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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 explained to me.

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. Endurance training with no easy days or 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.

My workout partner Jen Davis once signed a race consent form that said: “There are sections of the trail that travel along cliffs. If you’re not careful you could fall to your death. Very few runners go the distance without taking one painful spill. Most runners take lots of them.”

She found the trail so arduous and dangerous that she never did it again.

And there is no shortage of commercial fitness programs promising to push people beyond their limits.

“People think a good workout is, ‘I am in a pile of sweat and puking,’ ” said William Kraemer, a professor of kinesiology 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.”

When they train harder yet stop improving, even backslide, “they become alarmed and try to increase their training,” Dr. Raglin said.

He sees it over and over: An athlete will get into a training schedule and become very dogmatic, never taking a day off.

“The importance of recovery is a big topic in exercise science,” Dr. Raglin said. “It hasn’t filtered down to the serious recreational athlete.”

Muscles need to recover after they are stressed with heavy weights, Dr. Kraemer noted. Researchers have long known that the way to build strength is what they call periodization: Rest days and easier days and weeks are interspersed with periods when the weights are increased.

In endurance sports, muscles experience a different kind of stress, said Dr. Bengt Saltin, director of the Copenhagen Muscle Research Center at the University of Copenhagen. But the problem with intense exertion day after day is very similar.

Intense endurance exercise depletes muscles of their energy supply, glycogen. Muscles store enough glycogen only for an hour and a half to two hours of activity, Dr. Saltin said.

It takes a day for trained endurance athletes to replenish glycogen. Athletes with less training have less of the enzyme that restores glycogen — glycogen synthetase. It can take up to two days for them to restore this muscle fuel.

In addition, connective tissue in muscles can be damaged and needs time to recover. In a study of runners in an annual local race that is a bit longer than two marathons, Dr. Saltin and his colleagues found that the athletes’ muscles lost their elasticity as their connective tissues weakened.

Running got harder and harder, so much so that the energy required for a set pace at the end of the race was 50 percent higher than it had been at the start.

So how to avoid a self-defeating training program? There are no hard and fast rules, because individual athletes vary so much. A training program that one person thrives on will break another, equally talented athlete.

Dr. Raglin said even the experts, researchers like himself who study overtraining, had trouble defining the symptoms. Psychological changes are the most consistent signs of a problem, he said.

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.

And that does not mean that difficult regimens are out of the question, Dr. Raglin said. He should know — he’s training for a contest in March, the Arnold 5K Pump and Run, in which competitors must bench press their body weight up to 30 times and then run a five-kilometer race.

“I don’t think it is very extreme,” Dr. Raglin said.

At least there is no death waiver.