Rats running on exercise wheels were all over the news in 1968. Turned out the brains of the running rodents weighed more than the lazy rats.

It was the birth of a new concept, neuroplasticity, on the scientific scene.

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The fleet-footed rats challenged the previously firm scientific belief that the structure of the brain was relatively fixed after childhood — but direct proof of exercise’s benefits to the brain was slower to surface.

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“My zeal for the health benefits of physical activity was boundless in the 1980s, but there was no definitive evidence,” Dr. Walter Bortz, a physician and instructor of medicine at Stanford University, told LifeZette.

Related: Alzheimer’s Fact vs. Fiction

Things are different today.

“Now large-scale observational studies in groups including nurses, people living in rural areas, and older adults have found reduced risk of cognitive decline and dementia among people who exercise regularly,” Bortz, an expert in Alzheimer’s treatment and himself a trim, fit 80-something, told LifeZette.

The benefits are clear: A 28 percent reduction in risk of Alzheimer’s disease in physically active older adults

Just how much reduced risk? Dr. Jintai Yu from the Memory and Aging Center at the University of California, San Francisco, recently did an analysis of many of the most solid studies existing in this area. Yo found “a 28 percent reduction in risk of Alzheimer’s disease in physically active older adults compared to their non-active counterparts.”

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Nor were benefits restricted to those with a formal diagnosis.

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“We also found that physical activity had the potential to improve global cognitive function for subjects with mild cognitive impairment,” Yu said.

In other words — you and me.

Since the importance of physical activity in reducing the risk of Alzheimer’s is well documented, and most Americans have already heard the good news, why are there still so many people still sedentary?

Rita O’Brien, a retired high school teacher and small business owner, has some ideas of her own on this.

“The more time I spend on sedentary activities like reading, knitting, or iPad, the harder it is for me to get going,” she told LifeZette.

For O’Brien, the block is mental as well as physical.

“It’s like I have a sedentary self-image. I manufacture obstacles. I say to myself, ‘I’m not a walker.’ I tell myself I need to find special shoes that give the right support because I am afraid of falling,” she said.

Related: Pump it in the P.M.

She admits that even to herself, some of her “obstacles” start sounding like excuses.

“Getting dressed is another obstacle. I have to be presentable to walk in the neighborhood or in the mall, right? I can’t go because I don’t look good, and I don’t look good because I don’t get out there and exercise,” she said.

Many of us can relate to O’Brien’s story. More than two-thirds — 67.5 percent — of all adults watch two or more hours of television per day. Additionally, 25.2 percent of adults use a computer outside work, or play computer games an average of two hours or more per day.

O’Brien is determined to get active: “I want to take care of the things that God has put before me.”

The National Institute on Health recommends that some activity is better than none. Inactive adults should gradually increase their level of physical activity. Try to do a variety of exercises, but walking has been shown to provide health benefits at low risk of injury.

Other tips include:

Consider activities you were good at years ago and get back into them.

Link physical activity to the things you enjoy — window shopping, checking out new neighborhoods — or that you see as productive, like walking the dog or walking to do errands.

Related: Can Alzheimer’s Be Catching?

Don’t overlook having fun. Find an exercise buddy, and plan regularly scheduled physical activities that are cool.

Choose a role model and ask him or her to be your mentor.

If you get discouraged, set small, more realistic goals and start again.

Don’t be afraid to brag a little.

Reward yourself when you succeed.

And don’t give up. O’Brien isn’t.

“My mother died of Alzheimer’s at age 78,” she said. “I am now 73, and I want these years ahead to be the best of my life.”

She cites with regret — tinged with determination — a recent trip to California.

“I saw the redwoods and the coast from the car. Next time, I want to hike through those redwood forests … I want to take care of the things that God has put before me.”