Sports Doping's Effect May Be in the Mind

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But Ho pointed out that "there is actually no firm scientific proof that growth hormone actually does enhance athletic performance, despite a widespread belief in its ability to do so". In fact, a review of the literature on the subject, published in March in the Annals of Internal Medicine, found no evidence that HGH could boost athletic prowess.

Ho and his team wanted to explore whether the physical boost athletes attribute to HGH might be more psychological in nature.

To do so, they focused on 64 healthy recreational athletes, men and women between the ages of 20 and 40, who had been exercising at least two hours per week over the six months prior to the study.

After testing the participants for their athletic ability, the men and women were randomized into two groups. One group got growth hormone for eight weeks, and the second received a dummy substance, or placebo. Neither the researchers nor the athletes knew which group participants were in.

At the end of the two-month trial, all the participants were asked to guess whether they had been taking HGH or a placebo, and whether their sporting performance had changed during the study period. Athletic ability was then re-tested on a range of performance parameters.

Ho and his team found that about half of the participants who received a placebo incorrectly assumed they had been given HGH. Gender played a significant role in such perceptions: the male placebo athletes were much more likely than the female athletes to have mistakenly thought they were in the HGH group.

However, regardless of gender, athletes on placebos who thought they had taken HGH typically believed their performance had improved during the study.

What's more, these "incorrect guessers" actually did improve, albeit minimally, in all measures of performance, including endurance, strength, power, and sprint capacity. In one category -- high-jumping ability -- the improvement was significant.

People in the placebo group who correctly guessed that they had taken a placebo improved their performance by about 1 percent to 2 percent, Ho said. But those who mistakenly thought they had taken HGH showed twice that level of overall improvement -- about 2 percent to 4 percent.