Regular and strenuous exercise increases the risk of motor neurone disease in people who are genetically vulnerable, say scientists.
The team at the University of Sheffield said nobody should stop exercising as a result of their study.
But they hope the findings could lead to ways of screening people who may be at higher risk and give tailored advice.
Overall, around one in 300 people will develop motor neurone disease.
It affects people’s ability to move, talk and even breathe as the motor neurones that carry messages from the brain to the muscles fail. It can dramatically shorten people’s lives.
Who gets it and why is a complicated, poorly-understood mix of the genetic risk you are born with and other environmental factors that build up over a lifetime.
There has long been a connection between exercise and the disease, but whether it was a genuine “cause” or just a “coincidence” has been the source of fierce debate.
Studies in Italian footballers have suggested rates up to six times higher than normal. Athletes including Rob Burrow (rugby league), Stephen Darby (football) and Doddie Weir (rugby union) have all spoken openly about the disease.
“We have conclusively said exercise is a risk factor for motor neurone disease”, Dr Johnathan Cooper-Knock, one of the researchers, said.