Washington Think achy joints are the main reason we slow down as we get older? Blame the brain, too: The part in charge of motion may start a gradual downhill slide 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 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.