People find sport or exercise less strenuous if they believe it's doing them good, a new study has found.

German researchers from the Department of Sport Science at the University of Freiburg discovered a person's expectations have a major influence on just how strenuous they perceive exercise to be.

They also found that how the person felt about themselves played a big role in feeling the strain of the activity.

The researchers say the findings add further evidence that the placebo effect works in sport and could have important implications in the battle against obesity.

"Negative affective responses, in turn, decrease long-term exercise motivation and participation. If one wishes to counteract the detrimental effects of a sedentary lifestyle, it is thus important to understand factors that affect perceived exertion in exercising individuals," the authors wrote.

Led by psychologist Hendrik Mothes, the team or researchers studied a group of 78 men and women aged between 18 and 32 who were asked to ride a stationary bike for 30 minutes.

Before the cycling, they were asked to say how athletic they thought they were and were asked to put on a compression shirt produced by a well-known sporting goods manufacturer.

The participants were assigned to different groups and shown one of several short films that either stressed the positive health effects of cycling or dampened the expectations.

During the exercise, they were asked every five minutes what level of strain they were experiencing.

The results, published in journal PLOS ONE, showed the training was rated less strenuous for those who started out with a positive, and the more athletic the participants perceived themselves to be, the stronger this effect was.

However, positive expectations did not help participants who considered themselves not very athletic.

The researchers also found that believing in the compression shirt helped but to the men and women who considered themselves athletic, it made no difference.

"Merely the belief that the shirt would help, did help the 'unsporty' subjects to have a lower perception of strenuousness during the exercise," Mothes explained.