Think achy joints are the main reason we slow down, as we get older. New research says to blame the brain, too.
Neurologist Dr. George Bartzokis from the University of California, Los Angeles has gathered data that shows in middle age; even healthy people begin to lose some of their range of motion.
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.
After 40 you start losing that insulation in your brain.
"That helps explain why "it's hard to be a world-class athlete after 40," said Bartzokis in his report last month in the journal Neurobiology of Aging.
The research points to another good reason to stay physically and mentally active: An exercised brain may spot fraying insulation quicker and signal for repair cells to get to work.
Bartzokis compares the brain to 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.
"Someone like Michael Jordan is probably better myelinated than other humans," notes Bartzokis.
Myelin builds up during adolescence, but figuring out when production slow enough that we can't repair fraying, older insulation is the big question.
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.
Tapping speed and myelin health both peaked at age 39. Then both gradually declined with increasing age, the researchers reported.
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 like higher-level thinking, than in motor-control areas.
Bartzokis referred back to his example of Jordan, who last played professionally at age 40.
"That circuitry started breaking down a little," says Bartzokis. "He can become Michael Jordan the big-shot businessman ... but not be Michael Jordan the super-duper basketball player anymore."
Bartzokis ultimate goal is to fight Alzheimer's disease. The connection is that building memories requires high-frequency electrical bursts, too, and Bartzokis' earlier research suggests an Alzheimer's-linked gene may thwart myelin repair.
While much more research is needed, Bartzokis has some practical advice for folks.
Keep active and treat high blood pressure, high cholesterol and diabetes. Physical and mental activity may stimulate myelin repair.
"Remember, these are average people I tested," Bartzokis says. "Someone that's really practicing could make it (myelin) last longer because you're sending the signals to repair, repair, repair."