for your brain's wiring.

Now new US 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 George Bartzokis, a neurologist at 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 fibres.

Healthy myelin - good thick insulation wound tightly around those nerve fibres - 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?