Clay Aiken arrives at his Manhattan hotel. New York City, USA - 08.08.07

The singer admits his body is failing him as he learns complex tap dancing moves for the musical, in which he'll play comedy knight Sir Robin.

He tells Newsweek, "I'm sore. I couldn't even get off the toilet the other day. It hurts so bad. I don't know if it's I'm not co-ordinated or using muscles I never had to use before.

"My legs (are suffering). Having to remember 'three, four, five,' I can't move and think at the same time!"

Aiken, who makes will take to the stage for the first time this week (18Jan08), admits he's new to musical theatre and never even made the grade at school.

He adds, "I was cut from my high-school musical Guys and Dolls."

And he admits he never thought he'd ever take perform in Spamalot - because he once thought the show was ridiculous.

He explains, "The first time I saw it I thought it was the stupidest thing I'd ever seen in my entire life. My tour drummer is the Spamalot drummer, and (he) said, 'You've got to see it again'.

"I thought Monty Python was a (real) person until three months ago."

Source: WENN