Kelly Clarkson - Clarkson's Military Fitness Program

KELLY CLARKSON has embarked on a fitness regime used by police academies and the military in a bid to get fit.

The singer's fluctuating weight has attracted a string of comments after she was seen sporting a fuller figure in recent months.

Clarkson insists she's never felt uncomfortable about her waistline, despite critics sometimes panning her for her size.

She says, "My happy weight changes. Sometimes I eat more, sometimes I play more. I'll be different sizes all the time.

"When people talk about my weight, I'm like, 'You seem to have a problem with it'; I don't. I'm fine! I've never felt uncomfortable on the red carpet or anything."

The American Idol winner has now embarked on CrossFit, a cardio and strength training program, because it gives her so much more energy when she's performing.

She says, "I love it! It's like boot camp. You never work out more than 35 minutes, but it's so intense. You do things like box jumps, then 50 sit-ups, then 50 squats and then running.

"Me, my backup singers and my hair-and-make-up girl do it together. They laugh at me, because most of the time they're like, 'Oh, my God, I don't want to go,' and I'm the one saying, 'I'm ready!'. It makes me feel like GI Jane. It's very empowering. I feel like I can whup a**! And in my shows, everybody has noticed. They're like, 'Dude, you have 100 per cent more energy!'"