Transcript for Body Wars: How 'Fit Mom' Accidentally Sparked a Movement

Girls daring to bare it all, like Lena Dunham. We were saying how much we loved your bikini. That's not true, I heard you laughing at it. Joan rivers on Howard Stern calling her out for showing off her kfrs. That's wrong, you're sending a message it's okay, stay fat, get diabetes. And Mindy kaling laughing it all off. So refreshing that Mindy left herself go and be a fat sea monster. Fat shaming can also be serious. "Die young" singer Kesha said it led her to instant in rehab for a eating disorder. And now the fighting has filtered from the front pages to our home pages. When this mom posted what's your excuse? It sparked reaction from tens of thousands of women. Wow, this woman is so inspiring. You should be ashamed for fat shaming other mothers. Shame away my baby weight is coming off. Bad mom would be a better tag line for caring more about her body than her kids. Reporter: And that was excuse enough for us to meet the fit mom, maria kang, mother of three, fitness fanatic and accidental instigator in the battle of the body wars. I think people were taking my message out of context. That's unfortunate. This is a very powerful message. I heard I'm a bad mom, I heard I'm a fat shamer and bully. I'm saying what's your excuse for not making health a priority? Show America how tough moms are. Reporter: Turns out there are lots of moms that do agree with her. I lost 30 pounds and I'm a no excuse mom. I dropped five dresses and I'm a no excuse mom. Reporter: In just sigs months no excuse mom clubs popped up all over the world. No excuses! Reporter: Maria turned her controversial tag line into a movement. I don't think asking moms to work out is judgmental because everyone should find time in their day, at least 20 minutes to doo exercise. Reporter: But her unrelenting no excuse refrain is what makes so many mad. Her response on Facebook, sorry, I'm not sorry. That kind of talk drove a soft spoken Kansas City mom to post this. You don't know sleep deprivation until you worked a schedule like this. Reporter: Sarah juggles a job as overnight security guard and cares for her kids, she does 36 hours stretches with no sleep. Outraged by fit mom's post it was Sarah and some friend whose launched we don't need an excuse counteroffensive. This makes me feel shamed, like I'm supposed to live up to a standard. Reporter: And now, after months of battling it out, they've asked fit mom to come to Kansas City and challenged her to walk in their shoes for one day. This is a tough life. I wonder why they chose me for this assignment. I'm on maternity with this guy two and a half months. I feel the pressure to get back in shape. I'm wearing a compression tank to suck it all in and don't tell anything but these are still my maternity pants. First we meet up with maria in California who assures us she doesn't work out all day. She runs a chain of homes for the elderly. Are you going to exercise? Reporter: Chases her three boys around and even does a regular local news segment. Sleep is so important. It's a problem I'm seeing a lot of is complacency, battling that desire to sit around and do nothing and watch "American idol." Reporter: Next stop, Sarah's home turf where the moms will take it off the blog and to the kitchen table. She's pretty apologetic, I feel like it's going to go in one ear and out the other. Reporter: She's not outgunned, maria enlisted the leader of Kansas City's no excuse mom club as a backup. Seriously, I was like I didn't want to come into this situation and be by myself and feel like I'm being ambushed but these moms. I'm just so happy you're here. Let's do this. We have in blind courage, post post anonymously -- Reporter: I don't need to do much. You don't think maria is fat shaming? I don't at all. Reporter: Do you? Yes. Part of the reason I think that we have so many body issues, particularly in this country, is because we're -- Reporter: Reporter:le -- When people feel shame they feel powerless. Saying somebody needs an excuse implies they are at fault. I never said anyone should look like me. You don't have the disclaimers on that photo and you say you're not responsible for the thoughts in other people's minds but that's very similar to a celebrity saying I'm not a role model for children but you are. You plummeted yourself into this -- I'm not apologizing. Tell people that anyone mom's fitspiration went viral but mine did and they messed with the wrong mom and I have something to say. I attracted with you my face with my abs, with that caption but when it comes out of my mouth -- You did not tract me. I love you but you did not attract me. I'm attracting other people. Diapers. I got to get the -- Reporter: The next morning the distance between the two moms seems even greater. Good morning, Kansas City. Kind of early. Quick 20 minutes before a very long day. Didn't get the impression that she is willing to see any other side but hers. Sometimes I think people vilify me to the point that I don't think I know what it means to be busy and I actually do know a lot about that. Reporter: Three adults, one child, two car seats to install. Go. See how fast we can get in the car. What better way to bring people together than a day cooped up in the family minivan. And we're off to a day in the life of Sarah. Which means a lot of time in he here. I feel like come on, I'm a mom. I feel like -- zblike you took my mom badge away from me. I have three kids. I know what it's all about. I am sorry. I am so sorry that I made you feel that way. Reporter: Our first stop, Diane's home schooling group. Where does the rain come from? Reporter: Sarah contributes time here a few days a week and teaches her son at home in between, all in addition to her security guard shifts. I would never, ever say to anyone fitness is a bad thing or obesity is a good thing. You have to determine what works for each person because, again, I have a 36-hour day every week. Reporter: Because of this special needs, he's enrolled in activities all over town and it's here maria thinks she sees a clear window of opportunity in Sarah's schedule. Run around thelock for a good half hour because this is an hour class. You can incorporate fitness. 32 minutes away. Reporter: For maria being a good mom also means taking good care of yourself. My mom did so much for us, so much to provide for us and we came, my father is an immigrant, he came to this country, they didn't have anything. And watching her do everything for us made me sad because snefr took time out for herself. Reporter: In fact, she says it's why she started the into excuse mom club. Let's do this, baby. Reporter: She invited me, Sarah and Diane to the city's chapter to see what the movement is about. Let's start -- With the baby. Reporter: Moms working out with babies in tow, how that is for no excuses? It's hard with the kid, it throws your balance off. Reporter: This is tough. While the other moms pump it out, Sarah's soon on the sidelines uncomfortable and unable to join in. You think you could carve time out for yourself? Depends on what you define as time for myself. It's not something do I every day and it's not fitness related. If I took actual time to exercise that would seem like work to me. Reporter: They will say she just didn't get our point. So many people are mistrerping the mess and. Is it worth it? Pick it up. I think the majority of people get the message and the small group that doesn't, I don't focus on that. Reporter: You spent the last 24 hours with them, what did you learn? I'm absolutely enthralled with Sarah and her life and her kids. I think we're on very different paths in our lives. And I don't see them interse intersecting but I have a deep respect for what she's done.

This transcript has been automatically generated and may not be 100% accurate.