Kelly Clarkson has started a fitness regime used by police academies and the military in a bid to get fit.

The singer's fluctuating weight 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 programme, 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-makeup 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!'