A new study finds that pain or displeasure is the most accurate indicator that a person has crossed the threshold into too vigorous excersise.

"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.