Individuals with painful knee osteoarthritis (OA) experience restricted physical function. Few previous studies have investigated comprehensive sets of constructs and tests to investigate physical function in these patients. Most single studies of knee OA patients show limited scope for explaining physical function in the perspective of WHO's International Classification of Functioning, Disability and Health (ICF) 3 levels: Body functions (the bodily level), Activities (individual level), and Participation (societal level). On the bodily level there is limited case-control knowledge on muscle strength beyond the knee and endurance-strength in general. On the activity level, there is similar limitation of biomechanical bodily features in stair climbing. On the participation level the situation is similar for objective performance. Measures are needed that inter-relate all three levels of ICF. These should also include well discriminating tests e.g. knee strength, walking, and activities of daily life (ADL), as well as both objective and subjective measures.