Professional soccer players have a higher risk of suffering from the incurable degenerative illness motor neuron disease, according to a study on Wednesday.

Scientists at the University of Turin in Italy found that players develop the illness, also known as amyotrophic lateral sclerosis (ALS) or Lou Gehrig’s disease after the American baseball legend, earlier than normal.

The longer they played the game, the greater the risk.

“The cause of ALS remains unknown, as does the reason for the higher rate among footballers,” New Scientist magazine said.

ALS attacks nerve cells in the brain and spinal cord, leading to muscle weakness, difficulty speaking, swallowing and breathing, and eventually total paralysis.

Italian researcher Adriano Chio and his colleagues studied the medical records of 7,000 professional soccer players in Italy who played between 1970 and 2001.

Based on the players’ age and the normal incidence of the disease, they calculated there should have been 0.8 cases among the players. But in fact they found five.

Also, their average age of onset of ALS was 41 - 20 years earlier than usual.

“The study was prompted by what the Italian press dubbed ‘the motor neuron mystery’ - the discovery a few years ago of 33 cases of ALS during an investigation of illicit drug use among 24,000 pro and semi-pro players in Italy,” according to the magazine.

Two U.S. studies of veterans of the 1991 Gulf War showed they too appeared to have a high risk of ALS and that the illness appeared at younger-than-usual ages.