He and his coach tried to figure out what was wrong and seized upon the idea that perhaps it was the altitude training. He had been living and training in Boulder, Colo., hoping to take advantage of the thin air, which can increase the red blood cells that help deliver more oxygen to muscles. But maybe, Mr. Ritzenhein and his coach reasoned, training at Boulder's elevation (about 5,430 feet) was putting too much stress on Mr. Ritzenhein's body.
So Mr. Ritzenhein, his coach and his family moved to Eugene, Ore. (430 feet). "It didn't work," Mr. Ritzenhein said. He did not improve and, to his dismay, suffered another stress fracture.
In June, Mr. Ritzenhein joined a running group, a team of elite runners coached by Alberto Salazar, winner of three consecutive New York City marathons in the early 1980s. It made all the difference, Mr. Ritzenhein said. He was re-energized, excited about running again. And, he said, most important, he trained with fast runners who pushed him to work harder than he ever could alone.
At a track meet in Zurich on Aug. 28, Mr. Ritzenhein, 27, broke the American record for a 5,000-meter race, finishing in 12 minutes 56.27 seconds — a pace of 4:09 a mile in a race that is 3.1 miles long. The American record before that, 12:58.21, had stood for 13 years.
Mr. Ritzenhein is convinced his success is because of running and training with a group. Running alone, he said, "You can't push yourself as hard — you feed off the energy of other people."
Mr. Salazar said in an e-mail message that he is a firm believer in group training. He had trained with a group himself, he said, and group training "helped develop our great runners of the '70s and '80s."
Group training is an aspect of performance that has never been scientifically studied. Exercise physiologists say it can be impossible to demonstrate its value because usually too many things change simultaneously when people start to run in groups: the coach, the location, the training regimen. To do a proper study, it would be necessary to assign athletes at random to train alone or with a group, assessing their performances after a period of time — something that would be extremely hard, if not impossible.
But despite the lack of solid evidence that group training helps, more and more athletes are starting to think it does. And, they say, there are lessons for amateurs who want to run or swim or cycle faster. The right workout companions, they say, can make all the difference.
"In sports, you need to train at race pace," said Edward Coyle, an exercise physiologist at the University of Texas at Austin. "To do that, you need a coach and you need teammates to push you."
Recreational athletes can benefit, too, Dr. Coyle said. Many run by themselves or without a specific program. "They probably underestimate their ability," he said. Group runs "would help them tremendously.
Many amateurs already train with groups — masters swimmers, competitive road cyclists and runners who join clubs or groups that run together regularly.
And there can be drawbacks. Slower athletes may try to push themselves beyond their abilities, and faster ones may not be challenged enough.
Michael Berry, an exercise physiologist at Wake Forest University and a competitive cyclist, said he just can't ride with his group on his recovery days, when a workout should be easy. He always finds himself riding too hard.
Before he started cycling, he was a runner, and he had the same problem. "Say Monday would be a recovery day, an easy five-mile run," said Dr. Berry, 53. "Someone would show up who hadn't run all weekend. My competitive urge was such that I said to myself that I didn't need to recover." But, he added, "As I get older I realize that, yes, I do need to recover."
Kevin Hanson said he and his brother got the idea for the elite team when they began asking why American performances had declined so much in the 1990s from the golden days of the '70s and '80s.
"Frank Shorter, Bill Rodgers, Greg Meyer," who, in 1983, was the last American man to with the Boston Marathon, "all trained in groups," Mr. Hanson said.
But in the 1990s, distance runners began training on their own, with the guidance of a coach. And Americans were no longer among the best in the world.
"We started to see a decline in the sport," Mr. Hanson said. The countries whose distance runners were the best — Ethiopia, Kenya and Japan — all emphasized training in groups, he noted.
"You say: `Wait a minute. We were most successful in the U.S. when we trained in groups. The three most successful countries in the world are doing group training,' " Mr. Hanson said. There must be a message there.
So he and his brother started recruiting runners for their elite group. Its advantages, he said, are that athletes have "shared motivation, a shared sense of ideas." And they encourage one another.
"So often it may be hard to drag yourself outdoors," to go for a training run, Mr. Hanson said. "But when you have 8 or 10 or whatever number of teammates counting on you, then you're there."
That's also what Kara Goucher says. She ran her first marathon last year, in New York, and came in third among women. Her time, 2:32:25, was the fastest ever for an American woman running her first marathon. Ms. Goucher attributes her success to group training.
She graduated from college in 2001 and ran on her own, coached by her college coach, for three years.
"I really struggled," Ms. Goucher said. "I kept getting injured." She had multiple stress fractures, a knee injury and shin splints. Her husband, Adam Goucher, was also running alone, coached by his college coach, and was also struggling, she said.
In the fall of 2004, the Gouchers moved to Oregon and joined Mr. Salazar's team. It made all the difference, Ms. Goucher said.
"I think it's possible to train on your own, but I do think it is better in a group," she said. "You see success in each other. Everything seems more in reach."
"And it holds you accountable,
Kevin Hanson adds that when one person in a group has an outstanding performance, others gain confidence that they might be able to do it, too. They know how hard everyone works, they know they can run with that person in practice. If that person did it, if they ran that fast, then, team members think, why not me?
That happened this year when Desiree Davila, one of his team's members, ran the Berlin marathon in 2:27:53, finishing 11th. She was 26 years old; Mr. Hanson said the only other American woman to run a marathon that fast that young was Joan Benoit Samuelson in 1983.
"It was a huge motivating factor for all of our women," Mr. Hanson said.