The exercises only take about 10 minutes, and can be done without any extra equipment, researchers said.
Hamstring strains are the most common injury among soccer players, and "it's also the injury that takes out the most days from training and matches," said study author Dr Per Holmich, from the University of Copenhagen.
The injuries typically happen when players are sprinting, and the hamstrings -- the muscles that run down the back of the thigh to the knee – extend to prevent the knee from overstretching. That tension, while the muscle is extending, is known as eccentric contraction.
It involves very high pressure on the muscles, sometimes for long periods of time, Holmich said.
He and his colleagues figured that mimicking that type of pressure in a strengthening exercise might mean that the hamstrings were better prepared to deal with high force during practices or matches.
The researchers studied 50 professional and amateur men's soccer teams in Denmark during a full year of practice and play. They trained coaches on half of the teams to lead "eccentric training" exercises during a midseason break and then regularly during the season.
How to do 'eccentric training' exercises
To do the exercises, players pair up, with one athlete on his knees and the other holding the back of the first player's legs and ankles to the ground. The kneeling player slowly leans forward while holding his weight back with his hamstrings, until reaching a push-up position, then pushes himself back up once he hits the ground.
Teams did those exercises up to three times a week during the 10-week break between seasons, then once a week once matches were underway.
Other teams followed their normal training plan with no extra hamstring exercises.
The study involved a total of 942 players. By the end of the year, 67 of them had suffered a hamstring injury – 15 of whom did strengthening exercises, and 52 in the training-as-usual group. Athletes that had the strength training were less likely to get both new hamstring injuries and to aggravate a past hamstring injury.
When athletes did get injured, however, there was no difference in how long they were out as a result -- about four weeks in both groups, on average.
Researchers reported in the American Journal of Sports Medicine that, based on usual hamstring injury rates, 13 players would have to regularly undergo the strengthening exercises to prevent one injury.
None of the players got injured during the exercises themselves, although they did report muscle soreness after the early sessions.
Previous studies on 'eccentric training' exercise
Previous studies have suggested that this type of training helps players get stronger, but researchers hadn't randomly assigned teams to do or not do the exercises – so it was less clear that other factors unique to certain teams weren't behind its apparent benefits.
"This is more or less the final evidence that is needed to be sure that this exercise works," said Roald Bahr, one of the researchers who first proposed this type of training, from the Norwegian School of Sport Sciences in Oslo.
Although Holmich and his colleagues only looked at adult male soccer players, the author said there's no reason to think the strengthening exercises wouldn't also help prevent injuries in younger players and in women.
And he said, "Track and field sprinters as well would probably benefit from doing this exercise."
Bryan Heiderscheit, a physical therapist from the University of Wisconsin—Madison, said the exercises showed promise for many different kinds of athletes, including football running backs and wide receivers. But he added that people with a current hamstring strain should stay away from them until they recover.
"If you sustain a hamstring strain injury, these are not the type of exercises you want to do for the first three to four weeks because they're pretty intense," he said, adding that starting them too soon could make the injury worse.
One of the best things about the exercise, Bahr said, is that "you can do this efficiently, with the whole team, on the pitch, without any additional equipment except maybe a towel or some additional knee padding".
"Every soccer team in the world should be doing it."
(Reuters Health, Genevra Pittman, August 2011)