IOWA CITY, Iowa (AP) — The University of Iowa confirmed Wednesday that 13 football players were hospitalized this week with an unusual muscle disorder following grueling offseason workouts.

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

The school has said the players, whom they would not identify, were “in safe and stable condition” and responding well to treatment.

Moore said the cause of the disorder has not yet been determined. University of Iowa physician John Stokes said the common denominator is they had all participated in strenuous exercise, which commonly brings on the disorder in otherwise healthy young people.

Coach Kirk Ferentz and the team doctor, Ned Amendola, were not at the news conference. The university said Ferentz was out of town on a recruiting trip but was aware of the situation, while an aide to Amendola said he was traveling in Costa Rica on business.

Chris Doyle, the team’s strength and conditioning coach who has worked under Ferentz all 12 years of his tenure, did not immediately return phone and e-mail messages. He and other strength coaches were not made available to reporters.

Several Iowa players wrote on their Facebook pages that the workouts involved intense weightlifting. Freshman linebacker Jim Poggi wrote Saturday that he had done 100 squats and pushed a sled 100 yards. He said he was having trouble walking. His uncle, Bo Poggi, said his nephew remained hospitalized Wednesday and he had not heard an official explanation of the illness.

“All of us are extremely concerned,” Poggi said. “We’re hoping for the best and this is a temporary situation and he’ll make a full recovery. And then we’ll get to the next steps on how this happened, why it happened.”

Another player, freshman defensive lineman Carl Davis of Detroit, wrote Sunday that he couldn’t walk or feel his arms after performing 100 squats and 100 bench presses, and had “a whole weekend of soreness.” A third, freshman defensive back Tanner Miller of Kalona, Iowa, wrote on Tuesday that he had a “night in the hospital … couldn’t be a worse day.”

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.

“We have an excellent medical staff and training staff who will do due diligence to look at what did transpire and make sure we can avoid this in the future,” he said. “I’m quite sure they’ll have safeguards in place to make sure people aren’t harmed.”

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.