His back is weak because he won't try to sit up.

他的背很虚弱，因为他不愿试着坐起来。

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"Lift the latch," called out the grandmother, "I am too weak, and cannot get up."

“把门闩拉开，”奶奶喊道，“我太虚弱了，起不来。”

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Trembling and weak with emotion, Misses Smith hurried up to her husband.

VOA: special.2009.12.12

Sure those specs might seem weak up against the last smartphone, but its more than enough juice to run 325 (and growing) apps LeapFrog has for it, not to mention that the new unit really is noticeably quicker.

ENGADGET: LeapFrog LeapPad 2 and Leapster GS Explorer hands-on (video)

Industry officials are eager to point out that small cuts in the fuel consumption and emissions of millions of cars add up and have a greater impact overall than what might still be a relatively weak take-up of electric cars.

BBC: Electric motoring: a quiet revolution

Her mother, too weak to sit up in bed, lay against her pillows and inhaled deeply.

NEWYORKER: Temporary