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Phys Ed Gretchen Reynolds on the science of fitness.

Sturdy legs could mean healthy brains, according to a new study of British twins.

As I frequently have written in this column, exercise may cause robust improvements in brain health and slow age-related declines in memory and thinking. Study after study has shown correlations between physical activity, muscular health and mental acuity, even among people who are quite old.

But these studies have limitations and one of them is that some people may be luckier than others. They may have been born to have a more robust brain than someone else. Their genes and early home environment might have influenced their brain health as much as or more than their exercise habits. Their genes and early home environment also might have influenced those exercise habits, as well as how their bodies and brains responded to exercise.

In other words, genes and environment can seriously confound experimental results.

That problem makes twins so valuable for scientific purposes. (Full disclosure, I am a twin, although not an identical one.) Twins typically share the same early home environment and many of the same genes, and if they are identical, all their genes are the same.

So if one twin’s body, brain and thinking abilities begin to differ substantially over the years from the other’s, the cause is less likely to be solely genetic or the early environment, and more likely to be attributable to lifestyle, including exercise habits.

It was that possibility that recently prompted Claire Steves, a senior lecturer in twin research at King’s College London, to consider twins and their thighs.

Muscular power, especially in the legs — which are the largest muscles in the body — is widely accepted as a marker of healthy aging. Older people with relatively powerful leg muscles get around better than those with weak legs. They also tend to have sharper minds, studies show.

But whether people’s lifestyles, and in particular their exercise habits, had provided them with good legs and minds, or whether they had won the genetic lottery, remained unclear.

So for the new study, which was published this month in Gerontology, Dr. Steves and her colleagues turned to the TwinUK registry, which includes health and fitness data for thousands of British twins.

The scientists pulled records for 162 healthy, middle-aged, female twin pairs, some of whom were identical and some not.

The scientists looked for twins who, 10 years previously, had completed extensive computerized examinations of their memory and thinking abilities, as well as assessments of their metabolic health and leg-muscle power, which measure muscles’ force and speed.

The scientists focused on the twins’ muscles rather than their exercise habits largely because the power measures were objective, unlike people’s notoriously unreliable recollections of how much they have worked out. (There was a correlation, though, between more self-reported exercise and sturdier legs.)

The scientists then asked the twins to visit a laboratory and repeat the cognitive tests.

Twenty of the identical twin pairs also completed brain-imaging scans.

Then the researchers compared leg power 10 years earlier with changes in brain function over the same time period.

They found that of the 324 twins, those who had had the sturdiest legs a decade ago showed the least fall-off in thinking skills, even when the scientists controlled for such factors as fatty diets, high blood pressure and shaky blood-sugar control.

The differences in thinking skills were particularly striking within twin pairs. If one twin had been more powerful than the other 10 years before, she tended to be a much better thinker now.

In fact, on average, a muscularly powerful twin now performed about 18 percent better on memory and other cognitive tests than her weaker sister.

Similarly, in the brain imaging of the identical twins, if one genetically identical twin had had sturdier legs than the other at the start of the study, she now displayed significantly more brain volume and fewer “empty spaces in the brain” than her weaker sister, Dr. Steves said.

Over all, among both the identical and fraternal twins, fitter legs were strongly linked, 10 years later, to fitter brains.

Of course, this study involved only a single snapshot of the brain health of middle-aged female twins. The scientists did not directly study the effects of exercise on the women’s brains, or look at changes in muscular health over the 10 years and whether that affected how well the twins could think.

The study also was not designed to uncover how muscle power builds brainpower, Dr. Steves pointed out, although she said she suspects that working muscles release biochemicals that travel to the brain and affect cellular health there. And the sturdier the muscles, the more of these chemicals they create.

More experiments obviously are needed, however, to understand these mechanisms.

For now, she said, the results imply that whatever your genetic make-up, building muscles can strengthen your mind, and should your legs currently be spindly, you might want to consider walking, running, standing or dancing more often.

“I was quite surprised by the strength of the findings,” Dr. Steves said, “because to be honest, I am someone who has always in the past prioritized work of the mind over work of the body. This study brings home to me that the brain needs exercise to keep fit.”

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