As the world's top athletes prepare for Athens, researchers warn it is the brain, not the body, which controls when fatigue hits.
Athletes brains may tell them when to stop training
A team from the University of Cape Town in South Africa, writing in New Scientist, say the brain "steps in" to prevent muscle damage.
Another study by researchers at the same university looks at how injury-prone athletes could be spotted early.
The team developed a way to analyse movement to pick problems up.
Both could have a significant impact on sports performance, the scientists say.
The team which looked at fatigue say it is traditionally seen as the result of overworked muscles ceasing to work properly.
But they say it is actually the brain that controls the feeling.
The researchers say a body signalling molecule called interleukin-6 plays a key role in telling the brain when to slow down.
Blood levels of IL-6 are 60 to 100 times higher than normal following prolonged exercise, and injecting healthy people with IL-6 makes them feel tired.
The team injected seven club-level athletes with either IL-6 or a dummy injection and recorded their times over six kilometres.
A week later, the experiment was reversed.
It was found that, on average, athletes ran nearly a minute faster after receiving the dummy injection. Most ran the distance at around 41 minutes.
The researchers suggest it may be possible to block IL-6 receptors in the brain using antibodies.
This method has already been successful in treating chronic fatigue syndrome, but there are fears unscrupulous athletes could use the same technique to try too hard, New Scientist says.
But the molecule has many effects on the body, so blocking its action may be counterproductive or even dangerous.
These findings are also going to be published in the Canadian Journal of Applied Physiology.
Dr Paula Robson-Ansley, who led the research, has a personal interest in the issue. Her own career as an Olympic-standard rower was cut short in part by a condition called under-performance syndrome.
She was training for the 1996 games when her obsessive schedule proved too much for her body.
Dr Robson-Ansley told the magazine: "Suddenly a five-kilometre run felt like I'd run a marathon the next day".
In a separate study, mathematics researchers from the University of Cape Town and the University of Antwerp in Belgium looked at why some sportsmen are prone to injury.
They used a technique called projective geometry where the relationship between two lines can be studied in terms of their orientation, but without having to find out their co-ordinates.
England's Andrew Flintoff recently began bowling again after injury
The researchers modelled the actions of cricket bowlers, many of whom suffer repeated injuries.
They modelled all the ways joint movements could be combined when a ball was bowled.
They found most combinations allowed the bowler to make small adjustments to their actions, but a few left no room for manoeuvre.
The researchers, Rudi Penne from Antwep and Henri Laurie from Cape Town, said this may play a special role in sports injuries - many of which begin as small problems which are worsened by repeated actions.
They say that, if a bowler could modify their actions, it may be possible to prevent aggravating such "micro-injuries".
The theory was tested on two bowlers, one of whom was especially prone to injury, and it was found his technique was less easy to modify than the bowler who was not injured.
Dr Penne said: "This may help us make an early diagnosis of injury-prone cricketers".
The researchers hope to widen their research to other sporting techniques such as how footballers take free kicks or how rugby union players scrummage.
But Dr Kit Vaughan, a biomedical engineer at the University of Cape Town said: "It is an elegant approach, but considerably more work is needed to apply it more generally."