June 05, 2015 01:00 PM

Four months after giving birth, Carrie Underwood showed off her fit post-baby body in a photo shoot for her fitness line Calia, and now her trainer is revealing how the new mom got back in shape.

A typical workout session for the singer — who welcomed son Isaiah Michael in February — includes “a warm up and a cool down,” says Erin Oprea, Underwood’s longtime trainer (more of her workout tips can be found here!).

“Sometimes we’ll go for a run with lots of hills, stop at a park to do some pull ups, push ups, step ups and other fun body weight stuff,” she adds.

Oprea also mixes it up for the Grammy winner to “keep it fun and keep shocking the body,” she says. “No two workouts seem to end up the same.”

Peter Stanglmayr/Dick’s Sporting Goods

Calling her client a “super motivated person with big goals,” Oprea sees the new mom three times a week for an hour each time, but adds that Underwood “tries to be active every day” depending on her schedule.

“Having a baby definitely changes your routine, and I have to work around his schedule,” Underwood, 32, told PEOPLE from the set of Calia’s fall campaign. “Whenever he goes to sleep, I do the things I need to do for myself and make workouts happen when they can.”

And though it might be tough for new moms to get back in the gym, one way to feel more motivated after giving birth is by staying active throughout the entire pregnancy, says Oprea.

“To be able to continue your regimen right after your pregnancy, you had to try your hardest to keep up with your workouts during your pregnancy,” she says. “Maintaining your fitness levels during will allow you to kick start your post much more quickly.”

And while women can’t hit the gym right after giving birth, one way to stay healthy is by “cutting out the junk food,” she says. “Focusing on nutrition first will really get your body ready for you and your baby. Once back in the gym, start slowly.”

— Melody Chiu with reporting by Katie Kauss
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