Lachey posted a black-and-white photo of herself sweating it out at the Heart & Hustle gym, a private personal training facility in Los Angeles.
"It took a solar eclipse to get me to the gym apparently!" the mother of three joked. "I haven't set foot in a gym in over a year because... well... life happens! And I wouldn't trade in mine for anything in the world."
"Balance is key, though," she continued. "My family is my No. 1 priority. My husband and our three kids deserve the best of me... which is why I'm doing this for ME. You gotta start at the foundation. I'm finding 'my' balance still every day, every week, every month and every year. It seems to get harder as I get older and life gets more complicated, but that's the beauty of it sometimes! Live in the moment! Enjoy the ride! And to all you working Mama's out there!!! LETS DO THIS!"
And while the brunette beauty's been working on her bod, her husband, Nick Lachey, has been working on a different type of project. He's reunited with his band, 98 Degrees, and together they are releasing a Christmas album, Let It Snow.
ET caught up with the boys ahead of the exciting news, where they teased what fans can expect from their second holiday album, which drops in October. Hear more from our exclusive chat in the video below!