Burden of disease:
The World Health Organization estimates that osteoarthritis (OA) is one of the leading causes of years lost to disability worldwide and one of the most common chronic diseases of the musculoskeletal system [1, 2]. Musculoskeletal disorders including knee OA, result in decreased activity which leads to increased incidence of chronic diseases. Moreover, knee OA leads to work-related inactivity and a vicious downward spiral of increased symptoms and a marked decrease in quality of life [3, 4]. Up to 40% of people aged ≥ 65 in the United Kingdom suffer from symptoms associated with hip or knee OA  and it is well recognized that the prevalence is increasing with age . The lifetime risk of developing symptomatic knee OA is estimated to be 45 % . Due to the high prevalence and substantial impact on people's health and working ability the economic burden of OA is high and in US the estimated costs are 3.4 to 13.2 billion US$ per year . In Denmark, the OA related costs to sickness and early retirement were estimated to 6.8 billion DKK in 2010 .
Total and unicompartmental knee arthroplasty, patient satisfaction, and physical activity:
Total knee arthroplasty (TKA) and unicompartmental knee arthroplasty (UKA) are two of the most frequently performed procedures in orthopedic surgery  and the lifetime risk of undergoing TKA or UKA if one suffers from OA is 30% . Worldwide over a million patients receive a TKA or UKA each year and in Denmark more than 10,000 TKA and UKA's are performed annually [10, 12]. Furthermore, these numbers have increased the past few years and are expected to increase further due to the growing elder population .
Despite, successful surgical procedures, patients following TKA or UKA still demonstrate decreased function and physical activity (PA), earlier retirement, less income, increased health costs and home care compared to matched controls [14, 15]. Nationwide data from the UK has shown that 18% of patients are dissatisfied 1 year post TKA or UKA surgery and poor postoperative knee function and postoperative pain were negatively associated with patient satisfaction .
Early mobilization on the day of surgery and short length of hospital stay (1 day) have been achieved using the so-called fast-track protocols for TKA or UKA [17, 18]. However, little is known about the actual PA and return to an active daily living of the patients after discharge and in the early rehabilitation phase.
Measuring physical activity:
The past decade has focused on optimizing the specific surgical procedure on patients with OA, including the 'fast-track surgery' for TKA or UKA [18-21] but evidence on improving PA with the perspective of a better post-surgical outcome is urgently needed. Low levels of PA have consequences such as increased all-cause mortality and chronic co-morbidity. This is also evident for knee OA. On the contrary, regular PA at moderate or high intensity is associated with substantial health benefits . De Groot et al. (2007) observed that PA levels of Dutch patients with end-stage OA of the hip and knee was reduced (11%) compared to matched healthy controls . Surprisingly, few studies have objectively evaluated PA in OA patients and thus, the influence of optimizing PA following TKA or UKA remains unanswered. Reduced PA was found in Swedish patients scheduled for TKA in comparison with a healthy population . In addition, it was shown that age and body mass index (BMI) were negatively associated with PA leading to the conclusion that specific interventions to improve PA, especially for heavier and older OA patients are needed. Recently, a Danish study on fast-track TKA observed that PA was significantly reduced three weeks following TKA compared to preoperatively calling for early stratified physiotherapeutic interventions . In summary, even though TKA/UKA and subsequently rehabilitation is performed to relieve pain and increase physical function, an overall improvement in PA has been questioned [23-26], suggesting a need for improved and individually stratified rehabilitation strategies in patients at risk for reduced postoperative PA and function .
Health technology, wearables, and gamification:
Health technology such as wearables and motivational feed-back using principles of gamification are new features within health science. There is an acute need for data-driven support and a technological framework that objectively monitors and motivates for PA in patients undergoing elective surgery [15, 24]. Incorporating wearables into health science could catalyze an individualized approach focused on increasing patient motivation for PA with expected increased physical functioning, faster and safer return to work, and increased quality of life. Over a longer course, by integrating individual rehabilitation goals into a tailored patient app, wearables will prioritize a more personalized approach and thereby potentially improve the effectiveness of rehabilitation. SENS Motion (SENS Innovation ApS, Copenhagen, Denmark) is a wireless medical accelerometer for collecting objectively PA data from large cohorts of patients. Lately, SENS Motion has developed a patient app with gamification/motivational feed-back to enhance PA. Gamification has already been proven to increase PA in overweight and obese adults . Furthermore, the SENS system has been validated on knee-OA  patients and very recently in a pilot study demonstrated that it has motivated hospitalized elderly patients with heart and lung diseases to be physical active for 50 minutes more each day . Thus, SENS motion contains a clinically relevant potential to increase patient self-mobilisation following discharge from TKA and UKA.
Objectives of the studies:
Therefore, the aim of this randomized controlled trial (RCT) nested in a prospective cohort is to investigate whether PA following TKA or UKA can be optimized by the use of an activity tracking device including motivational feedback in comparison with activity tracking without feedback. Furthermore, the project will investigate the predictive value of PA level prior to TKA/UKA for the length of stay, return to work, and quality of life.