Michelle Obama has sent social media into a frenzy after she posted a rare Instagram snap showing off her toned tummy.

The 55-year-old former First Lady shared the image on Sunday, posing in a forward lunge holding a medicine ball above her head. Talk about impressive.

Rocking a black cap, sports bra and matching tights, Michelle captioned the post: "It doesn't always feel good in the moment. But after the fact, I'm always glad I hit the gym."

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"How did you all take care of yourself on this #SelfCareSunday?" the mum-of-two added in her first Instagram workout snap.

With her tight core and abs on full display, Michelle's fans couldn't help but comment on her fit physique, with the post attracting more than one million likes in a matter of hours.

"Beast!!!" one person simply wrote.

"Come through queen," another commented.

"You put me to shame," a third commented, while others said she was a huge inspiration.

"I'm going to plank because of this post."

"I also hit the gym! You are such an inspiration."

Others joked that unlike the former FLOTUS, they spent their "Self-Care Sunday" sleeping in.

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In July, Michelle was interviewed on stage by her friend (and CBS News anchor) Gayle King in light of her memoir Becoming and spoke of the importance of how women need to "un-train" each other and not put their wellness last.

"When it comes to our health as women, we are so busy giving and doing for others that we almost feel guilty to take that time out for ourselves," Michelle said, getting personal about her own experiences.

"A lot of mothers will understand this because I found myself looking around after I had my kids, and I didn't have time for me, but my husband was at the gym every day. And I was like, 'Well, how are you going to the gym?' He was like, 'I make time for the gym'. I was like, 'What?'"

She said many women had a hard time putting themselves first, and if "we don't act together as women, as mothers, as grandmothers, we aren't going to be able to get our kids on track".

You will find Michelle doing a lot more yoga, having discovered her love for the physical movement back in 2014 when she turned 50.

"I'm seeing myself shift from weight-bearing stuff — even though that continues to be important — and the heavy cardio and running, to things like yoga that will keep me flexible," she told People magazine.

Michelle is also very diligent when it comes to regular check-ups, telling the publication she has never missed a mammogram or pap smear.

"I've had a colonoscopy. I don't obsess about what I eat, but I do make sure that I'm eating vegetables and fruit," she said.

Michelle led a massive White House campaign for child health and fitness during the eight years of her husband Barack's administration.