Children ages 13 to 17 will compete on the upcoming season of "The Biggest Loser"
Michaels says 'The Doctors' gig wasn't a good fit
Tennis star not always a hit with contestants
"The Biggest Loser" has found an appropriately famous fitness celebrity to replace Jillian Michaels: magazine cover star and occasional tennis player Anna Kournikova
Departing "Biggest Loser" trainer will share her fitness expertise on two series
Trainer says her replacement has already been hired
Trainer Michaels to host "Losing It With Jillian."