By GINA KOLATA Dathan Ritzenhein, one of America��s most talented runners, was in a slump. He had been a national star since high school but he felt as if he had reached a plateau several years ago. He wasn��t improving the way he��d hoped, and had been suffering stress fractures in his left foot.

In June, Mr. Ritzenhein joined a running group, a team of elite runners in Eugene, Oregon, coached by Alberto Salazar, winner of three consecutive New York City marathons in the early 1980s. Mr. Ritzenhein said was re-energized, excited about running again.

At a track meet in Zurich on August 28, Mr. Ritzenhein, 27, broke the American record for a 5,000-meter race.

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.��

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 too many things change simultaneously when people start to run in groups: the coach, the location, the training regimen.

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.

There can be drawbacks. Slower athletes may try to push themselves beyond their abilities, and faster ones may not be challenged enough.

Yet the power of groups easily outweighs their drawbacks, says Kevin Hanson. He and his brother Keith start running groups that draw hundreds in Rochester, Michigan, and in 1999 started a team of elite runners, the Hansons-Brooks Distance Project.

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.

The countries whose distance runners were the best - Ethiopia, Kenya and Japan - all emphasized training in groups, he noted. 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.��

��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.��