Blake Lively's personal trainer reveals the one exercise you need to do to look good in a bikini

Blake Lively is currently all over the place promoting her upcoming movie "The Shallows," which tells the tale of a surfer in a standoff against a great white shark.

Of course, that means we're seeing pictures and clips of Lively in a bikini everywhere. And to no one's surprise, she looks absolutely amazing - even though she's covered in jelly fish stings and shark bites, and hanging out on a buoy.

Lively's fitness trainer Don Saladino, owner of Drive Clubs gyms in New York City, told INSIDER that the actress worked hard to get in shape for the movie after giving birth.

The one move that helped Lively more than anything was the kettlebell sumo deadlift, Saladino revealed.

The kettlebell sumo deadlift is a total body workout.

Before her movie, Lively was doing kettlebell sumo deadlifts once a week, as part of her regular training regimen.

"It is a deadlift, but it's a wide stance deadlift, with a kettlebell between your legs," Saladino said. "It's great for the muscles of the posterior chain, which is everything that makes up the backside of your body."

The trainer explained that the exercise also works out the whole body, with the upper body working to stabilize the lift of the kettlebell, and the lower body and core working hard to lift the weight up to the ground.

Lively worked her butt off with these lifts.

The actress would do five sets of five of the lifts, according to Saladino.

She'd usually start with two warmup sets with lower weights, and then work up to higher weight levels, but she never lifted to the point of technical failure - which is when the body just can't lift anymore

"I try to get her to the point where she struggles for five sets of five, but her technique is always perfect," Saladino said.

But even though Lively uses kettlebell in the exercises, the trainer stressed that these are not the same as kettlebell workouts. Instead, the exercise is a traditional deadlift set - picking up a weight from the floor to a standing position, with proper form.

"Blake's not doing kettlebell workouts; Blake was doing workouts with a kettlebell," Saladino explained. "You can do that same lift with a barbell, with a dumbbell. We just happened to use a kettlebell."

But doing this workout will only help give you Lively's stellar body if you're eating well, too.

Although sumo kettlebell deadlifts will definitely improve a person's strength, adding it to your workout once a week won't lead to any aesthetic changes unless you change your diet, as well.

"You're going to see strength results in two to three workouts, so two to three weeks," Saladino said. "For aesthetic results, there are so many variables. I believe if your diet is really, really good, you should start seeing some type of change in two weeks."