• Pain [ Time Frame: week 0, 13, 39 and 65 ]
  
• Physical function [ Time Frame: week 0, 13, 39 and 65 ]
  
• Patient global assessment [ Time Frame: week 13, 39 and 65 ]
  

• Tiredness [ Time Frame: week 0, 13, 39 and 65 ]
  
• Stiffness [ Time Frame: week 0, 13, 39 and 65 ]
  
• Joint mobility [ Time Frame: week 0, 13, and 65 ]
  
• Muscle strength [ Time Frame: week 0, 13, and 65 ]
  
• Patient-specific physical function [ Time Frame: week 0, 13, 39 and 65 ]
  
• Walking test [ Time Frame: week 0, 13, and 65 ]
  
• Pain coping [ Time Frame: week 0, and 65 ]
  
• Locus of control [ Time Frame: week 0 and 65 ]
  
• Quality of life [ Time Frame: week 0, 13, 39 and 65 ]
  
• Exercise adherence [ Time Frame: week 13, 39 and 65 ]
  
• Social support [ Time Frame: week 0, and 65 ]
  
• Level of performed activities [ Time Frame: week 0, 13, 39 and 65 ]
  

The principle objectives of managing OA are to control pain adequately, improve function and reduce disability. There is strong evidence that exercise therapy has a short term benefit for OA. However, these beneficial effects decrease over time and finally disappear. This decline is thought to be related to the difficulties people have in maintaining adherence to prescribed exercises. Therefore, to enhance long term benefit, adherence to exercise therapy is of utmost importance. Recently, the focus of attention within physiotherapy has shifted towards behaviorally oriented treatment, like Behavioral Graded Activity (BGA), which focuses less on pain and includes psychological and social factors in the treatment-process. Such intervention seems appropriate to increase the level of activities of patients with OA in a time-contingent way and to increase patients' adherence to these activities.

However, at the start of the present study the scientific evidence for the effectiveness of BGA in patients with a progressive and specific chronic disease, like OA of the hip and knee, was not available. Therefore, we performed a randomized controlled trial to study the effectiveness of BGA in patients with OA of the hip or knee. It was hypothesised that in the long term BGA results in less pain, less limitations in activities, and better patient global assessment (i.e. the effect of treatment perceived by patients themselves), compared to usual care of physiotherapists (UC). UC was operationalized as physiotherapeutic care according to the Dutch physiotherapy guideline for patients with hip and/or knee OA. It was also investigated whether specific subgroups of patients benefited more from BGA and which factors influenced the success of BGA-treatment. Also, it will be investigated whether differences exist in exercise adherence and whether there is a relationship between exercise adherence and long-term effectiveness.

Inclusion Criteria:

• Osteoarthritis of hip or knee according to the clinical criteria of the American College of Rheumatology

Exclusion Criteria:

• other pathology explaining the complaints;
• complaints in less than 10 out of 30 days;
• treatment for these complaints with exercise therapy in the preceding six months;
• under 50 or over 80 years of age;
• indication for hip or knee replacement within one year;
• contraindication for exercise therapy;
• inability to understand the Dutch language;
• a high level of physical function (since patients who perform at a high level of physical function at baseline do not need to increase their level of physical function. A high level of physical function was operationalized on a score of less than two on the sections walking ability and physical function of the Algofunctional index)