Scientists are developing a drug which relieves fatigue after strenuous exercise - and it could benefit heart failure patients as well.
Existing theories on muscle fatigue have been challenged
A Columbia University team found fatigue following exercise is caused by calcium leaking inside muscle cells.
They formulated a drug to plug the leaks, and successfully used it to relieve fatigue in exhausted mice.
They also believe that calcium leaks are responsible for severe exhaustion in heart failure patients.
Their study, published online by Proceedings of the National Academy of Sciences, raises hopes that the drug could help alleviate symptoms that can be completely debilitating.
It had previously been thought that fatigue following intense, sustained exercise such as marathon running was due to the accumulation of lactic acid in the muscles.
However, this theory has recently been challenged by physiologists.
The latest research pinpoints a tiny leak inside the muscle cells as the culprit.
The leak - which allows calcium to seep continuously inside the muscle cells - weakens the force produced by the muscle and also turns on a protein-digesting enzyme that damages the muscle fibres.
The researchers found the leak was present in the muscle of mice after an intense three-week daily regime of swimming, and in human athletes after three days of intense cycling.
The Columbia team had previously identified the same leak in the muscles of animals with heart failure.
The researchers stressed the leak was only spotted in humans and mice that had been exhausted by exercise.
In normal circumstances the body is able to repair any damage to the muscles.
However, the research suggests patients with chronic heart failure have to endure constant leaks, which never give their muscles a chance to recover.
It suggests that it is this, rather than a reduction in the amount of blood and oxygen supplied to the muscles by the heart that causes the debilitating fatigue often associated with the condition.
This fatigue can be so bad that patients cannot get out of bed, brush their teeth or feed themselves.
The researchers gave their experimental drug to mice before the animals started a three-week regime of swimming.
Without the drugs the animals were exhausted after three weeks, but those that received the drug were still energetic at the end of the exercise programme, and their muscles showed less signs of damage.
Plans are now under way to test the drug in patients with heart failure.
However, the researchers warn that even if successful it will take several years before the drug will be commercially available.
John Brewer, director of the Lucozade Sports Science Academy, said the research was interesting, but would need to be confirmed by further studies in humans.
He said: "There is a wealth of scientific evidence that lactic acid is a major cause of fatigue in high intensity exercise - I suspect that the effect of calcium leaks is in addition to, rather than instead of lactic acid build up."
Ellen Mason, a cardiac nurse at the British Heart Foundation, said heart failure affected 690,000 people in the UK.
"If scientists can find out what causes healthy muscle not to cope well with exercise, then the hope is that one day it may be possible to improve muscle function.
"This could potentially be of benefit to treating people with heart failure."