The researchers found that every hour of light exercise on top of recommended weekly levels of more intense activity reduced brain aging by about a year.

"This study emphasizes the relationship we are seeing between people doing more light-intensity physical activity and also having maintained brain structures," said lead researcher Nicole Spartano.

She added that the study shows a link but does not prove that physical activity keeps the brain healthy.

"We can't be certain that physical activity is causing people to have a better brain structure," said Spartano, a research assistant professor at Boston University School of Medicine.

Though it's not clear how much exercise is needed to keep your brain in peak condition, this study suggests that the bar is lower than thought. Spartano's team suspects a moderate amount of low-intensity activity may do the trick.

For the study, Spartano and her colleagues collected data on more than 2,300 men and women, average age 53, who each wore a device to measure their physical activity. The participants were all part of the Framingham Heart Study, a long-term, ongoing cardiovascular study of residents in Framingham, Mass.

The 2018 Physical Activity Guidelines for Americans say any activity is better than none, and more than 150 minutes of moderate-to-vigorous activity per week provides the biggest health benefit.

But Spartano's team found that each extra hour of light-intensity exercise was tied to larger brain size, which represented about a year less of brain aging.

People who achieved 10,000 or more steps a day had larger brain volumes than those with less than 5,000 steps, the findings showed.