Aiken plays one of the leads, Sir Robin, in the Tony Award-winning musical in a stint from Friday through May 4. He told the magazine he was so sore from rehearsals he "couldn't even get off the toilet the other day."
"It hurts so bad. I don't know if it's I'm not coordinated or using muscles I never had to use before," he said.
The show is based on the film "Monty Python and the Holy Grail," which came out in 1975, a little before the 29-year-old singer's time.
"I thought Monty Python was a person until three months ago," Aiken told Newsweek for editions on newsstands on Monday.