Jillian Michaels on Weight Loss Struggle: "It Was Pure Hell"

NEW YORK (CBS) Jillian Michaels is TV's toughest and fittest trainer. But, the "Biggest Loser" trainer wasn't always in the best shape.

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In the July issue of Redbook, Michaels reveals that she was once overweight. Michaels, who is now 5'2" and 120 pounds of muscle, said she weighed 170 pounds at her heaviest.

"Oh, my God, it was pure hell," says Michaels, who began to gain weight at the age of 12 after her parents divorced. "I spent all of eighth grade in my classroom because God forbid I ever left. I was terrorized."

The 36-year-old fitness expert credits martial arts for her weight loss success. "Karate saved my life. It all stopped the day I broke two boards with a kick," she tells the magazine.

After she shed the pounds, Michaels eventually transformed herself into America's fitness drill-sergeant. Additionally to being the certified fitness guru for "The Biggest Loser," Michaels has published a series of books and stars in exercise DVDs.

Along with karate, Michaels lives by a few unbreakable rules that have helped her successfully lose the weight. They include calorie counting, avoiding processed foods and doing five intense 30-minute workouts a week.

She will next star in her own show, "Losing It with Jillian," which will air on NBC.

The magazine hits newsstands on June 22.