Recruitment too slow, probably due to the procedure considered too complicated.

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.

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?

Inclusion Criteria:

1. Clinical and radiological osteoarthritis of the knee and referred to physiotherapists.
2. Symptomatic daily during the last month.
3. Symptomatic for more than one year.
4. Able to walk up and down a flight of 10 stairs with or without walking aids.
5. Able to walk 3 stairs up and down without walking aids.
6. Understand Norwegian orally and in writing.
7. Body mass index below 35.

Exclusion Criteria:

1. Neurological, rheumatic, orthopedic, or respiratory diagnosis, other than osteoarthritis of the knee which can negatively affect the walking ability, balance or pain.
2. Body mass index above or equal to 35.
3. Fracture of the femur or shank, or arthroscopic surgery in the osteoarthritic knee.
4. Chronic generalized pain (American College of Rheumatology Criteria, 2010).
5. Inability to understand Norwegian language.

Knee OA patients recruited from a private physiotherapy practice in Trondheim, Norway.