At the Cooper Tire plant in Findlay, Ohio, Jack Hartley, who is 58, works a 12-hour shift assembling tires: pulling piles of rubber and lining over a drum, cutting the material with a hot knife, lifting the half-finished tire, which weighs 10 to 20 pounds, and throwing it onto a rack.

Mr. Hartley performs these steps nearly 30 times an hour, or 300 times in a shift. "The pain started about the time I was 50," he said. "Dessert with lunch is ibuprofen. Your knees start going bad, your lower back, your elbows, your shoulders."

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