Sales of laptop computers passed desktops in the U.S. for the first time ever this fall, according to market-research firm IDC.

　　That's bad news for backs, necks and shoulders.

　　'Laptops are inherently unergonomic -- unless you're 2 feet tall,' says physician Norman J. Marcus, a muscle-pain specialist in New York City.

　　When you work at a computer, the keyboard should be at elbow height, so your upper and lower arms form an angle of 90 degrees or more and your forearms are supported by armrests. The monitor should be roughly at eye level so you can lean back in a chair with back support.

　　But most users simply set their laptops on a desk or table. The keyboard is too high, which makes your arms reach up, your shoulders hunch and your wrists bend down. The monitor is too low, which pulls your head and neck forward and down and puts a strain on your back.

　　That's OK if you use your laptop occasionally, for short periods. But if you use one for hours at a stretch -- as do millions of college students, business travelers, telecommuters, video-gamers and growing numbers of office workers -- you're setting yourself up for muscle problems that can make your entire upper body hurt.

　　Ergonomics experts have warned about laptop problems for years -- mostly in vain. People continue to abandon bulky desktops in favor of the ever-sleeker, lighter portables. And WiFi connections let us use laptops anywhere -- in bed, on the floor -- in all kinds of contorted positions.

　　'People think, 'How can a mouse or a keyboard hurt you?'' says Thomas Caffrey, founder of Myofactors LLC, which does ergonomic consultations for factories and offices (including The Wall Street Journal). 'But poor technique can significantly overload the anatomy over time.'

　　A wrong position can cause pain and stiffness in the neck, shoulders, back and arms, as well as headaches, pains in the temporomandibular joint (TMJ) and carpal tunnel syndrome, in which pressure on wrist nerves causes tingling and numbness in the hands.liuxuepaper.com