Iowa football players hospitalized with a muscle disorder this week could have been pushing their bodies too hard after a break, which is apparently what happened to seven University of South Carolina swimmers who came down with the same ailment in 2007, a doctor involved in the earlier case said Friday.

Rupert Galvez, a Denver doctor who specializes in sports medicine, helped treat the South Carolina swimmers as a medical fellow and wrote a 2008 paper about their case in the Clinical Journal of Sports Medicine warning coaches to avoid the same problem with rigorous workouts after extended layoffs.

Like the 13 Iowa football players, the swimmers had rhabdomyoloysis, which involves the release of muscle fibers into the bloodstream and can cause kidney damage. Extreme physical exertion is one of several causes of the disorder.

The Iowa players checked into the hospital Monday after complaining of discolored urine and extreme soreness. They all had participated in intense off-season workouts that started last week after the players returned from winter break. Iowa Coach Kirk Ferentz said five of the players were discharged on Friday.

The university has launched an investigation into what caused the hospitalizations, which has raised questions about whether college football players are being pushed too hard.

In the South Carolina case, Galvez said the male and female swimmers were among 41 student-athletes who participated in an intense training program that involved pushups, squats and weightlifting after they returned from summer break in 2007. After a few days, some of them were complaining of swelling and weakness in their biceps and triceps and discolored urine.

Galvez said seven swimmers were sent to the hospital to recover and rehydrate and were discharged after three or four days of treatment to rejoin the team. Galvez said he remains somewhat baffled why those individuals were affected by the disorder while others were not. He said those affected were not taking supplements and did not have underlying medical conditions that may have exacerbated the disorder.

"If everything checks out that they are not taking anything and are healthy otherwise, it usually is just them pushing themselves," Galvez said. "It may be individual competitiveness or peer pressure from other individuals or coaches to say 'you've got to max out.'"

University of Iowa spokesman Tom Moore said the cases sound "like remarkably similar circumstances."

"That will be something for the review team to determine. But as someone who is not an expert, that would sound remarkably similar to the circumstances we're facing here," he said Friday. "Certainly can't be ruled out at this stage."

Galvez said his paper was meant to educate coaches and athletics officials to be aware of the disorder and cautious about putting college athletes through rigorous training programs when they get back from a break. He said coaches should make sure athletes are doing some exercise during breaks and ease them into their workouts when they return.

The father of one of the Iowa players affected, freshman linebacker Jim Poggi, said this week his son had done little exercise during winter break. He said his son started feeling sore after the first workout last week in which he performed 100 squats in 17 minutes and pulled a sled 100 yards, and the pain got worse after an upper-body workout the next day.

Paul Federici, Iowa's director of football operations, said at a news conference Wednesday that the players knew what to expect when they returned from break. He described the workouts as strenuous and ambitious but no different than in past years.