A 2018 report in the JBJS found a lower incidence of knee and hip damage (osteoarthritis) in active marathon runners. 675 Marathoners were questioned. These runners on average had run for 19 years logging an average of 36 miles per week. Arthritis prevalence was only 9 % for runners vs. 18 % for the US population. Seven of the marathoners were able to keep running after hip or knee replacement surgery. So the question is why ? Is it lower BMI, better muscle mass, better bone density, or is impact loading exercise actually protective to our weight bearing joints. At this time, the answer is not known. What is known is that running is good for knees and hips.