The 'Dallas Buyers Club' star believes swimming competitively and ballet dancing as a youngster, reported Hello! magazine.
"I've always been active. When I was growing up I was a ballet dancer and I also swam competitively, so, although I didn't realise it at the time, I was setting myself up for a lifetime of activity," she said.
Garner, 42, who has three children, Violet, eight, Samuel, two, and Seraphina, five, with her actor husband Ben Affleck, is also grateful to her personal trainer, Valerie Waters, for helping her stay in shape.