Psychologists asked two groups of male students to practice a specific exercise, one by carrying it out and the other by imagining they were doing so. After eight sessions over four weeks they found the imaginary exercises had improved their strength by 16 per cent.
The finding opens the way for the creation of virtual gyms in which suited clients can pump images of iron and drip mental sweat.
Although the improvement was only half as good as that achieved by those who actually performed the exercise, it was better than the researchers had expected.
"While mental practice is not as good as physical practice it is certainly better than nothing," said Dave Smith of Manchester Metropolitan University who carried out the study.
The exercise involved pressing sideways with the little finger with as much force as possible by moving it away from the other four fingers. This curious activity was selected because there was little risk of the subjects practising it inadvertently during the four-week period of the study, which would have invalidated the results.
Mr Smith said: "We wanted to see if mentally practising something could make a muscle stronger. We used the little finger abductor muscle because it is rarely used. We couldn't choose something like a leg muscle because people then go out and play football and you can't tell what effect the mental practice has had."
Despite the unusual nature of the movement the findings could be applied to any kind of exercise, Mr Smith told the British Psychological Society's conference in Brighton yesterday.
"All muscles work in the same way. If this works for the little finger there is no reason it won't work for other muscles."
In each practice session the students were sat in a chair with their hands strapped to a table and they were asked to perform - or imagine they were performing - 20 presses of five seconds each with their little finger.
"The crucial thing for the mental exercises is not just seeing the movement in the mind's eye but feeling the physiological response in the muscle. The motor areas in the brain don't know the difference between doing it and imagining you are doing it but the more similar the feelings are the more you fool the brain."
Mr Smith said strength depended on the muscles as well as on the brain's ability to send messages to them. Building strength was partly dependent on exercising the brain.
"When training, as well as working out in the gym, you have to work out in your head. Evidence shows that mental and physical practice is better than physical practice alone," he said.
"There is nothing mystical about it. There are physical reasons for the effect of mental exercise and we are trying to find out more about them."Reuse content