A new study has revealed that the prevalence of osteoarthritis in the knees of humans has doubled since the mid-20th century — and not because we're living longer.

Dr. Ian Wallace, a postdoctoral research fellow in the Department of Human Evolutionary Biology at Harvard University, studied over 2,000 skeletons from three different time periods in order to reach that conclusion.

He looked at 176 prehistoric skeletons, and many more from both the early industrial era and the post-industrial era, which included specimens from the early 2000's.

Modern specimens were more likely to have knee arthritis than either group of older skeletons.

Research Paper - Knee Osteoarthritis has doubled in prevalence since the mid-20th century

What was really surprising was that Dr. Wallace found that the increase in arthritis was there even if he controlled for the fact that we're living longer, and the general increase in obesity in modern times, which is a risk factor for osteoarthritis.

Dr. Wallace thinks the most obvious candidate to explain the increase in knee osteoarthritis is the modern decline in physical activity. This also means that osteoarthritis may be more preventable than previously thought.