We determined that the low physical activity criterion of frailty can be effectively assessed using a subset of the up to 20 activities in the current questionnaire. We recommend including 6 activities (walking, chores, gardening, general exercise, mowing and golfing) in a surrogate. This surrogate has strong validity because: (1
) it was developed using baseline CHS data and has similar predictive accuracy at follow up, (2
) the resultant reclassifications in terms of low physical activity status and categorical (robust/prefrail/frail) frailty status are relatively low and (3
) frailty status – as defined by the surrogate – has an association with the risk of mortality in older adults similar to that found with the original frailty criteria. Use of this streamlined surrogate will ease participant burden, simplify the assignment of low physical activity status and speed frailty assessment for epidemiological studies, and could potentially lead to the development of a frailty screening tool for routine clinical exams.
We aimed to simplify assessment, so our ultimate recommendation uses the same activities for both men and women. However, we found that women required fewer activities than men for accurate assessment, so studies involving only women can reasonably use a smaller set of activities (chores, walking, general exercising and gardening were the 4 activities with highest relative influence for women).
We confirmed that the WHAS surrogate includes key activities for determining physical activity status in an older adult population similar to the CHS. The surrogate corresponds well with the original criterion and our cross-validation using data from the CHS provides evidence to support its continued application. For future applications of the WHAS surrogate, we recommend that investigators consider applying the higher threshold on kcal/week expenditures (105 kcal/week for women and 148 for men) for assignment of low physical activity status. The original WHAS threshold produces a lower prevalence of low physical activity and might attenuate frailty-related findings.
A limitation of our study lies in the selection of the number of activities to include in a surrogate. We informally weighed the tradeoff between efficiency (fewer activities) and good predictive accuracy (more activities). If additional information had been available on the relative costs of false positives and false negatives in low physical activity misclassification and on the cost of administering the current questionnaire, a cost-benefit analysis could have been performed to determine the optimal number of activities and the optimal discrimination threshold. It is unclear how streamlining will affect the face validity of physical activity assessment. Participants may miss the opportunity to respond on activities of their choice. Applications of the 6 activity surrogate in ongoing or new studies will provide insight.
Our work assumes that the current standardized MLTA questionnaire and criterion for low physical activity is a gold standard. This may not be true. For the CHS study population, the WHAS and 6 activity surrogates produced frailty status hazard ratios for 3 year mortality that were larger than for frailty as originally defined. This provides evidence that other physical activity measures may potentially produce a refined frailty categorization.
In this study, we had a narrowly defined goal of identifying and evaluating surrogates based on subsets of activities in the current questionnaire. In related work (23
), we investigated whether surrogates can be constructed from other physical activity information available in the CHS that was not used in the original criterion, including: the number of city blocks walked outside the home in the last week, usual walking pace outside the home, the number of flights of stairs climbed in the last week, and the number of hours spent seated or lying down in a typical 24 hour period. Neither of these studies answers the broader and important question of how to best assess low physical activity for frailty. However, this work does produce information that can be used to address the question in the future, possibly with an entirely novel omnibus questionnaire, although questionnaires may not be the best technology with which to assess activity.
Physical activity is challenging to assess in older adults, as evinced by the large body of literature on the subject and the range of assessment methods applied in other studies to determine the low physical activity criterion of frailty (24
). The assessment of physical activity for frailty differs from much of this work because it aims only to identify older adults at the lowest end of the energy expenditure spectrum. Future work on developing alternative assessments might consider focusing on a small number of age-appropriate low to moderate energy activities (not limited to leisure-time activities) that effectively distinguish between low and not low physical activity adults. It is also of interest to develop a screening tool that can be applied to older adults who are not living in the community but, for example, who are hospitalized or institutionalized. Differences in culture, socioeconomic status and the built environment may require additional consideration. The results in this paper should be interpreted as pertaining to populations similar to CHS participants.