Jessica Simpson has "sore buns"

Jessica Simpson has “sore buns” after a rigorous workout.

The singer complained her personal trainer Mike Alexander pushed her too hard during an exercise session on Wednesday, leaving her with a tender butt.

"Why am I still hurting from our workout earlier?" Jessica wrote in a Twitter post to Mike. "I'm getting old! See you tomorrow with a pouty face and sore buns."

However, Mike ignored Jessica’s pleas and refused to go easy on the 29-year-old star in their next workout. "You are a bada*s,” he wrote on her Twitter page. "Your efforts know no limitations. And I can deal with looking at that pouty mug..."

Jessica embarked on a strict exercise programme after she was criticised for appearing to have piled on the pounds earlier this year.

The blonde beauty’s sister Ashlee Simpson-Wentz has previously defended Jessica’s weight and has jumped to her defence again. “It's disgusting that people would say those things,” she fumed to Women's Health magazine. “My sister has an incredible body. I feel sorry for anyone who would judge her, because she's one sexy lady.”

Ashlee is such a fan of Jessica’s voluptuous figure, she is even planning to relax her diet and develop some curves of her own.

The singer-and-actress – who has an 11-month-old son, Bronx, with husband Pete Wentz - added in the December issue of the publication: “There's going to be a time when I'm way curvier, and that'll be sexy, too.

“Pete isn't worried about stretch marks. He always makes me feel good.”