We found that positive affect was not associated with more daily walking among adults with knee OA overall. However in the presence of knee pain, respondents with high positive affect walked 711 more steps per day compared with those with low positive affect, representing about 8.5% more steps from a mean of 8400 steps per day. Increasing daily walking by this amount is clinically meaningful as it could mean attaining a higher level of physical activity based on pedometer-determined physical activity indices (e.g. being sedentary or at a low active level of physical activity)(36
). In contrast, respondents without knee pain walked a similar amount per day whether or not they had high or low positive affect. These findings suggest that the absence of depressive symptoms may not suffice to promote daily walking in people with knee pain. Rather high positive affect is an important psychological feature associated with daily walking and may be needed to increase daily walking among people with both knee OA and knee pain.
Our findings highlight that knee pain modifies the relationship between positive affect and walking among adults with knee OA. Positive psychological factors could be particularly important when health is challenged, and may be why positive affect was associated with daily walking in respondents with knee pain, as opposed to those without knee pain. These findings support the ‘broaden and build’ theory, which hypothesizes that persons with positive affect are able to better marshal a broad range of physical and psychological resources in response to stress compared to persons without positive affect.(37
) For example, it may be that people with high positive affect are better able to cope to ‘work through’ knee pain and achieve more daily walking compared to people with low positive affect. However, given the cross-sectional nature of our study, we cannot determine the temporal relationships between positive affect and walking; longitudinal studies are needed to better understand this association.
Our results are consistent with previous observational studies that found protective effects for positive affect with health related outcomes in older adults.(10
) Evidence exists that negative psychological attributes, such as fear-avoidance beliefs and catastrophizing, influence functional outcomes in adults with low back pain .(39
) Our findings extend this research to positive psychological attributes influencing beneficial health behaviors among adults with knee OA. These findings have important clinical implications. In contrast to theories that view positive affect as a trait and therefore unlikely to be modifiable, recent intervention studies have increased positive affect in persons with cardiopulmonary disease(40
) and with newly-diagnosed HIV.(41
) Moreover, recent trials have shown that interventions aimed at increasing positive affect resulted in increased physical activity in people with asthma(42
) and adults after percutaneous coronary intervention(43
), as well as increased adherence to medication in African Americans with hypertension.(44
) These studies suggest that people with OA could be taught techniques to increase positive affect and other positive emotions, which might improve walking and other health behaviors, in addition to psychological well-being.
We found that depressive symptoms were associated with less daily walking among respondents without knee pain, which is consistent with previous studies.(45
) However, we did not find this association among people with knee pain. One possible reason for this was that walking overall decreased for people with knee pain, and there was little difference in walking between those with low positive affect and depressive symptoms. For instance, in crude analysis people with low positive affect walked 9168.5 steps/day without knee pain, and 7383.8 steps/day with knee pain. For those with depressive symptoms, there was less difference between those with and without knee pain. Hence, using persons with low positive affect as the referent group may have led to larger observed differences in walking in the group with depressive symptoms without knee pain, and smaller differences in the group with depressive symptoms with knee pain. Nonetheless, the crude analyses showed that persons with depressive symptoms walked less than other respondents in each subgroup, suggesting that depressed persons with knee OA walk less than other respondents, regardless of the amount of knee pain.
The observed step counts in this sample were comparable to previous studies in older adults measured with a StepWatch. For instance, Cavanaugh et al reported an average of 9981 steps/day among healthy older adults and 7681 steps/day among older adults with self reported functional limitations.(47
) Our sample took on average 8395 steps per day, fitting between the healthy and functionally limited groups. This result was expected given that our sample was comprised of respondents with and without functional limitation.(48
) Furthermore, it is important to note that step/day collected in our study cannot be easily compared with previous studies utilizing pedometers. Pedometers are known to underestimate the number of steps taken at slow speeds up to 33% compared with a StepWatch.(49
) Hence, step counts in our study are higher than those reported for pedometer-based studies.
Limitations of our study should be acknowledged. First, 24% of the MOST study cohort with radiographic knee OA who attended the 60-month visit chose not to wear the accelerometer or had insufficient data for these analyses. Those included in our study were more likely to have better health indicators compared to those not included in the analysis. Hence, our study findings may have limited generalizability to all people with knee OA. Second, we defined the presence of knee pain using the presence of pain at two time points, a clinic visit and a preceding telephone screen. We performed a sensitivity analysis defining knee pain using severity measured on a Visual Analogue Scale with a cut-point of 10/100, and found similar results. Furthermore, we repeated analysis including all study participants, irrespective of whether they had radiographic knee OA, and found similar results among those with knee pain defined both ways. Third, in addition to knee pain, lower body pain at the feet, ankles, or hips can also alter daily walking. Though we adjusted for widespread pain in our analyses, this may not fully account for lower body pain. We performed a sensitivity analysis adjusting for the presence of lower body pain and found similar results. Fourth, participants may have changed daily walking habits with the knowledge that their habitual walking was being recorded. Previous research suggests that this “testing effect” is greatest when participants wear an unsealed monitor, i.e., when participants are aware of how many steps are being recorded.(50
) We believe any increases in daily walking due to a testing effect were minimized since the StepWatch did not display recorded data to study participants. Fifth, other measures of positive affect exist,(51
) however these measures were not available within the MOST 60 month exam. Using questions from the CES-D has been found to be a valid measure of positive affect(52
) and has been employed as such in previous studies.(10
) Lastly, our study was cross-sectional, which precluded inferring causality from our findings.
Despite these limitations, our study had several important implications. Our findings support the notion that positive affect is associated with daily walking, though this association appears dependent on the presence of knee pain. Furthermore, people with high positive affect and low positive affect walk considerably more if they do not have knee pain. Thus, in the context of knee pain, high positive affect enhances walking among persons with knee pain. Thus, future research examining the association of psychological health with physical activity or other health outcomes in older adults should consider the effect of pain as a potential modifier.
In conclusion, we found that high positive affect was associated with daily walking among adults with knee OA and knee pain. Given that physical activity is recommended to reduce the risk of poor health outcomes in adults, it is important to identify factors such as high positive affect, to promote an active lifestyle. Our findings suggest that the absence of depressive symptoms may not be enough to promote daily walking and that high positive affect may be an important factor among older adults with knee pain. Longitudinal studies are needed to investigate the temporal relationship between positive affect and daily walking among older adults.