IOWA CITY, Iowa — 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.