The brain may explain why people slow down as they get older, starting at age 40.

How fast you can throw a ball or run or swerve a steering wheel depends on how speedily brain cells fire off commands to muscles. Fast firing depends on good insulation for your brain's wiring.

Now new U.S. research suggests that in middle age, even healthy people begin to lose some of that insulation in a motor-control part of the brain — at the same rate that their speed subtly slows.

That helps explain why "it's hard to be a world-class athlete after 40," concludes Dr. George Bartzokis, a neurologist at the University of California, Los Angeles, who led the work.

And while that may sound depressing, keep reading. The research points to yet another reason to stay physically and mentally active: An exercised brain may spot fraying insulation quicker and signal for repair cells to get to work.

To Bartzokis, the brain is like the Internet. Speedy movement depends on bandwidth, which in the brain is myelin, a special sheet of fat that coats nerve fibers.

Healthy myelin — good thick insulation wound tightly around those nerve fibers — allows prompt conduction of the electrical signals the brain uses to send commands. Higher-frequency electrical discharges, known as "actional potentials," speed movement — any movement, from a basketball rebound to a finger tap.

Consider someone like Michael Jordan. "The circuitry that made him a great basketball player was probably myelinated better than most other mortals," Bartzokis notes.

But while myelin builds up during adolescence, when does production slow enough that we fall behind in the race to repair fraying, older insulation?

Enter the new research. First, Bartzokis recruited 72 healthy men, ages 23 to 80, to perform a simple test: How fast they tapped an index finger. Anyone can do this; it doesn't depend on strength or fitness.

Researchers counted how many taps the men made in 10 seconds, recording the two fastest of 10 attempts. Then, brain scans checked for myelin in need of repair in the region that orders a finger to tap.

Strikingly, tapping speed and myelin health both peaked at age 39. Then both gradually declined with increasing age, the researchers reported last month in the journal Neurobiology of Aging.

That doesn't mean the rest of the brain is equally affected. Bartzokis has some evidence that myelin starts to fray a decade or so later in brain regions responsible for cognitive functions — higher-level thinking — than in motor-control areas.

So back to his example of Jordan, who last played professionally at age 40: "Even he started getting older. That circuitry started breaking down a little," contends Bartzokis. "He can become Michael Jordan the big-shot businessman ... but not be Michael Jordan the super-duper basketball player anymore."

Bartzokis is not looking to build a better athlete. His ultimate goal is to fight Alzheimer's disease. The connection: Building memories requires high-frequency electrical bursts, too, and Bartzokis' earlier research suggests an Alzheimer's-linked gene may thwart myelin repair.

But the new research has broader implications because it sheds light on normal aging, says Dr. Zoe Arvanitakis, a neurologist at Chicago's Rush University Medical Center.

"We knew at some age you peak and there's a sense it would disintegrate as you grow older. But we didn't have a sense of where that age would be," says Arvanitakis, who next wants to see if myelin and cognitive functions show a similar trajectory.

Bartzokis' research supports a recent report from German scientists, that with age comes a weakening of the system that's supposed to repair broken myelin, adds Dr. Bradley Wise of the National Institute on Aging.

"Any disruption in these neural circuits and networks will have problems for functioning," says Wise, who says the two reports are spurring increased interest into myelin's role in aging. Until recently, most myelin research has focused on multiple sclerosis, where myelin does not gradually degrade but disappears.