The knee is the most commonly invaded large joint in osteoarthritis (OA), and knee OA is an important cause of joint function loss and physical activity limitation in the elderly. Joint laxity is considered to be one of the important factors in the development as well as the progression of knee OA. To evaluate whether increased knee joint laxity in patients with knee OA after daily physical activity (e.g., stair climbing) is associated with progression of osteoarthritis, Dr. TsuyoshiMiyazaki from Fukui University, Japan, conducted a prospective study. The results of the study were published in the December 2012 issue of ARTHRITIS & RHEUMATISM (Arthritis & Rheumatism). The study concluded that those with progressive knee OA were more likely to have greater changes in knee laxity and higher body mass index (BMI) values. Between 2001 and 2003, 136 patients with bilateral primary medial degenerative knee osteoarthritis were enrolled in this prospective study. Baseline data recorded included BMI, muscle strength, joint space width on imaging, physiological force axis during standing on imaging, and anterior-posterior (AP) knee laxity before and after physical activity. Through 8 years of follow-up observation, 84 patients underwent relevant examinations again to assess imaging changes. Imaging progression was defined as a progression of >1 grade of Kellgren/Lawrence classification of the joint compared to the initial examination. There was no significant difference in age, gender, quadriceps muscle strength at baseline, physiological force axis, joint space width, or anterior-posterior joint laxity before physical activity between patients with progressive OA compared to those without progressive disease. There were significant differences between these two groups in terms of BMI at baseline and changes in anterior-posterior joint laxity due to physical activity. Each 1 mm increase in anterior-posterior joint laxity due to physical activity was associated with a 4.15-fold increase in the risk of progression of osteoarthritis of the knee, while each 1 increase in BMI was associated with a 1.24-fold increase in the risk of progression. The results of the study confirmed that patients with progressive disease exhibited greater changes in knee laxity associated with physical activity and higher BMI values than patients with OA without progressive disease. The greater knee laxity changes caused by repeated physical activity and higher BMI may play an important role in knee OA progression.