Nationals shortstop Ian Desmond is expected to return to the starting lineup on Friday after missing five starts because of an oblique strain. He also missed the All-Star Game attempting to rest the injury. (John McDonnell/THE WASHINGTON POST)

Utility man Mark DeRosa was the first Washington National stricken with an oblique injury, in late April. Then, in early May, it spread to Adam LaRoche’s right oblique. Ian Desmond is expected to return to the starting lineup on Friday after resting most of the previous five games because of a left oblique strain. The same annoying injury landed catcher Jhonatan Solano on the disabled list on Thursday.

All of this got LaRoche thinking. He mused on the topic with teammates, wondering why this kept popping up. Were players training their core muscles too hard? Were they relying on too many potentially dehydrating caffeine drinks? “Maybe we’re doing too much?” he asked. “Something is not working,” he added.

If oblique injuries seem like a new ailment, they’re not. Injuries to the set of three flat muscles in the side of the body that essentially connect the rib cage and the pelvis, have always existed. But, according to medical experts in the sport, injuries to that part of the body are occurring more often than before — so much so that it has caught the attention of league officials.

“The oblique is in in 2012,” Nationals Manager Davey Johnson said. “Everybody seems to have one. I didn’t know that those things were contagious.”

There is no conclusive reason why more oblique injuries are occurring. Experts and players offer a variety of theories, from the rise in core training to incorrect training to not enough preparation prior to the season.

Stan Conte, the Los Angeles Dodgers’ senior director of medical services, has sorted through the injuries of thousands of players who landed on the disabled list since 1991 and grouped together similar injuries such as intercostals, rib cage and abdominal strains.

Conte found that core injuries have been trending up over the past two decades. From 1991 to 1999, only in one year did more than 20 players land on the disabled list with abdominal muscle strains. The rate of such injuries increased from 2000 to 2010, with at least 20 players affected in eight of those years. Then in the 2011, the number exploded to 50, nearly double the total of 26 from the year before. This season, 32 players have already landed on the disabled list with the injury, he said.

Increased awareness of the injury could be part of the explanation. In the past, the injuries were often known as rib-cage strains or side injuries. Now, the term “oblique” has risen into the sports lexicon, Conte said. Those within baseball have gotten better at identifying oblique injuries. And when top players miss time because of them, such as the Tampa Bay Rays’ Evan Longoria last season, such injuries further shoot into the public consciousness, Conte said.

The injury has recently become so noteworthy that it has caught the attention of Major League Baseball. Gary Green, the league’s medical director, says researching oblique injuries will be among its next focuses. “It’s becoming more and more of a common injury and it’s leading to significant loss of time away from the game,” he said.

Conte said that more than 30 percent of oblique injuries occur in April, often a carry-over from spring training, when no disabled list is kept. He suggested that players might not be adequately prepared for the season. “It’s a phenomenon where players are not quite ready for the increase in speed and intensity in April coming from spring training games to major league baseball games,” he said.

Conte also found that position players are more likely to suffer the injury, and more often than not on the lead side of their body when they swing. For pitchers, it’s most likely to occur on the side opposite the pitching arm, where the body twists forward in a throwing motion.

Training core muscles became popular in the past decade. For baseball players, it’s an essential part of the body, needed for the quick, powerful twisting in throwing and hitting. LaRoche said he has done more core work in the past two seasons than ever in his career. But, Conte said, questions are raised about that: “Are we not doing enough core work, abdominal and trunk? Are you doing too much? Or, you’re doing the wrong type?”

One theory suggests that creatine, an approved supplement that aids recovery from weightlifting, can dehydrate muscles. Another theory suggests that players now rely more on energy drinks, such as caffeine shots, to provide a quick boost before a game, especially since the implementation of stricter drug testing. These theories can’t be dismissed or proven yet, Conte said.

Conte added that caffeine could lead to muscle dehydration.

“The answer is we do not have an answer,” he said. “Everybody has a theory, everybody has an opinion but we haven’t proved definitively that it’s one thing or the other.”

This season alone, four players on the Nationals’ active roster are dealing with the injury. Last season, Rick Ankiel suffered a strained intercostal muscle and Ryan Zimmerman required surgery for his abdominal strain.

General Manager Mike Rizzo said he didn’t think the Nationals were suffering from a rash of oblique injuries. Instead he believes there is more attention paid to the injury. He had no theories to what is causing them.

“If I had an answer for it, I wouldn’t tell you about it,” he said. “We’d write a book or make a movie about it.”

Desmond is unsure how exactly he injured his oblique, the first such injury of his career, but to him, the injury isn’t new at all.

“It’s around baseball,” he said. “It’s an epidemic the last couple years. Everybody. But it’s part of the game. You use your torso and core so much that you’re bound to have something.”