Trainer Gunnar Peterson doesn’t believe in working out for a fleeting moment, then letting yourself go. “What are you going to do, say, ‘Please snap the photo! Now I’m going downhill?’”

So in the nine years he’s worked with Jennifer Lopez, have you ever seen her looking, well, less than fabulous? Still, even she has times when she wants to crank it up.

With a new film, El Cantante, to promote, a tour, and a new album this fall, Lopez has been meeting with Peterson for hour-long sweat sessions six days a week.

(Unlikely secret location: a track on Staten Island, N.Y.) There, Lopez, alternates sprints and distance-runs with strength training. To maintain her famously curvy derriere, Peterson devised “clock lunges” with Lopez stepping out to the angle of each hour.

To emphasize her hourglass shape, he prescribes pull-ups. As for diet, he says Lopez is smart: She doesn’t use exercise as a pass to pig out. “The workout is like a really good lawyer,” he jokes. “But if you constantly commit [diet] felonies, you’re going to do time.” [via]