"It just locked up on me," Buck said.
After returning to the dugout, Buck first went to check on shortstop Luis Hernandez who had tried to take the catcher's errant throw as a runner slid into second base on a stolen-base attempt.
"I walked down to see if Luis was all right -- I saw him get his hand ripped. I went to ask him if he was all right," Buck recalled.
"I took off my chest protector, then I bent down to take off my shin guards. Then I bent to get the donut [for his bat] and it happened."
This was just outside the dugout.
"I went down to my knees," he said. "Then I started crawling back to the dugout."
Coaches, teammates and trainers quickly sprang to his aid and he was taken to the training room.
Buck said that he had never had any back problems prior to this incident. He was still confined to the hospital on Sunday night.
Buck, relegated this year to a secondary role behind Miguel Olivo, has batted .226 with three home runs and 19 RBIs in 28 games. He made a special effort to streamline his body during the winter and was in excellent condition this spring.
Pena was batting .307 with four homers and 18 RBIs for Omaha in 22 games, although he caught in five games and had played the outfield in nine. He appeared in four games for the Royals, going 2-for-7, before being designated for assignment on April 24. Pena cleared waivers and was assigned to Omaha on May 5.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.