Kramer says changing the size and the function of your brain is as easy as taking a few steps.
"We found in our study that walking will increase the volume of the brain, increase the efficiency of the brain and increase improvements in the number of cognitive functions such as memory and attention," Kramer says.
Seventy-eight-year-old Grace Miller walked three days a week for six months. She says she "really" thinks she's noticed a difference.
"My husband and I, we kind of try to remember things," Miller explains. "Lately, I have been doing all the answering."
"We know that over a six-month period you can get a 50 percent improvement in memory and attention," Kramer says. "That is pretty remarkable."
Kramer imaged the brains of 60 participants before and after six months of walking — and saw an increase in crucial areas of the brain responsible for memory and decision making.
Only On The Web: Psychology professor Arthur Kramer discusses the study results with Dr. Sanjay Gupta.
TIME: How The Brain Rewires Itself
"I was surprised how much plasticity, how much flexibility older brains have, because the general belief up until a decade ago was that brains deteriorated as we age. That's not true," he says.
Not true at all. Plasticity is the actual strengthening of connections between neurons, stopping, yes, even reversing memory loss. Physical exercise helps, and so do mental exercises.
"The brain is actually revising itself. It is actually plastically changing itself as you develop new skills and abilities, as you learn new things," says Mike Merzenich, a professor at the University of California, San Francisco.
Merzenich has, in many ways, turned neuroscience on its head by championing this idea of plasticity. He has started a company called Posit Science that has developed a computer program called Brain Fitness, which is commercially available for about $400. Program users exercise their brain by doing thousands of different mental tasks per hour. The goal is a younger, more active mind.
Seniors at Leisure Cares' Heritage Estates community in California say Posit Science's brain fitness programs have made a difference.
"It stimulates your brain and really makes you remember and want to remember," says Grace Curran, one of the residents.
To be clear, says Gupta, what Merzenich is selling hasn't been independently proven to work. But a recent study in the journal of the American Medical Association says the benefits of cognitive training like this can last five years.
Without question, there are already lots of good reasons to exercise. But now we know you may be training the most important "muscle" of all: your brain.