We're into week three of our four-part question and answer series with My Beauty Life's fitness expert, Brooke Marrone.

For this installment, we're going to focus on getting tighter, flatter tummies without even mentioning the words "crunch" or "sit up" -- sound good?

You have Melanie from Decatur, Ill. to thank because she wrote in and said, "I have a really bad back and can't do sit ups! Is there any other way to get good abs?"

We'll let Marrone take it from here �

"You're not alone with having a bad back, Melanie. But, luckily for you -- and the hoards of other people with bad backs -- crunches aren't even the most effective way to tone your core.

My go-to abs exercise is the low plank. The plank is great because it works your deepest abdominal muscles, your core-stabilizing muscles, and even your arms, chest, back, and hips. Yes, all those groups with just one exercise. There are so many different, fun, and challenging versions of the plank, but this one is the best to start with:

Begin on your hands and knees and then place your elbows on the floor or mat, directly under your shoulders. Extend legs out, keeping your body in a straight line and all of your weight pushed back into your heels and out of your shoulders. Keep your head a natural extension of your spine, abs tight, and never let your hips sink toward the floor. Imagine pulling your belly button into your spine. Make sure you keep your shoulders down and back and squeeze your legs together for additional support.

Start by holding this position for four 15-second rounds, then increase the time you hold your low plank, trying to get up to a few one-minute intervals.