No pain, no gain! Jillian Michaels has made a name for herself as a fitness guru on The Biggest Loser, where she's whipped contestants into shape for 12 seasons. As she revealed during a photo shoot for her sexy spread in Us Weekly's Body Issue, however, she doesn't actually like working out.

"When I'm at the gym, feeling nauseous, with burning quads, I'm not going, 'Oh, this is heaven!'" the L.A.-based trainer, 40, joked with Us. "But the payoff is worth it."

The trick, Michaels said, is to keep your eye on the prize and remember why you started to work out in the first place. "Maybe you want to be a better role model for your kids, or to wear a two-piece instead of a one-piece," the mom of daughter Lukensia, 4, and son Phoenix, 2 (with partner Heidi Rhoades), told Us. "But no reason is too superficial."

Michaels' reason? "I would give anything to have a booty that a rapper sings about," she admitted. "I swear, that's my dream. You just feel sexier with a firm ass." 

Watch the video above to see outtakes from the exercise pro's Us Weekly shoot!