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Nearly a quarter of the Mets’ roster from opening day — a collection of players making more than $50 million this season — is out because of injuries. In their place, the Mets have resorted to a ragtag group that includes several minor leaguers.

The problem is representative of a larger trend in Major League Baseball that has left people in the sport grasping for an explanation: the amount of time players were out with injuries increased 26 percent from 2006 to 2008. This year’s numbers, midway through the season, are similar to those from last year, when players spent more time on the disabled list than ever before.

Injuries can hurt teams on the field and in the pocketbook; baseball estimated that teams paid a half-billion dollars last season to players on the disabled list, the ledger of players considered too injured to compete. Players are breaking down at such a rate that teams are basing contracts more on performance incentives like games played and innings pitched. That way they can limit their exposure to having to pay players who are out with injuries.

Without a rigorous examination of what is behind the increase, the most popular explanation is that testing for performance-enhancing drugs, particularly stimulants, has deterred players from taking substances that for many years enabled them to play through pain.

Other theories include the belief that major leaguers, whose salaries are higher than ever, are less inclined to play with pain for fear of jeopardizing their lucrative careers; the idea that advances in imaging technology have enabled team doctors to diagnose more injuries; or the possibility that teams are putting uninjured players on the disabled list as a way to expand their rosters.

“There are a lot of theories around about why it has gone up, and a lot of them make sense, but I am not convinced that it’s one thing,” said Stan Conte, the director of medical services and the head athletic trainer for the Los Angeles Dodgers. “Anyone who thinks they have the one answer is lying or wrong.”

Long before baseball players began taking muscle-building steroids, they were openly using stimulants like amphetamines in clubhouses, to help them through the grind of a 162-game season. But in 2006, the league began testing for amphetamines, which mask fatigue and pain and increase reaction time. The testing is not believed to have become a deterrent until 2007.

“What stimulants do is create an unrealistic self-confidence for the athlete, and they think their body can do things that it, at times, really can’t,” said John Hoberman, a professor at the University of Texas who has written frequently on doping. “These drugs postpone the symptoms of injuries, symptoms that restrain athletes from playing hurt. They are no longer there, and players are realizing that they can’t play hurt.”

In the 1990s, a decade of significant advances in sports medicine and training, the number of reported injuries steadily increased in baseball.

Barry Axelrod, a longtime player agent who is the treasurer of the United States Anti-Doping Agency, attributed the rise of injuries in the 1990s to the use of steroids. “I know the powers in baseball said they had no idea, but everyone in baseball knew dating back to the late ’80s it was prevalent,” Axelrod said. “The side effects from the steroids was the increased injuries because players were more fragile and their muscles were becoming too big, and the training was too intense for the body.”

In 2002, a year before baseball began testing for steroids, the number of injuries dropped drastically. It stayed low until 2007, when the number spiked; it set a record last season.

The problem, exemplified by the Mets, has remained acute this season. The Mets (39-42) are missing the heart of their lineup — shortstop Jose Reyes, center fielder Carlos Beltran and first baseman Carlos Delgado — and are four and a half games off the pace in the National League East standings. They lost all three of their games against the first-place Philadelphia Phillies over the weekend.

The Atlanta Braves (39-43), who are a half-game behind the third-place Mets in the five-team N.L. East, , are in a similar situation. They have played the entire season without one of their best starting pitchers, Tim Hudson, and have seven other players on the disabled list.

“Teams could be being more cautious with players, not letting guys play through things,” said Mets catcher Brian Schneider, who spent time on the disabled list this season with a back injury. “Everyone’s always heard of a phantom D.L. It might be that they’re doing it more for roster spots, not to lose guys to waivers, to keep guys up. There’s just so many different rules, so many different ways to go about it. It might be a coincidence. It’s just such a complicated question because there’s so much that can go into a D.L. situation.”

If a pitcher needs to work on his mechanics, for example, a general manager could say that he is injured and put him on the disabled list, allowing the team to replace the pitcher while he works out his problems.

“Making good use of the D.L. allows you to expand your 25-man roster,” Axelrod said.

Baseball, which has compiled the yearly injury totals, has not done so this season. But Conte, the Dodgers trainer who is leading an effort to build mathematical formulas to predict injuries, said that based on his data, the numbers are down slightly from last season. They have not, he said, returned to the levels of 2002 to 2006.

Peter Nash, the director of underwriting for Sportscover, a syndicate of Lloyd’s of London that insures contracts for teams and players, said that over the last decade, teams have been giving more contracts based on performance. A hitter, for example, may receive more money if he plays in at least 100 games.

“We used to have to cover clubs much more when they were giving large contracts without the incentives,” Nash said. “Now we are being approached by the players and being asked to insure the incentives in their contracts to make sure they make money even if they get hurt and can’t play.”

Nash said that his company had noticed an increase of injuries in baseball in recent years and thought it was part of a larger trend.

“We believe that injuries move in five-year increments: they go up for five years, then plateau, then go up for five years, and plateau, and over time, they are going higher and higher,” he said. “As long as athletes are pushing themselves as hard as they can, the number will continue to go higher and higher.”

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