Interesting paper from the RSNA conference, where patients at risk for knee arthritis who performed light exercise had less onset of arthritis and patients involved in pounding activities like running had more knee arthritis. This makes sense and is consistent with some of the published data on how knee mesenchymal stem cells actually differentiate better with activities similar to walking. This also fits with other studies of animal models of cartilage healing showing that animals left to their own activities after surgery produced better cartilage healing than those who were completely restricted from any activity (casted or immobilized). This body of mounting evidence may call into question our orthopedic surgery concept of extensive immobilization and non weight bearing after surgery. This is the reason we generally will encourage light activity after our stem cell procedures for the knee and other weight bearing joints.