There’s nothing magical about the number 10,000.

In fact, the idea of walking at least 10,000 steps a day for health goes back decades to a marketing campaign launched in Japan to promote a pedometer. And, in subsequent years, it was adopted in the U.S. as a goal to promote good health. It’s often the default setting on fitness trackers, but what’s it really based on?

“The original basis of the number was not scientifically determined,” says researcher I-Min Lee of Brigham and Women’s Hospital.

She was curious to know how many steps you need to take a day to maintain good health and live a long life, so she and her colleagues designed a study that included about 17,000 older women. Their average age was 72. The women all agreed to clip on wearable devices to track their steps as they went about their day-to-day activities.

It turns out that women who took about 4,000 steps per day got a boost in longevity, compared with women who took fewer steps. “It was sort of surprising,” Lee says.