Here's the next of the short essays I wrote for the "Year in Ideas" issue of the New York Times Magazine this Sunday:

Text Messager's Thumb

Dr. Robert Bacon says he can recognize the symptoms right away: ''Employees coming in complaining of sore thumbs.'' Bacon, an in-house chiropractor for Rogers Wireless Communications in Toronto, says that in the last year and a half he has handed out 16 ''thumb braces'' to help employees who have inflamed the tendons that snake along the hand and wrist -- a painful condition known as ''tenosynovitis.''

The culprit? Incessant ''text messaging'' -- using your thumb to peck out thousands of short messages on a mobile phone's 12-button keypad or on a Blackberry-style hand-held communicator. These days, peripatetic students and workers send messages all day long as they walk down the hallway or ride the subway; over 1.4 billion short messages are sent each month in Britain alone. But as hand and wrist complaints multiplied this year, experts began to wonder whether we're facing a strange new 21st-century health hazard: text messager's thumb.

If our thumbs are feeling the pain, it's because of a strange cultural evolution. They have suddenly become our most important digit. In Japan, where kids band together in ''thumb tribes,'' one company actually invented a phone-style keypad that plugs into your computer, because kids now prefer that to the traditional (and more ergonomic) qwerty keyboard. Since young people are the most fanatic texters, doctors worry that they're on the verge of a new tenosynovitis outbreak. ''They'll be developing workplace-style injuries before they've ever set foot in a workplace,'' says Andrew Chadwick, head of the British Repetitive Strain Injury Association.

Virgin Mobile has even started an ad campaign called ''How to Practice Safe Text,'' offering shoulder-shrugging exercises and a phone-shaped squeeze toy in hopes of getting its subscribers to change their hand-crippling ways.

''We have not evolved as fast as our technologies,'' Bacon concludes. -- Clive Thompson