Sportsmen trying to perfect that tee shot, or wicket-taking delivery have a new excuse for slices and wides - their own brains.
Even hours of training don't guarantee the perfect delivery
Stanford University research suggests we are not capable of repeating exactly the same movement over and over again.
The Neuron study found monkeys trained to repeat simple movements produced slight variations every time.
The experts said training can improve the way the brain controls the muscles, but practice will never make perfect.
The Stanford team persuaded their monkeys to perform a reaching task - when they showed them a green spot, reaching slowly to touch it would produce a reward, but when a red spot was shown, reaching quickly was the route to success.
As the monkeys repeated the tasks thousands of times, not only were their movements analysed, but activity in the parts of the brain governing movement was recorded too.
They found that there were always subtle differences in a monkey's speed of reach, and further analysis revealed a pattern of brain activity which corresponded with how long it took the monkey to make its move.
"This is the first study to successfully record neural activity during the planning period and link it on a trial-by-trial basis to performance during those trials," said researcher Mark Churchland.
He speculated that the brain has evolved this way in order to respond better to new situations, which are more important than the ability to perform repetitive movements accurately.
He said: "The nervous system was not designed to do the same thing over and over again.
"The nervous system was designed to be flexible. You typically find yourself doing things you've never done before."
Professor Alan Wing, who carries out research into movement neuroscience at the University of Aberystwyth, agreed.
"This might be seen as a problem by sportsman, for whom the ability to reproduce exact movements is an advantage, but for humans in general, this range of variation allows us to adapt better to new environments and situations."