Fitness experts agree that sit-ups are worthless - here are 9 moves they recommend instead

Celebrity fitness trainer Anna Kaiser says many people are doing their crunches wrong, and it's making their bellies bulge.

"When they do a crunch, they push their abs out," Kaiser told Business Insider recently. "Which actually will assist in that rounded lower belly shape."

The key to a strong core, Kaiser says, is a fit transverse abdominus — that's a deep-layered muscle that sits between your ribcage and your hips.

If you want your abs to appear flatter and be stronger, you have to strengthen those deep core stabilizers, Kaiser says. Try this move:

Sit on the floor with your knees bent, feet flat. Take hold of the area behind your knees with your hands and pull in your abs while tilting your pelvis forward. This will create a c-shaped curve in your spine.

Now, raise your arms and press them towards the back of the room in little pulses.

Push-ups are also a great multi-purpose exercise that gets at your core, and other parts of your body, too.

Maloney said the benefits of push-ups are unbeatable. From your arms and shoulders down into your core, they strengthen the entire spine:

"You're getting that upper body toning effect, but you're still working the trunk," he said.

A healthy core is an important goal, not just for good looks. "If our core muscles weren't there we'd fall right over," Maloney said. Think of your core muscles as spine stabilizers, he says.

"When you do a sit up, of course, you're creating motion," he said. "I prefer things that stop motion. So planks are what we consider an anti-extension exercise."

Maloney encourages people to get up into a plank, and stay there for 30 to 40 seconds to help build a solid core.

Another reason fitness experts love the plank: much like the push-up, it gets at a bunch of ab muscles all at once.

Heather Milton, a senior exercise physiologist at NYU Langone Health recently told Business Insider that planks are a solid way to develop the strength you need to hold yourself up every day, and get a whole host of muscles toned up:

"Not only just the six-pack abs — which is your rectus abdominis — but your transverse abdominis and your obliques."

Once you're feeling like a plank pro, you can start lifting one elbow or leg at a time for an extra challenge. "You can add a whole bunch of things to a plank to make them a lot more fun, and more effective" Maloney said.

"Just by lifting one elbow, is going to challenge the system big time," he said.

Side planks are equally awesome.

In a plank, "you're holding and stabilizing the spine," Maloney said. "That's really what the core is supposed to do."

Keep your hips pressing up, and now try lifting a leg.

If you're really aiming for a defined six pack, Milton recommends hanging from a bar and doing some leg raises.

Keep your core tucked, and raise your legs straight out in front of you, feet flexed. If you're doing it right, it should look like you're making an "L" shape in midair.

Milton recommends raises over crunches, because you're less likely to hurt your back.

Olympian Dara Torres has won 12 swimming medals for Team USA, including four golds. Like so many of the other exercise pros we've talked to, Torres loves the core-boosting benefits of a good plank.

One extra challenge she often adds in is alternating from side to side with her hips.

"This will really really get you those six pack abs because you're working your obliques," she said.

Torres also does flutter kicks and scissor kicks on her back, but critically, she keeps her hands under her glutes the whole time, protecting her spine.

"You don't want to go too high because then you're not engaging your abs. You want to keep your feet down about 6 inches off the ground," she said.

And keep that belly tucked in.

Follow this simple advice from the pros, and you'll have a steely core in no time.

Your back will thank you, too.