In a new study, pain or displeasure was the most accurate indicator that a person had crossed a threshold believed to be the optimum level of exercise.

"As astonishingly simple as it sounds, perhaps the most appropriate level of exercise intensity for health-oriented exercise is the intensity that does not feel unpleasant," lead author Dr. Panteleimon Ekkekakis of Iowa State University, Ames, told Reuters Health.

Ekkekakis noted that it is natural for people not to want to continue doing things that are consistently unpleasant or uncomfortable.

"People will go to the gym after New Year's resolutions, but, if exercise hurts the first few times, after a while they will opt to stay home and watch TV," he said.

Most Americans could benefit from more physical activity, but Ekkekakis said that most people are not very good about estimating how hard they are exercising. People often do more or less than what is recommended, he said.

People who do too little miss out on the full benefits of exercise, but those who do too much may become exhausted and give up.

Ekkekakis explained that there is a specific level of exercise intensity that seems to be appropriate for a wide variety of people. This intensity corresponds to the level of the transition from aerobic to anaerobic metabolism--when the body switches over from burning fuel from aerobic (with oxygen) to anaerobic (without oxygen) sources.

This level of intensity is desirable for several reasons, including the fact that previously sedentary middle-aged and older people seem to get the same benefits at or just below this level than when they exercise at a higher intensity, Ekkekakis said. In addition, people who exercise significantly above this threshold quickly run out of steam.

People are often told to gauge their exercise intensity by measuring their heart rates, but this can be inconvenient, according to Ekkekakis. Another method of measuring exercise intensity involves rating a person's perceived exertion level, but doing this can be tricky, the Iowa researcher noted.

Ekkekakis' team set out to determine the best way to know when a person has reached this exercise threshold. The researchers believed that if a person exceeded the level of the aerobic-anaerobic transition, "that's precisely the point where they will start feeling gradually worse," Ekkekakis said.

"This is exactly what we found in the two samples of college students we tested," he said.

In two groups of 30 students who underwent exercise testing, feelings of pleasure and displeasure were more accurate than heart rate and other measures at determining the aerobic-anaerobic transition, the researchers report in the February issue of the journal Preventive Medicine.

For people who are just starting an exercise program, particularly those who are overweight, the intensity that corresponds to their individual aerobic-anaerobic transition may be very low, "perhaps not faster than a stroll," Ekkekakis said.

He added, "It is important that they stick to the intensity that feels comfortable rather than trying to match cultural expectations of what exercise should look like or feel like to be effective."