IOWA CITY, Iowa — Iowa Coach Kirk Ferentz said he remains puzzled about why 13 players were hospitalized with a muscle disorder following intense workouts last month.

Ferentz told reporters on Wednesday his teams had done similar or even tougher workout routines three times before — in 2000, 2004 and 2007 — without incident.

He said he has no idea why last month’s workout apparently gave the players rhabdomyolysis, which releases muscle fibers into the bloodstream and can cause kidney damage. They’ve all been released from the hospital.

Ferentz said the workouts will not be repeated.