These findings report on the prevalence of meeting the Physical Activity Guidelines for Americans aerobic recommendations and on time spent in different physical activity intensity levels using objectively measured accelerometer data among adults with radiographic knee OA. Despite substantial evidence showing that health benefits are related to physical activity, persons with knee OA participated in little physical activity. Less than one of seven men and one of twelve women with knee OA accumulated sufficient physical activity to meet the Guidelines. More than one-third of men and more than half the women were completely inactive, doing no sustained MV activity that lasted 10 minutes or more. Although men compared to women engaged in significantly more daily minutes of MV intensity activity they also accrued significantly more time engaged in no/very light intensity activities than women. These findings point to the urgent need for widespread dissemination of public health interventions to reduce the sedentary lifestyle of the 27 million adults with knee OA.
This study based on objectively measured physical activity using accelerometers found that 12.9% of men and 7.7% of women with radiographic knee OA met the current Guidelines. This finding indicates the vast majority of adults with knee OA are not participating in guideline recommendations to benefit overall health (e.g. mortality reduction from cardiovascular disease). National data on self-reported physical activity from the 2002 National Health Interview Survey (NHIS) found that 30% of adults age 18+ with self-reported doctor-diagnosed arthritis met physical activity recommendations.16
Self-reported physical activity information from the 2000 and 2001 Behaviors Risk Factor Surveillance Survey (BRFSS) state surveys showed that 22%-40% of adults age 45+ with self-reported doctor-diagnosed arthritis met recommended levels.35,36
One clinical study that objectively assessed physical activity using accelerometers (n=259) found that 30% of adults aged 35–65 years with confirmed early knee OA accumulated at least 30 minutes/day MV activity. However that study did not assess bouted MV activity, the metric for public health recommendations, which comprises less than one-half of overall MV activity in adults over age 40.17
Our results applying the current Guidelines (at least 150 bouted minutes MV activity per week
) to objective accelerometer assessments from a knee OA cohort indicated that a much lower proportion met physical activity recommendations than these previous reports in arthritis populations. Consistent with these knee OA findings, a sensitivity analysis using the OAI accelerometer sample without
baseline knee OA showed a lower proportion met current Guidelines (15.7%) based on objective assessment than estimates from the 2002 NHIS sample without arthritis (38% met recommendations) based on self-reported physical activity.16
There may be several reasons why our prevalence estimates of meeting physical activity recommendations among adults with arthritis are lower than previous prevalence estimates. First and arguably the strongest contributor to differences is that these previous population-based estimates used self-reports of physical activity to assess if physical activity guidelines were met, in contrast to the current study based on objectively measured physical activity. Self-reports of physical activity have been shown to overestimate objectively measured physical activity in the general population, particularly in older, obese individuals.37;38
Second, prior to the 2008 Guidelines, meeting physical activity recommendations required stricter criteria based on engaging in 30 minutes or more per day
of bouted [episodes lasting at least 10 minutes] moderate intensity activity on 5 or more days of the week (5×30) or 20 minutes of vigorous intensity activity on 3 or more days per week (3×20).39
Persons that did some moderate intensity or vigorous intensity activity but not enough to meet either the 5×30 or the 3×20 criteria would be classified as not meeting physical activity recommendations. In practice, this issue likely has little effect since we summed all moderate and vigorous intensity activity before applying the 150 minutes/week Guideline recommendation and still found very low rates of meeting recommendations. Third, the case definition of arthritis used in national health surveys such as the NHIS and BRFSS include persons with OA, rheumatoid arthritis, lupus, fibromyalgia, and gout, while our study used a strict case definition of radiographic knee OA. It is not known how much the prevalence of meeting physical activity recommendations varies among different types of arthritis. However, OA is the most common type of arthritis in the US and would be the predominant type of arthritis represented in those national samples. Last, the NHIS national health survey included adults with arthritis as young as 18 years, while our study evaluated participants aged 49+ who may be less active than younger adults. However 30% of that NHIS arthritis population met physical activity guidelines, which is comparable to the 22%–40% of the BRFSS arthritis population of adults age 45+ meeting guidelines, so it is likely that differences due to a wider age span are small.
