Physical fitness is the key to keeping your brain young. This is according to a new study that looked at brain activity in people as they age.

We already know that exercise is good for our bodies. Daily exercise fights the loss of muscle tone and flexibility that tends to creep up with age. And it turns out, it has the same fountain-of-youth affects on our brains.

Studies show that over time our minds also become less flexible and nimble. We lose the ability to process new information or switch between mental tasks quickly — and now we know why.

Neuroscientists have looked at the brain activity of young people and compared it with the brain activity of people over 40. While completing cognitive tasks, younger participants had brain activity that was highly localized. Depending upon what they were thinking about, the scans lit up exclusively in either the right or the left prefrontal cortex.

But the older participants used far more brain power to complete the same tasks. No matter the activity, the older volunteers tended to use both hemispheres of their brain.

Researchers from Japan's University of Tsukuba wanted to explore this idea by looking at the effect of exercise on this weakening of the brain. They tested the aerobic capacity of 60 Japanese men between the ages of 64 and 75 who did not show any signs of dementia or other mental issues. Next, they looked at the brain activity of each man as he completed a mental task. They found that the fitter a participant was, the less brain activity he used to complete the tasks when compared with his out-of-shape peers.

Of course, this study alone can't draw a direct line between exercise and what's happening in the human brain. But it does build upon similar research, such as a study first mentioned by MNN's Lloyd Alter that looked at how exercise improved cognitive ability in older adults, and a study I wrote about earlier this year that showed how daily exercise in your 40s can led to less brain shrinkage as you age.

Less brain shrinkage and more brain power? Sounds like some pretty good reasons to lace up.