Physical activity levels in patients with early knee osteoarthritis measured by accelerometry

Osteoarthritis (OA) is the most prevalent form of arthritis in the United States (1). Moderate-to-severe OA affects at least 15.8 million American adults between ages 25 and 74 years, accounting for more than 12% of the population in this age range (2). OA is more prevalent in women than men (3). Indeed, OA is currently the most prevalent chronic condition in women and the rate of self-reported cases is projected to increase in future decades (4). Along with age and being female, obesity is a major risk for OA incidence and severity (5). Joint pain and physical impairment from OA are responsible for extensive use of medical and surgical resources in the U.S. The burden of OA is projected to afflict 40 million Americans by the year 2020 (3). Clearly OA has emerged as a major public health concern negatively impacting individual's lives and socioeconomic situations. The primary goals of OA management are to reduce pain and improve functional ability and quality of life. Patient education and exercise are recommended as two mainstays of treatment. The health benefits of physical activity (PA) are undisputed (6). Regular PA is associated with lower prevalence of obesity and co-morbidities such as diabetes, cardiovascular disease, and hypertension (6-8). Accordingly, the U.S. Centers for Disease Control and Prevention (CDC) (9) and the American College of Sports Medicine (ACSM) (10) have implemented recommendations for PA to promote health benefits for the general public. These recommendations are based on intensity of PA reported in standard units of metabolic equivalents (METs) (9;10). Activities are categorized by intensity as light (LPA 6 METs) (9). According to CDC/ACSM recommendations, adults should engage in a minimum of 30 min/day of at least moderate-intensity (≥ 3 METs ) PA on at least 5 days of the week (9;10). Work group recommendations from the 2002 Exercise and Physical Activity Conference (EPAC), advise knee OA patients to accumulate 30 minutes of at least moderate-intensity (≥ 3 METs) PA on at least 3 days of the week (11). VPA is associated with increased health benefits (6). The pain and discomfort felt during certain types of weight bearing VPA (i.e. running) may inhibit knee OA patients from choosing these modes of activity. However, non-weight bearing modes of VPA (e.g. resistance exercise) may be safe and feasible for knee OA patients, depending on the local mechanical environment (12) and thus, the accumulation of 30 min/day of a combination of moderate- and vigorous-intensity PA, hereafter called MVPA (≥ 3 METs), is likely a beneficial and realistic PA goal for knee OA patients. Published intervention studies have shown beneficial effects of PA on relevant outcomes in patients with knee OA (13-16). While it is generally assumed that the pain and impairment associated with knee OA limits PA, typical levels and patterns of PA in knee OA patients are poorly described. Thus far, no studies have objectively measured PA levels in the knee OA population nor have they determined whether this population achieves recommended PA levels (9;10). Existing studies are limited in their ability to identify specific knee OA populations that could benefit from more PA due to problems inherent in self-report measures of PA (17) and small samples with heterogeneity in disease severity and duration. Furthermore, despite the higher prevalence of OA in women (3), potential sex differences in the level and pattern of PA that may be implicated have yet to be examined. Decisions regarding the allocation of resources for promoting PA in knee OA patients should be informed by accurate estimates of knee OA population PA levels. Thus, the primary aim of this study was to objectively assess the level and pattern of health-enhancing MVPA in early knee OA patients and to determine the proportion of subjects who achieve the PA recommendations implemented by the CDC/ACSM (9;10) and EPAC (11). The secondary aim was to compare PA in male and female knee OA patients.