As it turns out, the question of whether weight training matters to serious endurance athletes is a matter of debate.

Researchers who study weight lifting, or resistance training as it often is called, are adamant. It definitely helps, they say. But other experts in the field are not so sure.

Gary R. Hunter, a professor of exercise physiology at the University of Alabama at Birmingham, is a believer. He cites, for example, a recent study involving middle-distance runners. Three months of resistance training, he said, improved their leg strength and running efficiency, a measure of how much effort it took to run.

And, he said, it is not just runners who become more efficient.

“There is no doubt that an appropriate weight-training program would improve efficiency in pretty much any athlete,” Dr. Hunter said.

William J. Kraemer, a kinesiology professor at the University of Connecticut in Storrs, said lifting weights also can increase endurance and reduce the risk of injury, especially to connective tissue.

And don’t worry about becoming too muscular, Dr. Kraemer said.

“The fear of getting really big is not plausible for most people,” he said. Competitive distance runners and cyclists, who are naturally slender and light, “don’t have the muscle fiber number to get really big,” Dr. Kraemer said. “I can train them until the cows come home and they are not going to have big muscles.”

But other researchers, like Patrick O’Connor, an exercise scientist at the University of Georgia, are not convinced.

Dr. O’Connor points out that the weight-lifting studies, as is typical in exercise science, are small. And each seems to examine a different regimen, to measure outcome differently and to study different subjects — trained athletes, sedentary people, recreational athletes. It becomes almost impossible to draw conclusions, he said.

That may be one reason why different athletes end up doing different weight-lifting exercises. Chris Martin, a 31-year-old chemical engineer who has an elite racing license from USA Triathlon, the governing body for the sport, works on his entire body. But for his legs, he does exercises like leg extensions using one leg at a time, to correct any muscle imbalances or weaknesses. Mr. Martin, who lives in Lawrenceville, N.J., said he got the idea from coaches and from his own reading.

“Cycling and running are one-leg-at-a-time activities,” he explained. And one-legged exercises “recruit more muscles that help the hips.”

Steve Spence, who won a bronze medal in the marathon at the 1991 track and field world championships in Tokyo, is also a proponent of one-legged exercises. Now 45 years old and the head cross-country coach at Shippensburg University in Pennsylvania, Mr. Spence enters local 5-kilometer races and typically finishes in about 15 ½ minutes.

“I feel that every major breakthrough with my running has come after a period of strength training,” he said. He attributes this to the emphasis he puts on leg exercises, but he also believes that working his upper body and abdomen helped.

Other athletes concentrate on exercises that require them to jump or leap to develop explosive power.

And many top athletes spend lots of time in gyms lifting weights, and many trainers and coaches swear by it.

For example, the distance runners who are part of Team Running USA do resistance training for 30 to 60 minutes six days a week, said Terrence Mahon, a coach for the team. This group includes marathon stars Deena Kastor and Ryan Hall, the winner of the Olympic marathon trials last November.

“We do it all,” Mr. Mahon said. “We do upper body, core and lower body. The stronger the athlete is in a total body perspective, the more efficient they become as a runner.”

The Team USA runners do five to six exercises per session, he said. For example, upper body exercises may include pull-ups, the overhead press, bench press, rowing and exercises for the biceps and triceps. Lower body exercises include step-ups, squats, single leg squats, snatches and the leg press.

The main problem with weight lifting is that many people do it all wrong, said Kent Adams, the director of the exercise physiology laboratory at California State University at Monterey Bay. They don’t have a program or a goal. Technique may be sloppy. Or, Dr. Adams said, they use weights that are too light. Muscles need to be stressed if they are to respond, he said.

Dr. Kraemer is on the same page. One study, he said, found that women tend to lift half or less of what they could lift. And this happened even when women were working with personal trainers, he said.

“There is so much misinformation,” Dr. Kraemer said. “It’s a quagmire out there.” He recommends trainers certified by the National Strength and Conditioning Association, which also supplies educational information. Dr. Kraemer is a past president of the organization.

The right trainer, these researchers say, can be helpful when people are learning to lift weights. Not only can trainers teach proper technique, but they also can help people develop programs that meet their goals.

“I hate to say that a trainer is required for everybody,” Dr. Adams said. “But I think it is an excellent way to learn.”

That said, though, the evidence that weight lifting can improve performance is equivocal enough to leave plenty of room for the skeptics. And not every successful athlete spends serious time lifting weights.

DR. O’CONNOR, for example, lifts weights for health, for enjoyment and for vanity’s sake (he does not want an emaciated upper body, he said), but stops lifting when he is training to run a marathon. Those muscles, he said, “are just dead weight you have to carry around.” He adds that a sport like rowing, swimming or running requires specific muscles and nerve-firing patterns that may best be developed by actually doing the sport.

“If your goal is to improve running performance, then weight training should probably mimic the running pattern,” he said. “If you do leg extensions, you can get stronger, but people don’t run like that.”

That’s pretty much what Cathy O’Brien, a 40-year-old distance runner, thinks. She started racing when she was 12 and ran the marathon in the 1988 and 1992 Olympic Games.

“As far as resistance training, I have always been a minimalist,” she said. She does push-ups, pull-ups and dips for her upper body, and abdominal exercises, but does not work her legs.

“I think that running is the best thing for running results, ” Ms. O’Brien said.

Kevin Hanson, a coach for the Hansons-Brooks team of distance runners, is of like mind.

“We do some weight training,” he said. But other than some abdominal exercises, “everything we do is for the upper body.”

That doesn't dismiss the benefits of lower body weight training for different athletic populations, it simply means that many runners benefit more from sport-specific training.