You’ve probably heard the saying, “abs are made in the kitchen.” But that's really only partly true. While a healthy diet goes a long way toward uncovering your six-pack (it's in there … somewhere), you’ve got to put in work at the gym (or studio!) too. And when it comes to developing a rippling core, the lower abs might be the toughest zone to sculpt.

“This is an area many women want to target,” says trainer Susanna Kalnes, who teaches fitness classes at Studiomix in San Francisco. “I get asked by many of my students how to flatten this area of the body.”

Below, Kalnes and trainer pal Kristi Williams, who teaches fitness classes at XSport Fitness in Chicago, share their picks for the best lower ab exercises to add to your spring shape-up routine.

The eight bodyweight moves require no equipment (which means no excuses). And rather than counting your reps and sets, Kalnes recommends timing your workout. "I like to do these [moves]–and teach them–in time intervals," she explains. Do as many of each exercise as you can with good form for 45 seconds. Rest for 15 seconds, then move on to the next one.

Get ready to feel the burn.

Heel Taps

Lie on your back with your hands behind your head, your elbows wide, and your knees raised in a 90-degree angle with your feet off the floor. Lower one leg down, maintaining the bend in the knee, and tap your heel on the floor. Return to starting position, then repeat with the opposite leg. Continue to alternate legs.

Butterfly Sit-Ups

Lie on your back with the soles of your feet touching and your knees bent out to the sides. Lift into a sit-up while reaching your arms beyond your feet to tap the floor. Lower down one vertabra at a time to protect your spine. Repeat.

Scissor Kicks

Lie on your back with legs straight and shoulder blades lifted. Draw one leg up to the ceiling as the other hovers an inch or two off the ground. Alternate legs, switching like two scissor blades. Use your arms to support your thighs if you need a little assistance.

RELATED: 27 Fat-Burning Ab Exercises (No Crunches!)

Leg Drop

Lie on your back with your hands tucked under your rear and your legs lifted straight up toward the ceiling. Flex your feet, then lower your legs toward the ground while keeping them as straight as possible. Lift back up to starting position and repeat.

Low Plank Rock

Start in a low plank position balancing on your forearms with your spine straight. Using your abs, rock forward two to three inches on your toes, then rock back to starting position. Repeat.

RELATED: 20 Ways to Do a Plank

Mountain Climbers

Start in a high plank position with your wrists directly below your shoulders. Draw one knee into your chest, keeping your spine straight, then return to starting position. Repeat with the opposite leg, then continue to alternate as quickly or as slowly as you want. The faster you go, the more of a cardio move it becomes, Kalnes notes.

Ski Abs

Start in a high plank position with your wrists directly below your shoulders. Use your lower abs to lift your hips and jump both feet forward and to the right side of your body. Jump back to plank position, then repeat on the opposite side. Continue to alternate.

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Reverse Crunch

Lie on your back with your hands tucked under your rear and your legs lifted straight up toward the ceiling. Using your lower abs, lift your legs, butt, and lower back off your mat closer to the ceiling. Lower back down with as much control as possible. Repeat.