The number of hamstring injuries suffered by male professional footballers has doubled in Europe over the last 21 years, a study published on Wednesday has found.
The research at Linkoping University in Sweden, funded by the European governing body UEFA and published in the British Journal of Sports Medicine, found that the number of matches professional players miss with hamstring problems has also doubled.
Nearly 20 per cent of hamstring injuries are recurrences, of which more than two-thirds occur within two months of a player returning to play.
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The study collected data from medical teams who reported 2,636 hamstring injuries over 21 seasons. It found that the risk was 10 times higher in games than in training.
Of the total of injuries, 1,714 (or 66 per cent) occurred in 343,738 hours of match play hours and 922 (34 per cent) in 1,787,823 hours of training.
Hamstring injuries caused an average lay-off of 13 days.
Over the 21 years, more and more of the injuries suffered by players were to the hamstring. In the first year, they were 12 per cent of all injuries but rose to 24 per cent by the last.
Over the same period, days lost to hamstring injuries doubled from 10 per cent to 20 per cent.
A 25-player squad can expect about eight hamstring injuries every season.
The researchers said they did not set out to find causes but suggested that a more physical play, demanding tactics and a crowded fixtures calendar may be to blame.
"The intensity of elite men's football has increased," the researchers told the British Journal of Sports Medicine.
"Professional players now undertake more high-intensity activities per match than they did previously, and they also run faster than their predecessors."
"Professional players now work year-round apart from a 4-6 week break between seasons. Even during the traditional break between seasons, players are often required to undertake pre-season tours which require intercontinental travel," the researchers added.