The University of Iowa said Wednesday that 13 football players had to be hospitalized this week with a muscle disorder following grueling offseason workouts that left them with extreme soreness and discolored urine.

The players have rhabdomyolysis, a stress-induced syndrome that can damage cells and cause kidney damage and even failure in severe cases, school spokesman Tom Moore said at a news conference two days after players were hospitalized at the University of Iowa Hospitals and Clinics in Iowa City.

School officials said the players, whom they would not identify, were in stable condition and responding well to treatment, which includes bed rest and the administration of hydrating fluids. Moore said he did not know when the players would be discharged.

Director of football operations Paul Federici said the players participated in workouts that started last Thursday after they returned from winter break. Some of them complained to medical staff after a workout on Monday and symptoms included soreness throughout the body and tea-colored urine, and other players were told they should receive treatment if they had similar problems, he said.

A former athletic trainer, Federici said he's never seen the syndrome among student-athletes at Iowa before. He said he was still looking into the details of the workouts but said they were no different than those from previous years during what he called a critical seven-week stretch of training.

"It is strenuous. It is ambitious. The student athletes know that," he said. "This is an anomaly. We just haven't seen this type of response before."

He said the players range from freshmen to upperclassmen and include a range of positions.

One of those hospitalized is freshman linebacker Jim Poggi of Towson, Md., whose father, Biff Poggi, said his son complained to trainers on Monday after several days of soreness. He said his son's pain started last Thursday with a lower-body workout that involved performing 100 squats in a certain amount of time and pulling a sled 100 yards. It got worse Friday after an upper-body workout, and Monday's workout "didn't go well." His urine was discolored, and the team's medical staff sent him for treatment.

Iowa coach Kirk Ferentz, athletic director Gary Barta and team doctor Ned Amendola were all out of town on business and not at the press conference. Chris Doyle, the team's strength and conditioning coach who has worked under Ferentz all 12 years of his tenure, and other strength coaches who oversaw the workouts were not made available to reporters.

Doyle is nationally known as a leader in player development, turning sometimes scrawny freshmen into physical specimens by the time they graduate. Ferentz has said that is a key part of his program's success.

Biff Poggi, a high school football coach in Baltimore, said he was concerned about the situation but also confident his son would recover and rejoin the team. He said the hospitalized players are disappointed and eager to resume practice.

University of Iowa doctor John Stokes, a kidney specialist who is not involved in the players' treatment, said the common denominator is they had all participated in strenuous exercise, which commonly brings on the disorder in otherwise healthy young people. He said the symptom is common among military recruits in boot camp and treatment usually focuses on trying to limit kidney damage.

"I've been at UI for 32 years and I don't think I've seen 13 people get rhabdomyolysis," he said. "It's a fairly common diagnosis .This cluster would be unusual."

Athletes routinely recover from the disorder and go back to playing, but they may change their exercise routines and ensure better hydration, he said.

Associate athletics director Fred Mims said school officials would take steps to "ensure it doesn't happen again." Mims, who is in charge of the department's compliance with NCAA rules, said the matter did not need to be reported since the workouts were allowed and routine.

He said the case is a "good lesson" for why university officials should ask players about how they are feeling after strenuous workouts. He said Iowa will also try to avoid problems after players return from school breaks and might not have kept up with fitness routines by making sure expectations are clear.