A substantial 40.1% of men and 56.5% of women were classified inactive, having no MV episodes that lasted 10 minutes or longer. These results are consistent with the 2002 NHIS findings that classified 40.0% of men and 45.8% of women with arthritis as inactive based on their self-reported activity.16
Our study confirms these previous estimates using an objective measure of activity. Although no minimum dose of MV activity that results in health benefits has been identified for adults with arthritis specifically, just moving from inactive to low active (1–149 bouted MV minutes per week) has been shown to have substantial benefits including reduced mortality and risk for incident coronary heart disease, hypertension and diabetes. For example, as little as 60–90 minutes per week of MV physical activity lowers the risk of premature mortality by approximately 25%.27
This health benefit, coupled with the fact that moderate, low impact exercise has been proven safe and effective for adults with arthritis, affirms the Healthy People 2020 recommendation that adults with OA should be counseled to be as physically active as possible (i.e., avoid inactivity) even if they may never intend to engage in sufficient activity to meet recommendations.40
Indeed, the avoidance of time spent in no or very light activity may be the first realistic goal for those with knee pain/mobility issues.
Research on physical activity has largely concentrated on time spent in MV activity. However that focus neglects the potential health benefits of time spent in light-intensity activities over no or very light intensity activity such as sitting. Physiologically, unbroken absence of activity suppresses skeletal muscle lipoprotein lipase (LPL) activity and reduces glucose uptake41,42
. Time spent no/very light activity is associated with a larger waist circumference, poor 2-hour plasma glucose levels, triglyceride profiles, and increased metabolic risk scores.43,44
Being sedentary can have adverse effects even among otherwise physically active people. In a study of over 4000 adults from the Australian Diabetes, Obesity, and Lifestyle study, longer television time was significantly associated with larger waist circumference, higher systolic blood pressure and 2-hour plasma glucose, even among active adults who reported activity levels compatible with guidelines.44
We found a negative correlation between no/very light activity time and light activity in this knee OA cohort (r=−0.48), which was stronger than its correlation with MV activity time. Trading sitting activities for light intensity activities such as gardening or leisurely walking may be an intermediate step to change undesirable behaviors mediated through improved self-efficacy. This strategy may be particularly helpful for adults with arthritis who fear their symptoms will become worse through increased activity levels. Once they have been successful at replacing time spent in no/very light intensity activities with light activity, counseling efforts can be targeted to increasing the intensity of activity. In addition, any movement beyond lying or sitting contributes to total daily energy expenditure; increased activity when coupled with dietary caloric restriction may produce weight loss. These potential benefits motivate the promotion of light to moderate intensity activities in persons with knee OA as a feasible approach to reduce no/very light activity time and possibly improve health outcomes.
This study had substantial strengths which include the large sample size, the objective accelerometer assessment of physical activity, radiographic verification of knee OA, and the age and gender diversity of this OA cohort. There are limitations to acknowledge in the present study. Accelerometers do not provide qualitative information on context of the physical activity (e.g., household, transportation, outdoor location), information which may be helpful to target interventions. While accelerometer information could assess the aerobic component of the physical activity guideline recommendation, it is not known if that activity was accomplished using low impact activities as advised for people with arthritis; nor could it assess the muscle-strengthening component of the recommendation. The accelerometer model used in this study cannot capture water activities and may underestimate upper body movement or vertical acceleration/deceleration activities, such as cycling. Diary information indicated that the median time this sample spent in water and cycling activities was 0 minutes/day, so the potential underestimate is negligible. It is possible that wearing an accelerometer may made individuals more aware of activity, providing a stimulus to participate in physical activity. To minimize such effects the accelerometer provided no feedback to the participant on monitored activity. If participation rates were inflated due to the presence of the monitor, the true day to day physical activity levels would be even lower than those observed. Radiographic data on joint damage were only available from baseline, 4 years prior to the current study. Sensitivity analyses that controlled for baseline radiographic status showed similar statistical differences to the reported findings. Last, it is recognized that adults not measured in the OAI physical activity ancillary study were more likely to be female, African American, and had greater baseline pain than study participants. Because these differential characteristics are associated with lower levels of physical activity, our findings represent a conservative upper bound on physical activity levels for adults with knee OA. However sensitivity analyses that accounted for these differential characteristics among the unmeasured group yielded similar estimates and identical trends, suggesting that any bias in our findings is small.