However, in the absence of a control group, estimating direct associations by longitudinal analysis is the best alternative for the purpose of this study. Secondly, this study specifically included patients with knee OA suffering from instability of the knee joint (i.e. biomechanically assessed and/or self-reported); therefore, the results may only be representative for this subgroup. Thirdly, it has been assumed that improved muscle strength resulted in reduced pain and activity limitations. However, this association may also be in the opposite direction (i.e. pain relief led to improved muscle strength).