The purpose of this study is to explore mechanisms that can explain physical function in patients with knee OA in primary care. The study's main and primary goal is to quantify which baseline factors and change scores are the strongest predictors of a positive treatment outcome in terms of patients' global perceived change after physiotherapy. Secondary research question is: which factors are the strongest predictors of a positive outcome of physiotherapy in knee OA, in terms of pain intensity, patient specific functional scale, and function in daily living?