Dr. Walter Bortz, at age 85, still runs an annual marathon, as he has for the last four decades. He’s been a professor of medicine at Stanford University, with a specialty in longevity and healthy aging, even longer. To him, the latest research linking leg fitness with brain fitness makes perfect sense.

“The legs are your most important ‘organ’ as you get older because they’re the bellwether that will reflect how fit you are,” the physician told LifeZette.

We might overlook leg strength when thinking about that other most important organ in the body: the brain. But compelling research suggests the fitness of one is closely tied to the fitness of the other.

A recently released study at King’s College in London followed the fitness of over 150 pairs of twin sisters between the ages of 43 and 73. At the start and end of the 10-year study, researchers used a simple leg press to gauge leg strength and a computer test to measure cognitive skills and memory.

They found that the twin who packed more power in her hindquarters had generally fewer age-related brain changes as the years went by.

Lead study author Dr. Claire Steves said that while other physical indicators, such as grip strength, are important, strong legs really stood out. Her daily work as a clinician who looks after older people has fueled her professional interest in the topic, but the issue also hits close to home.

“Like many people, I have also been personally touched by dementia, with my grandmother having suffered from it. That has, of course, deepened my drive in this kind of research,” Dr. Steves told LifeZette.

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Her findings have helped change the way she’s raising her family.

“I am bringing my children up differently,” she said, “stressing to them the importance of embedding physical exercise in their lifestyle.”

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While her study cohort was female, she’s looking forward to repeating her investigation with men, and expects similar results. The caveat is that they “have generally slightly higher levels of physical activity, so the effect may not be quite as easy to detect.”

She may want to sign up Dr. Bortz, whose physical feats and mental prowess clearly work in concert.

“I wrote a paper about the metabolic field, so I think the brain is just a part of this system,” said Dr. Bortz, whose numerous books on longevity and healthy aging include “The Roadmap to 100: The Breakthrough Science of Living a Long and Healthy Life.”

“The more you move, the more brain-derived neurotrophic factor (BDNF) you make. It’s a compound out of the hypothalamus, and the thing that stimulates it is exercise. It’s like miracle grow for the brain.”

Related: Don’t Want Cancer? Start Moving.

Leg strength indeed is often associated with greater aerobic fitness, observed one trainer who works with many seniors in the San Francisco bay area. “Most common, higher intensity, cardiovascular exercises require the use of your legs,” said Kristy Kinnon of Oakley Fitness.

For her clients, Kinnon recommends climbing stairs, walking briskly, riding a bike, using the rowing machine, or using an elliptical machine. All of these exercises “give your body and brain the benefit of the cardiovascular system working at a higher pace, and the legs become stronger in the process.”

Though many of the clients she works with are academics and highly cerebral by trade, Kinnon said that the scientific findings linking better mental health with better leg fitness may help motivate others who are also concerned about aging well.

The findings certainly had that effect on the study author herself.

“This has been a particular motivator for me,” said Dr. Steves. “I always knew exercise was healthy, but I prioritized the work of the mind over that of the body. Now I am much more aware of the interaction between them, and I keep much fitter by walking and cycling every day.”