Frequent and intense exercise increases the risk of motor neurone disease (MND) in those who are genetically vulnerable to developing the condition, say scientists.
Researchers at the University of Sheffield said no one should stop exercising as a result of their study, but they hope the findings could result in methods of screening people who could be at higher risk from the disease.
Around one in 300 people are at risk of developing the neurodegenerative condition, which affects movement, speech and even breathing as the motor neurones - which carry messages from the brain to the muscles - fail.
It can also lead to a shorter lifespan.
Some high-profile British sportsmen have shared their experience with the condition in recent years, including rugby union’s Doddie Weir, rugby league’s Rob Burrow and footballer Stephen Darby.
Approximately 5,000 people in the UK are affected by MND.
One of the authors of the study, Dr Johnathan Cooper-Knock, said he wouldn’t advise people to “change their exercise habits” because the team don’t know who is currently at risk.
Speaking to Sky News, he said: "This work shows frequent, strenuous exercise is causal for motor neurone disease... at the moment we don't know who is at risk but this lays the platform for looking at that."
‘We are trying to prevent suffering’
Dr Cooper-Knock, a senior lecturer in neurology at Sheffield University’s Neuroscience Institute, said he hopes the next step will be undertaking research to find out which individuals are genetically vulnerable to MND and how their levels of exercise raise the risk of developing it.
"About 10% of MND is inherited and 90% doesn't run in families but has a significant genetic component,” he added.
"Hopefully we can do some good, we are trying to help people and prevent suffering."
The Sheffield team, whose findings have been published in the journal EBioMedicine, wants doctors to be able to offer advice to families and patients about the risks so they can tailor their own exercise habits.
Dr Brian Dickie, director of research development at the Motor Neurone Disease Association, said: "In recent years, understanding of the genetics of MND has advanced, but there has been little progress in identifying the environmental and lifestyle factors that increase the risk of developing the disease.
"This is, in part, because the genetic and the environmental studies tend to be carried out in isolation by different research teams, so each is only working with part of the jigsaw.
"The power of this research from the University of Sheffield comes from bringing these pieces of the puzzle together."