounds. Now she does most of her running on a treadmill, and has no interest in competing in a weight category. “I’m proud of being in the game even though I’m overweight,” she says. “But I don’t think I’d be super thrilled at seeing I’m No. 1 in the over-200 weight division.”

Men, however, tend to be more irreverent about the weight class. On the Clydesdale Facebook page, which has about 80 members, one runner recently posted an ad to find two more 200-pound-plus runners for a team race.

“No girly-men or Kate Moss-esque type runners need apply,” read the Facebook post. “We aren’t fast but we can lift heavy things like sofas and armoires.”

A version of this article appeared in print on October 5, 2010, on page D5 of the New York edition of The Times.