Image caption Testosterone boosts the number of cell nuclei (in green) that help build muscle long after exposure to the drug

New research suggests that athletes who use steroids for a short period can benefit for their entire careers.

Experiments with mice showed that a brief exposure to testosterone allowed the mice to rapidly regain muscle later in their lives.

The scientist behind the study says that if a similar effect can be shown in humans, it should lead to a lifetime ban for dopers.

The research has been published in the Journal of Physiology.

Norwegian scientists had previously published work that showed the existence of a "muscle memory".

This suggested that if people exercised when they were young, their muscles grew more easily when they exercised later in life.

In science if you cheat you are out for life, and my personal view is that it should be similar for athletes Prof Kristian Gundersen, University of Oslo

The same team has now looked at the impact of a brief exposure to testosterone on mice.

They found that three months after the drug was withdrawn, their muscles grew by 30% after six days of exercise. A control group of mice saw growth of just 6% in the same time period.

Famous 'dopers' Cyclist Lance Armstrong was stripped of his seven Tour de France titles and banned for life in October 2012 by the US Anti-Doping Agency

was stripped of his seven Tour de France titles and banned for life in October 2012 by the US Anti-Doping Agency US Olympic medallist Marion Jones admitted in 2007 she had taken steroids ahead of the 2000 Sydney games

admitted in 2007 she had taken steroids ahead of the 2000 Sydney games Canadian medallist Ben Johnson was stripped of his 1988 100m title after testing positive for steroids

was stripped of his 1988 100m title after testing positive for steroids American football player Lyle Alzado, one of the first professional athletes to admit steroid use, died in 1992 of brain cancer aged 43

"It is rare to have data that clear cut, I was pretty satisfied with that," Prof Kristian Gundersen, from the University of Oslo, told BBC News.

He explained that the drugs boost the number of cell nuclei in the muscle fibres.

Ten year benefit

These nuclei are key to building strength in muscles when people exercise and the mouse study suggests that these extra nuclei gained through using testosterone remain in the long term.

Prof Gundersen believes the same holds true for humans.

Despite the long held belief that steroid users lose the benefit of the drugs when they stop taking them, the Norwegian research suggests that even a brief exposure to steroids could have a long-lasting effect.

"If it is sufficient to build muscle mass, I think it would be sufficient to give you this long term effect. I think it could last 10 years but I don't have the data to back that up. It would be my speculation yes," he said.

The Norwegians believe that their research calls into question the current proposal from the World Anti Doping Agency (Wada) to raise the penalty for dopers from two years to four.

"In science if you cheat, you are out for life, and my personal view is that it should be similar for athletes," he said.

"It is a harsh treatment but if you really are cheating, I think that's reasonable."

The team has been given a grant from Wada to carry out further research on humans. They are developing the protocols for a study involving students at a sports college in Oslo.

The research follows on from work that was carried out in Sweden in the past few years.

Researchers found that power lifters who have stopped taking steroids had an advantage in their sport years after they stopped using the drugs.

The researchers discovered that the lifters who had once taken the drugs had a comparable number of cell nuclei in their quadriceps as athletes who were currently performing high intensity training.

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