Jessica Simpson has proved she's made the most of lockdown.

The singer and mum of three showed off her dramatic 45kg weight loss on Instagram, posting a mirror selfie while wearing bike shorts and a sports bra.

"Woke up before all 3 kiddos to get my steps in and spend time with me, myself, and I," she captioned the photo.

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"Move move move for your own mental health."

In 2019 after the birth of her third child Birdie Mae, the 39-year-old undertook a dramatic lifestyle change and enlisted the help of trainer Harley Pasternak.

"The difference between this and the other times was that she worked out, yes, but her transformation was more about what she did on her own time," he told People.

"She was so motivated and positive. She was saying that her body has not belonged to her for the past decade. Not in a bad way, in a positive way – her body has been designated to create life and now it's hers again and she's going to make it fantastic in a really enjoyable way."

Pasternak told People he gave Simpson five daily tasks to maintain her discipline: getting in her steps, unplugging from technology for an hour, getting sleep, eating healthy and exercising.

However, he noted that despite her impressive shedding of pregnancy weight, Simpson's gym time was only 45 minutes for three days a week.