The injuries, also known as side strains, typically occur with twisting or pivoting -- such as during a pitcher's throwing motion or a batter's swing -- and are also common in tennis and golf.

"Part of this is just, you're doing something that's not a natural motion... so the body takes a beating," said Dr. Joshua Dines, from the Hospital for Special Surgery in New York, who worked on the study. If you keep doing those motions over and over, he said, your body is going to break down at some point.

"I think there's also a balance between working out and also staying limber -- a lot of this is dependent on flexibility," Dines told Reuters Health.

"It's great to work out, great to do your core stuff. But make sure you stretch."

Dines and his colleagues looked back at 20 years worth of records from Major League Baseball's disabled list, which includes athletes that are sidelined for 15 days or more. By placing players on the disabled list, or DL, teams open up a spot on their rosters for healthy athletes to fill in.

From the 1991 through 2010 seasons, 8,136 players were placed on the DL, according to findings published in the American Journal of Sports Medicine. Abdominal strains accounted for 393 of those injuries, or about five percent.

Strains kept pitchers out for an average of 35 days, and position players for an average of 27 days.

That's in comparison to more common sore arms and shoulders that typically only require a week or two of rest, said Kevin Wilk, a physical therapist who treats professional athletes at Champion Sports Medicine in Birmingham, Alabama.

"These don't happen all that often," said Wilk, who wasn't involved in the new research. "But when they happen, you're out for a while."

The research team noticed an increasing trend in abdominal injuries over the study period, and found that the overall rate of abdominal strains was 22 percent higher in the 2000s than in the 1990s. The most strains came in 2007, when 29 out of 1,278 pro players suffered the injury.

Athletes were most likely to strain muscles early in the season, in late March and April.

That could be because the weather is colder in those months and it's harder to get muscles loose, or because players aren't in as good shape as they are by mid-season.

"Our best guess is, (players are) just not ready yet for the demands of the baseball season," Dines said.

He added that the study can't explain the general increase in abdominal strains over time, but that elevating strength training above stretching and flexibility exercises may be throwing off the balance that helps prevent muscle injury.

Lack of adequate prevention could also be why the researchers found that one in eight athletes who had an abdominal strain during the study ended up on the DL multiple times with the injury.

That's probably an underestimate of the true number of re-injuries, Dines said, because it doesn't include the players with more minor strains who were out for less than two weeks the second time around.

Rehabilitation for abdominal strains typically involves rest, ice, painkillers and a gradual return to normal activity levels.

Researchers agreed that stretching is one of the key ways for athletes to prevent a first-time abdominal muscle strain, or a repeat strain.

"I think the mindset is changing a bit," Wilk told Reuters Health. "We want (athletes) strong but we want them stable and we want the flexibility and the endurance."

That includes improving flexibility of the leg and hip muscles, which, when they're tight, make the abdominals work extra hard and increase the chance of a strain, said Dr. W. Ben Kibler, medical director of the Lexington Clinic Sports Medicine Center in Kentucky, who wasn't involved in the study.

"Because of the number of days it takes to get back and the amount of money we're talking about, it's a fairly expensive thing and it really does affect the performance a lot more than it should," Kibler told Reuters Health.

"A lot of these should be preventable problems."

SOURCE: bit.ly/xEWk9j American Journal of Sports Medicine, online January 19, 2012.

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