We found BMI to be strongly associated with walking independent of knee pain. In particular, we found BMI to account for 9.7% of the variability of walking in comparison to only 2.9% for pain. These findings suggest that obesity has an important association with low levels of walking in people with or at high risk of knee OA independent of knee pain.
Knee pain accounted for little of the variability of walking when considered along with the effect of obesity. To put the relative effect of pain into perspective, knee pain accounted for only 10% of the total variability accounted by our model (pain, BMI, and all covariates). In contrast, obesity accounted for 35% of the total variability of the same model. We found a similar trend from the standardized beta coefficients with a one standard deviation increase in BMI accounting for more change in walking than the same increase in pain. This difference is notable given that knee pain is a major cause of functional limitation in people with knee OA [30
]. However, from a conceptual perspective, the performance of physical function, such as walking speed, is distinctly different than how much physical activity one performs on a daily basis. Furthermore, previous studies have reported a weak association between knee pain and physical activity [34
]. One possible reason for this is that people may have avoided walking for different reasons. Those with low levels of knee pain did not walk for fear of increasing their knee pain, while those with high knee pain were unable to walk due to current pain levels. Disentangling these associations is needed for future longitudinal studies.
We found obesity to have a strong association with walking, which has been reported previously in adults who are normal weight and obese and general population studies [37
]. Subjects in the highest BMI category walked over 3000 steps less per day than those in the lowest BMI category. The magnitude of this difference is clinically meaningful as it approaches a one standard deviation difference for walking in our sample. Given that our study is cross-sectional, we cannot infer causal direction, and association between obesity and walking is likely bidirectional. For instance, low levels of walking or physical activity could result in obesity. Similarly, people who are obese could have difficulty walking and hence have low levels of walking. Irrespective of the directionality, we found obesity to be strongly associated with walking independent of pain, which underscores the obesity epidemic in the United States and the importance of addressing obesity to avoid future poor health outcomes.
Step counts collected in our study cannot be compared with previous studies utilizing pedometers. Pedometers are known underestimate the number of steps taken by older adults up to 33% compared with a StepWatch [39
], hence step counts in our study are higher than pedometer based studies. However, the average step counts in our study are comparable with smaller studies that employed the StepWatch in people with knee or hip OA and older adults [40
]. For instance, Winter et al. reported 30 people with radiographic knee OA walked 9350 steps/day, which is similar to our finding of 9194 and 8598 steps/day for men and women, respectively.
Our study has several strengths. First, we report daily walking from a large cohort of people with or at high risk of knee OA who wore a validated walking monitor. Second, this is the first study to report the association of obesity with walking independent of knee pain in people with or at high risk of knee OA using a well-validated objective monitor. There are some limitations to our study. First, subjects may have changed walking habits with the knowledge that their habitual walking was being recorded. Previous study suggests this “testing effect” is greatest when subjects wear an unsealed monitor, that is, when subjects are aware of how many steps are being recorded [42
]. We believe any increases in walking due to a testing effect were minimized since study participants did not know the number of steps that were recorded. Second, we acknowledge we employed few psychological measures as covariates in our analyses. We were limited to measures already collected in the MOST study and were not able to add measures of self efficacy or fear avoidance, which are likely associated with walking. Lastly, our sample consisted of people both with and at high risk of knee osteoarthritis, therefore, it is not clear if our findings are directly generalizable to those with knee OA. We performed a sensitivity analysis stratifying our sample by those with and without ROA and found similar effects for pain and obesity within each strata compared with our main findings.