Bodybuilders are particularly vulnerable of becoming addicted to their sport - and the atmosphere in the gym may be to blame.

David Smith, a lecturer in sports psychology at Chester College, interviewed hundreds of bodybuilders in gyms around the country.

He believes that the results show why bodybuilders are prone to getting hooked on training.

This can lead to over training injuries and even immune system problems as they spend too long pumping iron, and not enough time at home or working.

Many people who take up bodybuilding, he said, do so because they do not like the way their bodies looked in the first place.

Because the sport produces tangible and increasing differences in the appearance, and other gym users are likely to give positive comments about these improvements, bodybuilders can get hooked on the atmosphere at gyms.

Poor self-image

Their poor self-image means they crave positive comments.

Mr Smith said: "Certainly being involved in bodybuilding can be fantastically healthy.

"But when people get addicted, there can be a variety of negative consequences, including neglecting their jobs or their families.

"I spoke to one man who said he couldn't get a job because it would interfere with his training schedule."

Some bodybuilders may actually be suffering from a form of "reverse anorexia", he said, in which they believe that their muscles are smaller than they actually are, and train harder as a result.

The most experienced bodybuilders surveyed showed more signs of exercise dependence.

The research was published in the British Journal of Sports Medicine.