This study contributes to public health efforts to improve health outcomes of persons with arthritis by examining the association of potentially modifiable risk factors associated with physical inactivity in a large cohort of adults with radiographic evidence of knee OA. An important strength of this study is the objective assessment of physical inactivity. Only one of ten adults with knee OA had recommended physical activity levels. Notably, over one-third of men and half the women were completely inactive, doing no sustained moderate-to-vigorous activity that lasted ≥10 minutes in a week. Modifiable factors significantly associated with inactivity were being overweight/obese and consuming a diet with inadequate fiber, the report of severe knee limitations, and severe knee pain. Over 23.8% of excess inactivity was related to being overweight/obese, and another 12.1% was related to inadequate dietary fiber consumption, after accounting for other descriptive and modifiable factors.
A low level of physical activity among adults with arthritis is a recognized public health concern. However, assessing the magnitude of the problem has been a challenge due to differing methods for assessing physical activity. Earlier studies that relied on self-reported physical activity levels estimate that 23.8–57.8% adults with arthritis in the Unites States are inactive.13,15,16,18–20
Imbedded into these estimates are differences related to the self-report of physical activity and how inactivity was defined. Inactivity was defined by no reported leisure time activity19,49
, less than 10 minutes/week MV leisure activities;13
less than 3 sessions/month lasting ≥15 minutes of activities associated with moderate intensity energy expenditure,16,18
and no reported activities lasting ≥10 minutes.15
In this study, definition of physical inactivity is anchored on the federal DHHS definition and is assessed by objective accelerometer monitoring.
Modifiable factors were evaluated from two perspectives. The first perspective identifies factors associated with physical inactivity at the level of the individual. Modifiable factors significantly associated with inactivity based on adjusted ORs were obesity, knee pain, knee dysfunction, and dietary fiber intake. Earlier studies on adults reporting an arthritis diagnosis18,19
found a significant relationship between inactivity and being overweight, but a 2002 NHIS study did not find a significant association.15
These reports evaluate broader arthritis populations than the current study and rely on self-report to assess inactivity and weight status. Self-report is related to underreporting of weight51
and over reporting of physical activity amount/intensity.51
It is not known how reporting accuracy may influence the apparent association between inactivity and weight status, but measurement issues may contribute to the mixed findings across earlier studies.
Significant associations between inactivity and self-reported knee dysfunction (OR=1.9) and pain (OR=1.7), were in line with findings from the NHIS15
studies. We also found a significant association between inactivity and inadequate fiber intake (OR=1.6). In the general adult population, low fiber intake has been associated with low physical activity levels.22,23
This association may partially reflect the association of a low fiber diet, representing low intake of fruits, vegetables, whole grains and high consumption of refined carbohydrate/sugar related to snacking and a sedentary lifestyle.52,53
Thus, a low fiber diet may be a marker for unhealthy behaviors that include inactivity and high-fat/high-sugar snacking.
A second public health perspective examined the influence of each modifiable factor on inactivity by estimating the AAF for the sample. The sample AAF has public health relevance because the metric incorporates population criteria related to the risk factor prevalence plus its association with the outcome. Recognizing that many individuals had multiple modifiable risk factors (e.g., 80.2% of overweight/obese adults had low fiber diets and 70.8% reported pain), we examined the simultaneous effect of all modifiable risk factors on inactivity. While pain (AAF=6.2%) and dysfunction (AAF=2.6%) are associated with lower levels of inactivity, being obese/overweight (AAF=23.8%) and inadequate dietary fiber (AAF=12.1%) explain a significant and larger proportion of inactivity. The results reinforce the contribution of excess weight and poor diet.
Pain and poor function are commonly viewed as barriers to being physically active for adults with knee OA.15,18,54
These findings indicate that being overweight/obese and an unhealthy diet are also important to consider. There is evidence that higher BMI is related to greater knee pain and poor function in adults with knee OA.55–58
In turn, high levels of pain are associated with binge eating.59
If obesity due to poor dietary patterns contributes to knee pain and resulting poor function through mechanical stress due to excess weight on the joint, then obesity supported by poor dietary choices may contribute to the relationship between knee pain and inactivity. However, randomized controlled trials show that exercise is safe and effective for overweight/obese adults with OA58
. Taken together, these results support incorporating weight loss and diet modification into interventions designed to promote health benefits from physical activity.
There are limitations to acknowledge in the present study. Accelerometers do not provide information on the type of the physical activity (e.g., household, transportation), information which may be helpful in targeting interventions. The accelerometer used in this study cannot capture water activities and may underestimate upper body movement or activities such as cycling. Diary information showed a median of 0 minutes/day spent in water and cycling activities, so the potential underestimate of inactivity is negligible. Radiographic data on knee joint damage and dietary information were only available from baseline, 4 years prior to the current study. Because joint damage does not improve over time and people with subsequent knee replacements were excluded, radiographic verification remains valid. Dietary fiber intake tends to remain stable or decrease slightly over a three to four year period, as demonstrated by control groups from large nutritional trials;60,61
a potential underestimate of inadequate fiber intake would likely understate the strength of its relationship with inactivity found in our analyses. Self-reported data are commonly subject to social desirability bias and recall bias. However, to minimize biases we have used validated questionnaires such as BLOCK 2000 for dietary variables, CES-D for depressive symptoms, WOMAC for knee function and pain, and KOOS for knee confidence. Our results will be strengthened if participants over-reported dietary fiber intake as we would have under-estimated the effects of dietary fiber. Finally, causality cannot be inferred from these cross-sectional data. These limitations must be balanced against the substantial strengths of this study which include the large sample size, clinical measures of height and weight as opposed to self-reports, radiographic verification of knee OA, and the age and gender diversity of this OA cohort. An important strength of this study is that federal DHHS definition of inactivity27
based on objective accelerometer monitoring was used.
Despite substantial health benefits related to physical activity, adults with radiographic knee OA were particularly inactive. A substantial 48.9% of adults with knee OA were classified as inactive, demonstrating no 10 minute episodes of moderate-to-vigorous activity in a week. There is a critical need to intensify public health efforts to reduce physical inactivity among adults with knee OA. Being obese/overweight, the quality of the diet, severe pain, severe dysfunction, and levels of physical activity are inter-related in adults with knee OA. One cannot hope to improve physical activity patterns in adults with knee OA without consideration for weight management, diet, and OA pain and disability, as all may affect successful achievement of activity goals. All components should be considered in developing physical activity interventions that target arthritis populations with low activity levels.