Los Angeles ? Chris Claypool was addicted to his BlackBerry wireless handheld. Like many users, he never thought twice about pecking away at lightning speed, replying to a wave of e-mails from clients around the globe.

Last year, the 37-year-old agricultural sales director from Post Falls, Idaho, noticed a throbbing sensation in this thumbs whenever he typed. He switched to tapping with his index finger, then his middle digit and finally his pinky. But his thumbs pained him to the point where he can’t even press the buttons on his TV remote control.

After months of aching, Claypool took a break. Now he only uses his BlackBerry to send short messages – typing with the tip of a pencil eraser whenever his thumbs get sore.

“It affects business because I can’t whack away on my BlackBerry like I used to,” he said. “It’s just too painful.”

Repetitive motion injuries, which have long afflicted desktop and laptop computer users, are invading the mobile handheld world.

There’s even an informal name for the malady – “BlackBerry Thumb” – a catch-all phrase that describes a repetitive stress injury of the thumb as a result of overusing small gadget keypads.

Business executives and tech-savvy consumers are increasingly using BlackBerries, Treos, Sidekicks and other devices with miniature keyboards designed for thumb-tapping to stay connected while on the go.

And that has some ergonomic and hand experts worried about injuries from overexertion.

No national statistics exist on how many people suffer from this type of thumb ailment, but some doctors say they are seeing an upswing in related cases, said Dr. Stuart Hirsch, an orthopedist at St. Joseph’s Hospital and Medical Center in Paterson, N.J.

“It’s mostly the road warrior who prefers to answer e-mails on a thumb keyboard,” Hirsch said. “If all you did was just answer with a simple yes and no, it would not be a dilemma.”

Earlier this year, the American Society of Hand Therapists issued a consumer alert, warning users of small electronic gadgets that heavy thumb use could lead to painful swelling of the sheath around the tendons in the thumb.

The group recommended taking frequent breaks during e-mailing and resting one’s arms on a pillow for support.

“People tend to use just one finger over and over again, and it’s that repetitive use with one digit that could lead to problems,” said Stacey Doyon, vice president of the American Society of Hand Therapists and a registered occupational therapist in Portland, Maine.

An executive for Research In Motion Ltd., which makes the BlackBerry, said the company considers ergonomic factors when designing its keyboards.

“Of course, any product can be overused … so people should listen to their own bodies and adjust their routine if necessary. But I would caution against confusing rare examples of overuse with the typical experience,” Mark Guibert, vice president of marketing, wrote in an e-mail.

Musculoskeletal disorders, which include repetitive strain injuries, accounted for a third of all workplace injuries and illnesses reported in 2003 – the latest data available, according to the U.S. Bureau of Labor Statistics.

Specialists say the thumb – considered by many as an island because it is set apart from the other fingers – is among the least dexterous digit and is not meant to be rigorously worked out.

For people who insist on typing more than a sentence with their thumbs, external keyboards that connect to the gadgets may be a less painful alternative, said Dr. Jennifer Weiss, assistant professor of orthopedics at the University of Southern California in Los Angeles.

Treatment for BlackBerry thumb may include wearing a splint and applying ice to the affected area. If the pain persists, doctors may opt to inject the thumb area with a cortisone shot.