Want strong abs? Personal trainers say to skip crunches and do these 6 exercises instead.

• Crunches and sit-ups aren't the only way to build strong abs.
• Underrated exercises like planks and flutter kicks are effective, low-impact alternatives.

If you want to build strong abs, focusing on underrated exercises that use anti-movements may be a more effective option than doing hundreds of daily crunches.

"A common belief in strength training is that performing a motion that uses the main muscle is the best way to train it," says Alex Rothstein, strength and conditioning coach and program coordinator for the Exercise Science Program at the New York Institute of Technology. "But when training abs, the main function of the muscle is to prevent motion, [so] anti-movement exercises tend to provide the greatest benefit."

Rothstein recommends focusing on exercises that represent anti-extension (planks), anti-flexion (deadlift), and anti-rotation (Pallof press). Luckily, most of these ab exercises require little to no equipment, minimal space, and are low impact.

Below are six of the best underrated exercises for building strong abs, as recommended by Rothstein, as well as trainer Nico Gonzalez from Vital Online Wellness.

Plank

The plank is a solid core exercise that relies on your anterior (the front of your body) to help prevent your posteriors (the back of your body) from taking excessive stress. "It works muscles from the shoulders to the feet," says Rothstein.

1. Start on your knees with your hands planted on the ground in front of you, creating a tabletop with your back.

2. Extend your legs behind you to create a straight plane from your shoulders to your feet.

3. Aim to keep your hips in this even plane; if your hips drop too low, it automatically defeats the purpose of the exercise as your back takes more stress than your front.

4. While holding, lean your body weight forwards toward your arms; be sure to avoid shifting your weight to your feet and making your legs do most of the work (something that can happen as you get tired, says Rothstein)

5. As you engage the core, imagine crunching your belly gently and pushing your shoulders forward so your shoulder blades separate.

Number of sets & reps: Aim to hold the plank for as many seconds as you can while maintaining solid form, and have the total number of sets you do add up to one minute. Work toward holding a plank for one minute straight.

Forearm plank hip dips

This plank variation is one of Gonzalez's go-tos as it effectively works your abdominal muscles, obliques, and lower back. Aim to keep an engaged core so your torso doesn't sag toward the ground or arch too high up.

1. Start by kneeling on the floor, then bring your forearms to the floor about shoulder-width apart.

2. Lengthen both legs behind you and assume a plank position; be mindful to brace your core and resist arching your back.

3. Twist your hips slightly to dip one side of your body towards the floor. Rotate everything from your belly button down to your legs, but keep the top half of your body steady and strong.

4. Return your hips level with the floor and back in the original forearm plank position.

5. Twist your hips in the other direction and dip that hip in the same motion.

6. Return to the forearm plank position.

Number of sets & reps: Perform 10 repetitions on each side.

Hollow hold flutter kicks

Flutter kicks are an underrated ab exercise because they not only work your stabilizing core muscles (upper, lower, and obliques), but they also engage your glutes, hip flexors, and quads.

Here's how to do them correctly:

1. Start on your back with your arms reaching overhead and legs long on the floor. Focus on keeping your abs engaged and allow your breathing to expand your ribs in and out.

2. On your exhale, curl your upper body off the floor, reach your arms to the side of your hips (similar to a crunch) and hover your legs slightly off the floor.

3. Breath in and out but stay braced in your midsection. Start to flutter kick your legs up and down at a pace that slightly agitates your balance. Work on keeping your hips and back steady.

4. Flutter kick for a count of 10, then slowly lower your upper body and legs back to the floor.

Number of sets & reps: Perform 10 sets of 10 kicks.

Quick tip: For an easy modification, bend your knees or flutter one leg at a time. To make the exercise harder, use ankle weights or hold a weight in your hands overhead.

Side plank twist

The side plank can be tricky to master since it requires both core strength and balance — but you can modify to stay on your forearms and knees for added stability. This exercise works your abs, obliques, shoulders, and hips, plus, it puts little pressure on your back and helps improve balance and coordination.

1. Start in a plank or forearm plank position.

2. Rotate your entire body open to the left, stack your legs on top of each other and support yourself with one hand or forearm

• • You can opt to keep your bottom leg straight or drop your bottom knee to the floor for support.

3. Extend your top arm toward the ceiling. Assume a straight line from the crown of your head toward your feet.

4. While keeping your hips stacked on top of one another, reach your top hand towards the floor in front of your body, as you allow your chest to turn.

• • You'll feel your oblique work as you twist, and remember to keep your hips and legs steady. Rotate only as far as you can stay balanced.

5. Return your arm up toward the ceiling and come back to your side plank to complete the rep.

Number of sets & reps: Perform 10 reps on each side of your body.

Pallof Press

The Pallof Press helps strengthen muscles from your legs through your torso and arms, providing a true full-body workout. Rothstein recommends using a cable station if you have access to one for this move. If you don't, a variation of the Pallof Press can be done using a resistance band tied to a chest-high anchor point.

1. Hold the handle attached to the cable station and instead of facing the cable, turn 90 degrees either left or right.

2. Hold the cable with both hands and against your chest and take a slightly wide legged, stable stance.

3. Slowly press the handle away from your chest; the weight will seem to get significantly heavier as you do this and your torso will begin to naturally turn.

4. Engage your core muscles as you turn to maintain proper form; this is a form of anti-rotation training that strengthens muscles in your legs, torso, and arms.

5. Rotate back to your starting position and repeat the move.

Number of sets & reps: Aim for three sets of eight to 12 reps.

Hanging leg or knee raise

Rothstein likes this workout because the act of hanging stretches the ab muscles and makes them slightly weaker, and as you perform the raising motion, the muscles are challenged to a greater extent.

You'll need access to a pull-up bar or captain's chair to do this exercise.

1. Begin by bending your knees and lifting them up toward your elbows.

• • Rothstein recommends bringing your knees all the way up to your elbows. You'll crunch your abs and even slightly tilt your hips to raise your knees this high.

2. Then lower your legs back down to the starting position, straightening your legs and knees as you do.

3. Repeat this motion at a medium pace.

For a challenging alternative: Perform a version of this move by straightening your legs and keep them straight while raising them in front of you.

Number of sets & reps: Perform eight to 10 reps.

Insider's takeaway

Ditch the crunches to add variety and more benefits to your ab workouts. Focus on moves that work all your core muscles, and even some hip or back muscles.

Your entire body will thank you for a stronger torso as you'll experience better balance, stability, and posture.