Brain slows at 40, starts body decline

(AP) -- 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.

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