Enlarge / An archaeological sample with traces of bone-on-bone contact. (credit: Heli Maijanen)
Painful, stiff, and grating knees may not just be a sign of aging—they may also be a sign of the times.
Degenerative knee arthritis, or knee osteoarthritis (OA), seems to be on the rise.
Since the early part of the 20th century, its prevalence has doubled in Americans, according to a new study in PNAS. Researchers estimate that at least 19 percent of all US adults over the age of 45 now suffer from the condition, and it’s a leading cause of chronic pain.

But, it’s unclear why.

The most obvious suspects—longer lifespans and heftier bodies—don’t fully explain the aching jump in joint problems, researchers report.
But one thing that does seem clear is that our frames haven’t kept pace with our lifestyles. “From an evolutionary perspective, knee OA thus fits the criteria of a “mismatch disease” that is more prevalent or severe because our bodies are inadequately or imperfectly adapted to modern environments,” the researchers behind the study conclude.
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