GINA KOLATA | NEW YORK TIMES

AT the gym last week I saw a guy lifting weights, working out his shoulders while two friends urged him on. He alternated two similar exercises with heavy weights, repeating one exercise 10 times and then the other one 10 times, never resting between sets.

“We want to burn out his shoulders,” one of the man’s friends.Exercise researchers would be appalled.While public health officials bemoan the tendency of most people to do little exercise, if any, physiologists are fretting over the opposite trend: an increasing focus on extreme exercise among some recreational athletes.

Weight lifting with no rest between sets and with no days off. Competitions that encourage excess.To enter a recent race, my friend Joel Wilbur had to sign a waiver acknowledging he could die. Still, Joel was disappointed to find the race wasn’t all that dangerous.

After signing a death waiver, he said, he expects some serious risks.

“People think a good workout is, I am in a pile of sweat and puking,” said William Kraemer, a professor at the University of Connecticut. But if that happens, he said, “it means you went much too quickly, and your body just can’t meet its demands.” It’s not so easy to strike the right balance between exertion and rest, researchers say. Do too little, and the results may be disappointing.

That may be what happened to my colleague Jason Stallman. He wanted to avoid the usual consequences of marathon training: injuries from overuse. So he invented his own training program.

He exercised on weekdays but did not run.

He ran just once a week, on the weekend, when he would do a long run.Jason felt great, and the long runs went well. But when it came time to race, he said, his legs just didn’t have it. His time was slower than for most of his previous marathons.

Experienced athletes know that the only way to improve is to push yourself. Lift weights that are heavier than those you’ve tried before. Run or cycle at a fast pace on some days, but focus on increasing your distance on others. Work out enough that you may not fully recover between sessions.”You should feel tired, said John Raglin, a sports psychologist at Indiana University. But if you do too much with too little rest, your performance gets worse, not better.

“Serious athletes recognize these issues whether they respond to them or not is another matter ,”Dr Raglin said. “A lot of recreational athletes really have no idea.” In the early stages of overtraining, athletes constantly feel tired; by the end stage, they may be nagged by depression.

Recreational athletes must be attuned to their fatigue, Dr Raglin said. If it persists for several days, they should take a day off or simply do a lot less during workouts. A diary or notes on how they feel can help.”