Sydney - Athletes given what they thought was a human growth hormone improved their performance markedly, Australia researchers found.

In contrast, those who guessed correctly they were being given a placebo showed no significant gain in strength, endurance, power or sprint capacity.

"It's a very simple message," said Ken Ho from Sydney's Garvan

Institute of Medical Research. "If someone, whether they are an athlete or a patient who is unwell, is receiving something they think gives them a benefit, they will feel better and do better."

Ho and his team duped some weekend athletes into thinking they were receiving a human growth hormone. A control group in the same eight-week trial knew that the substance they were taking had no active ingredient and was a placebo.

Performance was monitored and it was found that those who assumed they were getting an illicit dose showed a 3.5 to 7.5 per cent improvement in performance.

"This is a great demonstration of the power of the mind," Ho said. It was all about the power of self-belief and positive thinking. (dpa)