Frequent strenuous exercise increases the risk of developing motor neurone disease for those genetically vulnerable, British scientists found.
A University of Sheffield research team said nobody should stop exercising, but that the findings could allow for tailored advice to be given to those more at risk.
The MND Association says the odds of developing the disease are about one in 300, but there is a family history of the disease for 10 per cent of MND sufferers. Environmental factors can also cause vulnerability, the association said.
The debilitating illness affects a person’s ability to move, talk and breathe, and worsens over time as the motor neurons that carry messages from the brain to the muscles fail.
The study, published in the journal EBioMedicine, showed a connection between "exercise and MND, with high-intensity, physical activity likely to contribute to motor neuron injury, but only in individuals with a predisposing genetic profile".
High-profile sportsmen who developed MND include former England rugby league player Rob Burrow, Scotland rugby union international Doddie Weir, footballer Stephen Darby and South African rugby union World Cup winner Joost van der Westhuizen, who died aged 45 from the disease.
Dr Johnathan Cooper-Knock, one of the researchers, said the findings proved rigorous exercise was a risk factor for MND.
"We have suspected for some time that exercise was a risk factor for MND, but until now this link was considered controversial," he said.
"This study confirms that in some people, frequent strenuous exercise leads to an increase in the risk of MND.”
But, he said, most people who undertake vigorous exercise would not develop MND.
"The next step is to identify which individuals specifically are at risk of MND if they exercise frequently and intensively, and how much exercise increases that risk," he said.
The researchers analysed data from the UK Biobank project, which has detailed genetic samples from half a million people.
They used a technique called Mendelian randomisation to turn the data into an experiment, proving that those whose DNA made them more likely to do strenuous activity were at increased risk of developing MND.
It is thought that low levels of oxygen in the body during strenuous exercise could lead to a process called oxidative stress in the motor neurons.
The study reaffirmed that 10 per cent of MND cases were inherited and that the remaining 90 per cent were caused by “complex genetic and environmental interactions which are not well understood”.
Global awareness of MND peaked after the "ice bucket challenge", a viral video campaign in 2015 in which participants had a bucket of icy cold water poured over their head, replicating the feeling of breathlessness experienced by people with MND.
